gas mileage

R

ron keener

New member
Jul 8, 1999
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just saw the comment about getting 9.9 mpg in a mountain run

I have never been able to get more than 7.5 mpg with my 400 engine on
LEVEL ground with a tail wind - I recently replaced plugs in my 78
Birchaven - plugs were clean - no oil crude - has electronic ignition -
I have tried both cruise and manual - no diff - has lots of power and
pulls 60 mph easily on interstates - i dont jack rabbit start or ride
the pedal - it has 75k miles as of yesterday

what am I missing here?

ron - central texas
 
Seems that the average is about 8.5 mpg and that is what I get on my 455
- --- and that is on a 6000 mile overall average. Note that trying to get a
mileage figure on one tank of gas is near impossible since you can "always"
squeeze in another few gallons if you have the patience and that messes up
one-tank figures. Have you had your spedo checked?? It may not be
correct. Frankly, I'm suspicious of figures much in excess of 9 mpg.
Gary
North Bend, Oregon Coast

> just saw the comment about getting 9.9 mpg in a mountain run
>
> I have never been able to get more than 7.5 mpg with my 400 engine on
> LEVEL ground with a tail wind - I recently replaced plugs in my 78
> Birchaven - plugs were clean - no oil crude - has electronic ignition -
> I have tried both cruise and manual - no diff - has lots of power and
> pulls 60 mph easily on interstates - i dont jack rabbit start or ride
> the pedal - it has 75k miles as of yesterday
>
> what am I missing here?
>
> ron - central texas
 
Just thinking out loud here, but... get 16" E range tires with 75-80 pounds
of air cold, a good 6 wheel alignment and you'll probably see a difference
in your mileage. Look at your passenger side front tire for pre-mature wear
on the outside edge of the tire.

"Soft" D range 65psi tires may be hurting your mileage. Also, what temp
thermostat do you have? Is your distributor in good shape? (that is, is the
centrifugal advance working right?

My HEI was worn out with about that many miles. That made a big difference
in mileage.

Again, just some ideas.

Mark

| -----Original Message-----
| From: owner-gmcmotorhome
| [mailto:owner-gmcmotorhome]On Behalf Of Ron Keener
| Sent: Saturday, July 24, 1999 5:22 PM
| To: gmcmotorhome
| Subject: Re: GMC: gas mileage
|
|
| just saw the comment about getting 9.9 mpg in a mountain run
|
| I have never been able to get more than 7.5 mpg with my 400 engine on
| LEVEL ground with a tail wind - I recently replaced plugs in my 78
| Birchaven - plugs were clean - no oil crude - has electronic ignition -
| I have tried both cruise and manual - no diff - has lots of power and
| pulls 60 mph easily on interstates - i dont jack rabbit start or ride
| the pedal - it has 75k miles as of yesterday
|
| what am I missing here?
|
| ron - central texas
|
 
thanks for questions

I just checked speedometer on a 100 mile stretch - with new 9.50 tires
on front it is 5.5 % low - prior to 9.50's I had 8.75's and speedometer
was right on - I am running 60lbs - tire wear has been good on all 6
wheels (I have driven 14k miles with it) - I had to replace one front
tire due to blowout on right front so went to larger 9.50's on both and
moved left front back to spare

thermostat was replaced because it was running TOO cool - never over 1/4
meter - new thermostat is only slightly warmer than old one - I have
never seen meter even close to half scale

I have never checked timing but plugs removed before last trip were
clean and worn evenly - power is what I would expect from timing that is
in the ballpark - no problem with any octane gas

I am not complaining about 7.5 mpg - that is what I expected from past
experience - like you I question 9+ numbers

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
**************************************************************
 
Ron,
Several Comments on your E-Mail:
When you went to the larger time size you also change the ratio of the
Final drive, in your case by your calculation 5.5%. Bigger tires
causing a change in drive ratio your engine has to work harder to get it
down the road. The stock 3.07 final drive ratio has always been a
little high and not letting the engine get into it's torque curve
better. I presently have a 3.55 final drive with 8.75R16 on Alcoa's.
When I tow my mileage is 8 to 8.5 mpg and when not towing I get a little
