Final Drive Ratio

C

clark searle

New member
Dec 2, 1997
76
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0
Would be interested in comments from owners who have changed their final drive
ratio (especially 3.42 vs. 3.67). Wes C. reports that the 3.42 ratio was on the
drawing boards at GM before the demise of our coach, and it is optimized for
tooth strength. That gives an 11% increase in ratio, dropping the engine and
tranny torque at a given driving condition by that amount and raising available
engine torque and power by increasing the RPM from about 2100 at 55 MPH to 2400
RPM.

On the other hand the 3.67 ratio is almost a 20% change in ratio (vs. 3.07). Is
that overkill? Does anyone know of problems with pinion and ring gear wear or
excess noise with the 3.67 ratio? By my calculation at 55 MPH, the RPM should
be around 2600.

Max. engine torque (both 455 and 403 engines) is at 2400 RPM and generally
that's also about the speed that gives maximum efficiency (fuel economy).

TIA

Clark
78 Kingsley
 
Hi Clark, this is Chuck, 78 Eleganza II, I have the CASPRO Final Drive and his
transmission gears. My GMC runs fine. I just got a newly rebuilt 455 installed.
I had some very bad luck this past year. Three engines in 1,000 miles. This one
is ok. Tom Green built it and Joe Mondello in Paso robles provided the parts. I
drove the Eleganza probably 25 maybe 30 thousand with the final drive and gears.
The best trip I had was to Tennessee to the F-100 Nationals a couple a years ago.
No problems on the road until I was coming back and had a thermostat stick in
Wyoming. Who carries a spare Thermostat. I do now. I can't answer you question
about which gearing to use. I don't know what CASPRO supplied. But it works fine
especially going up hills (mountains) like to Las Vegas or Donner Pass or just
along the Pacific Coast. My old set up I used to get about 12 to 14 miles per
gallon. I don't know what this new engine will do. It has about a 8 to 1
compression. You can get Chuck Stoddard on the web look for CASPRO or one of the
other GMC links. I don't have any of this stuff here. Chuck

> Didn't get much response from my earlier post on this subject. So, let me try
> again.
> It seems to me that a higher final drive ratio than 3.07 would make a big
> difference in drivability, but I'm having trouble justifying the $1,500 to make
> the change.
> And if I did, which ratio is recommended.
> I was hoping to get some user input to help with these decisions.
> Any advice from users of 3.42, 3.55 or 3.67 ratios?
> Clark
> 78 Kingsley
 
Didn't get much response from my earlier post on this subject. So, let me try
again.
It seems to me that a higher final drive ratio than 3.07 would make a big
difference in drivability, but I'm having trouble justifying the $1,500 to make
the change.
And if I did, which ratio is recommended.
I was hoping to get some user input to help with these decisions.
Any advice from users of 3.42, 3.55 or 3.67 ratios?
Clark
78 Kingsley
 
3.67 & 3.46 are not available at this time. Marvin shut down his opporation due to
health ( and the death of his partner) he is looking for a buyer for his business
last I knew. I have a 3.42 we have been driving trouble free since '95. I agree it
seems like a LOT of money but I'll do it again on 'This olde Coach', but probably a
3.55. It just makes the motor feel better.
Stephen
73 X-Painted Desert

> Didn't get much response from my earlier post on this subject. So, let me try
> again.
> It seems to me that a higher final drive ratio than 3.07 would make a big
> difference in drivability, but I'm having trouble justifying the $1,500 to make
> the change.
> And if I did, which ratio is recommended.
> I was hoping to get some user input to help with these decisions.
> Any advice from users of 3.42, 3.55 or 3.67 ratios?
> Clark
> 78 Kingsley
 
How do you know what you have? I purchased CASPRO's final drive a couple
of years ago as well as his transmission gears, they have been installed
and seem to work fine. Chuck

> snip..........
> >It seems to me that a higher final drive ratio than 3.07 would make a
> >big
> >difference in drivability, but I'm having trouble justifying the
> >$1,500 to make
> >the change.
> >And if I did, which ratio is recommended.
> >I was hoping to get some user input to help with these decisions.
> >Any advice from users of 3.42, 3.55 or 3.67 ratios?
> >Clark
> >78 Kingsley
> >
> I am waiting for my 3.07 to crap out (the investment is more easily
> justified) but with my luck, it will last forever!
>
> I have heard:
> 3.42 All I know is, Steve Galovic likes his and that is good enough for
> me!
> 3.46 is noisy..........Sirum has sent lots of them back
> 3.55 also noisy to some; probably optimal ratio!
> 3.66 hear (from owners) it is quieter. Maybe too high!
>
> I would most likely go for 3.55 or 3.66 since I always to a car.
>
> What is the consensus?
>
> Dave Greenberg
> GMC Motorhome Registry
> 200 MacFarlane Dr PH4
> Delray Beach, FL 33483-6829
 
snip..........
>It seems to me that a higher final drive ratio than 3.07 would make a
>big
>difference in drivability, but I'm having trouble justifying the
>$1,500 to make
>the change.
>And if I did, which ratio is recommended.
>I was hoping to get some user input to help with these decisions.
>Any advice from users of 3.42, 3.55 or 3.67 ratios?
>Clark
>78 Kingsley
>
I am waiting for my 3.07 to crap out (the investment is more easily
justified) but with my luck, it will last forever!

I have heard:
3.42 All I know is, Steve Galovic likes his and that is good enough for
me!
3.46 is noisy..........Sirum has sent lots of them back
3.55 also noisy to some; probably optimal ratio!
3.66 hear (from owners) it is quieter. Maybe too high!

I would most likely go for 3.55 or 3.66 since I always to a car.

What is the consensus?

Dave Greenberg
GMC Motorhome Registry
200 MacFarlane Dr PH4
Delray Beach, FL 33483-6829
 
I just pulled out the Caspro catalog and was looking through it as you wrote. I
will go out and look in the GMC in the book of replacement parts etc. and see
what he sent me. Ouintrans built the transmission with his parts. A shift kit
and torque converter were also in the rebuild. I don't know what ones though.
It just goes good. With the new engine I am going to take a couple of days and
go up the coast to see how it handles. I think I have some sort of problem in
either the left wheel hub and knuckle or steering, after the 60/65 mph rate the
coach seems to want to wander a bit. I checked the air pressure and looked at
the shocks. They all look good and the air was equal in both front tires at 70
pounds of air in the "E" rated Michelins. Less than 1500 miles on them. Also
the ball joints were replaced at the same time the tires were installed. If I
have to replace the hub and knuckle on the left side I have the tools from Ken
Toma but was looking at the Caspro set up. It seems like a lot of money. I
don't know what a Toma set plus the bearing cost. I guess I will try to do some
comparison shopping first. Toma vs Caspro. Chuck

>
> > How do you know what you have? I purchased CASPRO's final drive a couple
> > of years ago as well as his transmission gears,
>
> Chuck,
> According to Caspro's latest catalog, their "transmission gears" (actually
> different torque converter to transmission sprockets and a chain to match)
> produce a 3.50:1 overall ratio when used with the OEM 3.07:1 final drive
> gears, therefore the sprocket ratio must be 3.50 / 3.07 = 1.14:1. They sell
> two different final drives; a rebuilt OEM 3.07:1 ratio, and a rebuilt early
> Toronado planetary gear final drive with a 3.21:1 ratio, identifiable by its
> 8-bolt cover, vs the 3.07's 10-bolt cover. Assuming this is the one you got,
> you end up with 3.21 X 1.14 = 3.66:1 ratio.
> Hope this clarifies it a little. ;-)
>
> Rick Staples,
> '75 Eleganza, Louisville, CO
 
I too, have a 3.42 final drive ratio. Unless one has operated a coach
with a succession of final drive ratios, I doubt that anyone can tell
you one is better than another.

I know one owner, Dick Crawford, who has had a succession (i. e.
original, 3.21:1, 3.46:1 and 3.67:1). Earlier this year when I asked
him about the 3.67:1 final drive, he told me he couldn't tell any
difference from the 3.46:1 one.

In my case, as long as I'm not driving on mountain highways, I rarely
lose any speed anymore navigating hills along my routes. However, the
exhaust system I installed (i. e. headers, Flowmaster mufflers and 3"
exhaust piping) may be more responsible for what I experience.

As I have previously related on a coup[le of occasions "While convoying
with Bobby Moore (in his 76 GMC with a 455 cid engine and me with a 78
GMC with a 403 cid engine) from Louisville KY to the Boerne TX GMCMI
Rally last year, about 25 miles s of Louisville on I-65, there is a long
3-4 mile sweeping S-curve in the road that climbs a healthy grade toward
Elizabethtown KY. As we approached the S-curve, Bobby, who was in the
lead, said over the CB "set your cruise on 65 mph and lets see what
happens". About two-thirds of the way into the S-curve, I caught Bobby
and passed him while still on cruise That should give you some idea of
the effect a 3.42:1 ring and pinion gear will give you".
Prior to installing the larger exhaust system, but after installing the
new ring and pinion gears, I was able to notice a definite change in
passing ability at highway speeds (i. e. 60 mph) when pulling out to
pass a vehicle in front of me. Acceleration in that situation was much
improved and noticeable right off the bat.

Also, I notice more acceleration effect when I'm at or below 65 mph
compared to above that speed, particularly on hills.

Paul Bartz

From: Stephen J. Galovic III [mailto:galovic]
Sent: Wednesday, November 04, 1998 11:16 PM
Subject: Re: GMC: Final Drive Ratio

3.67 & 3.46 are not available at this time. Marvin shut down his
opporation due to health ( and the death of his partner) he is looking
for a buyer for his business last I knew. I have a 3.42 we have been
driving trouble free since '95. I agree it seems like a LOT of money but
I'll do it again on 'This olde Coach', but probably a 3.55. It just
makes the motor feel better.
Stephen
73 X-Painted Desert

Didn't get much response from my earlier post on this subject. So, let
me try again.
It seems to me that a higher final drive ratio than 3.07 would make a
big difference in drivability, but I'm having trouble justifying the
$1,500 to make the change.
And if I did, which ratio is recommended.
I was hoping to get some user input to help with these decisions.
Any advice from users of 3.42, 3.55 or 3.67 ratios?

Clark
78 Kingsley
 
JR:

Thought you were a working man during the day???????? How do you get to
play with the computer?????

Just curious what engine rpm you're turning @ 70 mph with your 3:67?
Are you not way above the top of the torque curve rpm wise then.

In my case with the 3:42 ratio, @ 65 mph I'm up near 2800 rpm.

Paul Bartz

From: Jrslaten [mailto:Jrslaten]
Sent: Thursday, November 05, 1998 1:23 PM
Subject: Re: GMC: Final Drive Ratio

In a message dated 11/4/98 9:40:25 PM Eastern Standard Time,

Any advice from users of 3.42, 3.55 or 3.67 ratios?
Clark,
I installed a 3.42 final drive two years ago and was impressed with the
performance. Just prior to the Las Vegas rally I replaced the 3.42 with
a 3.67 and was equally impressed with the difference in all around
performance between these two units. I always tow, a Miata or a Tracker,
and run equal too or a teensey bit above the posted speed limits. This
improvement ranks right up with Headers, Fuel Injection, Alcoa Wheels
and a hi-speed paint job. My personal opinion is that they are worth
every penny spent. Boy, I've used up a whole months supply of
words
 
Clark,
I really wouldn't recommend any higher ratios for GMC's unless you are
towing something. We always tow a trailer, and did install the 3.42 from
Cinnabar. From reports we had seen, we thought it might improve the gas
mileage. It really did not make that much of a difference in power, and
the gas mileage did not get any better (possibly a little worse). The
differences in the ratios are really not that big. The difference is no
greater than changing from a commonly used larger than stock sized tire
back to a stock sized tire. IMHO, the money is much better spent in
obtaining more power from the engine (cam and intake) instead of trying to
mask it with higher ratios.
Another idea is a downshift button. This idea was written up in one of
the back issues of the GMCMM mag. This modification made driving our GMC
much more pleasurable. This button can be wired off the full throttle
downshift switch on the gas pedal. It allows you to hold gears as if you
were at full throttle even if you are not. This allows you to stay in
first and second much longer when starting from a stop while allowing you
to also pull away without flooring it to do so. The button can also be
used to downshift for going up hills. This makes it much easier (and
safer) to downshift than moving the column shifter and makes the driver
more likely to do so. In our GMC, 2nd gear is good for around 55 MPH which
should be fine for going up hills and results in a much higher final ratio
than any of the final drive upgrades. NOTE OF CAUTION: One side-effect of
the downshift button is that if it is depressed while decelerating, the
transmission will shift into a neutral state until a speed of about 65 MPH
is achieved. At this point, the trans will shift back into high gear until
the speed drops. Operation is normal if the button is not depressed,
however.

Zachary Zehnacker
Seemann Motorsports

>Didn't get much response from my earlier post on this subject. So, let me
try
>again.
>It seems to me that a higher final drive ratio than 3.07 would make a big
>difference in drivability, but I'm having trouble justifying the $1,500 to
make
>the change.
>And if I did, which ratio is recommended.
>I was hoping to get some user input to help with these decisions.
>Any advice from users of 3.42, 3.55 or 3.67 ratios?
>Clark
>78 Kingsley
>
 
Clark,

I just had a 3.55 ratio final drive installed in my 77 Eleganza II with
a 403 engine last weekend. Buskirk's took about 2 hours in put it in.
The unit is now much more responsive and peppy. I have a large high
bridge to go over and in the past my speed would drop from 67 mph to 56
mph. With the new gear my speed only fell off about 2 MPH. The engine
was not lugging and my throttle did not have to be depressed as much. I
haven't got enough miles to check fuel usage, but it appears that I use
less fuel coming back than went I went to Buskirks. They are all usable
gear ratios. If you are planning to tow a car I would recommend a higher
ratio. The 3.42 would work well with the 455 and not towing or towing a
very light unit. Clarence Buskirk has a 3.67 ratio in his extended
unit. The 3.55 is probably the best all around, the engine rpm get to
2950 at 65 mph which is in the engines best power band.

The choice is yours
 
Honest answer, "No I can't". My 3.42 was an opportunity impulse purchase. We
were at Darrell Winterfeldts in September of '95 killing time between an
international conv and a western states roundup. He had one that somebody had
special ordered and then was unable to be there for the installation. I told him
to put in our coach and he did. I never looked inside the case. I had a deposit
on a 3.42 from Marvin, (in fact still do) that I figured I'd put in 'this olde
coach' when he came out with the 3.67 planned on getting that instead of the
3.46. In October he told me he was out of inventory with no plans to have more
made (mentioned in a earlier note). The early 3.46's were just the gear set and
the purchaser had to have them installed (slowed me down because I didn't have
anybody in my local area I felt comfortable with to do the installation). By the
time he started selling the complete final drive I fell into the DW opportunity.

Stephen
73 X-Painted Desert
'This Olde Coach' is a 73 X-Sequoia

> Steve
>
> Could you tell me if the 3.42 ring and pinion were stronger looking
> than the 3.07? I have read the 3.42 is a stronger gear.---is it?
>
> Take Care
> Arch 76 GB IL
>
> >
> > 3.67 & 3.46 are not available at this time. Marvin shut down his
> > opporation due to
> > health ( and the death of his partner) he is looking for a buyer for his
> > business
> > last I knew. I have a 3.42 we have been driving trouble free since '95. I
> > agree it
> > seems like a LOT of money but I'll do it again on 'This olde Coach', but
> > probably a
> > 3.55. It just makes the motor feel better.
> > Stephen
> > 73 X-Painted Desert
> >
 
With all the information on final drives. Just what is the difference between the
replacement gears and the Caspro set up.? Thanks in advance for all the response
and infor. Chuck

> Honest answer, "No I can't". My 3.42 was an opportunity impulse purchase. We
> were at Darrell Winterfeldts in September of '95 killing time between an
> international conv and a western states roundup. He had one that somebody had
> special ordered and then was unable to be there for the installation. I told him
> to put in our coach and he did. I never looked inside the case. I had a deposit
> on a 3.42 from Marvin, (in fact still do) that I figured I'd put in 'this olde
> coach' when he came out with the 3.67 planned on getting that instead of the
> 3.46. In October he told me he was out of inventory with no plans to have more
> made (mentioned in a earlier note). The early 3.46's were just the gear set and
> the purchaser had to have them installed (slowed me down because I didn't have
> anybody in my local area I felt comfortable with to do the installation). By the
> time he started selling the complete final drive I fell into the DW opportunity.
>
> Stephen
> 73 X-Painted Desert
> 'This Olde Coach' is a 73 X-Sequoia
>

>
> > Steve
> >
> > Could you tell me if the 3.42 ring and pinion were stronger looking
> > than the 3.07? I have read the 3.42 is a stronger gear.---is it?
> >
> > Take Care
> > Arch 76 GB IL
> >
> > >
> > > 3.67 & 3.46 are not available at this time. Marvin shut down his
> > > opporation due to
> > > health ( and the death of his partner) he is looking for a buyer for his
> > > business
> > > last I knew. I have a 3.42 we have been driving trouble free since '95. I
> > > agree it
> > > seems like a LOT of money but I'll do it again on 'This olde Coach', but
> > > probably a
> > > 3.55. It just makes the motor feel better.
> > > Stephen
> > > 73 X-Painted Desert
> > >
 
Many thanks for all the responses about final drive ratios. Not really what you
could call a solid concensus about which is best, but a lot more were in favor
of higher ratios than staying with the 3.07. I'm sure it's true that unless
you've experienced more than one, it's hard to say which is best. Therefore I
was particularly interested in the comments from anyone who upgraded ratios more
than once. I can certainly understand why the change from 3.07 to 3.21 (less
than 5%) wouldn't be very noticeable. But several people reported good results
with the 3.42 ratio by Cinabar. Then there was at least one guy who tried the
3.42 and later swapped that for the 3.67 ratio. I'm wondering if this was
because the 3.42 didn't satisfy them. And if so, why or in what way?

I personally think it's better to make the ratio change in the final drive,
rather than in the engine to tranny sprocket, because this also drops the torque
through the tranny and should reduce its heat build-up somewhat.
Clark
78 Kingsley
Mid-Michigan
 
I was just on the website for the Sunshine Statesmen (GMC Club in Florida)
and saw that their newsletter contained a nice write-up of an actual trip
made with a new final drive of 3.67. The link is:
http://www.songtek.com/gmcss/nov_98.htm and I would encourage everyone to
take a read through it. There is also a nice article on the Marion Rally.

>Many thanks for all the responses about final drive ratios. Not really
what you
>could call a solid concensus about which is best, but a lot more were in
favor
>of higher ratios than staying with the 3.07. I'm sure it's true that unless

- ----------------------------------'
Gary Thurlow
email to thurlow
St. Louis, MO
- ----------------------------------'
 
Don:

First, the original ring and pinion gears are 3.07:1 vs. 3.21:1.

Secondly, I notice no change in mileage.

Paul Bartz

From: DVM22727 [mailto:DVM22727]
Sent: Friday, November 06, 1998 10:27 PM
Subject: Re: GMC: Final Drive Ratio

Paul:
Reference you change to the 3.42 Ring & Pinion Gears.

Did this change effect your gas mileage, either plus of minus, over
operation with the original 3.21 Gear Ratios?
 
JR:

Hellooooooooooo!

Paul

From: Bartz, Paul [mailto:s9d3452]
Sent: Thursday, November 05, 1998 1:47 PM
To: 'gmcmotorhome'
Subject: RE: GMC: Final Drive Ratio

JR:
Thought you were a working man during the day???????? How do you get to
play with the computer?????
Just curious what engine rpm you're turning @ 70 mph with your 3:67?
Are you not way above the top of the torque curve rpm wise then.
In my case with the 3:42 ratio, @ 65 mph I'm up near 2800 rpm.
Paul Bartz
From: Jrslaten [mailto:Jrslaten]
Sent: Thursday, November 05, 1998 1:23 PM
Subject: Re: GMC: Final Drive Ratio

In a message dated 11/4/98 9:40:25 PM Eastern Standard Time,

Any advice from users of 3.42, 3.55 or 3.67 ratios?

Clark,

I installed a 3.42 final drive two years ago and was impressed with the
performance. Just prior to the Las Vegas rally I replaced the 3.42 with
a 3.67 and was equally impressed with the difference in all around
performance between these two units. I always tow, a Miata or a Tracker,
and run equal too or a teensey bit above the posted speed limits. This
improvement ranks right up with Headers, Fuel Injection, Alcoa Wheels
and a hi-speed paint job. My personal opinion is that they are worth
every penny spent. Boy, I've used up a whole months supply of
words
 
JR:

Last Sunday, I drove over to Dick Crawford's place with the coach to
dump and recorded my rpm's from 45-75 mph, in 5 mph increments, I'll
get that info from home this evening and get back to you on what numbers
are.

I didn't receive your e-mail response.

Paul

P. S. Bobby and I have been on the phone the last two evenings
discussing the supplemental braking vacuum pump and how to plumb and
wire it. Seems there is some conflicting guidance on how to do it.
Also talked to Bob Cook, who gave the GMC Junkyard Gems seminar @
Marion, last night about it. There is a drawing inside the back cover
of his handout showing the hookup. However, it was reduced from the
original and not all that readable. He is going to sent me a copy of
the original sized drawing.

Do you have the pump installed?

- -----Original Message-----
From: Jrslaten [mailto:Jrslaten]
Sent: Wednesday, November 18, 1998 1:06 PM
To: gmcmotorhome
Subject: Re: GMC: Final Drive Ratio

Paul, since I have to be on line anyway I may as well catch up on my
mail. And
the pay is not bad either. Huh Paul !!
I sent you a note saying I didn't have the exact #'s you requested. I
know
better than to give you an estimate. The next time I get the coach out
of the
garage I will document all the vitals and send that data to you.
Take care...........JR
 
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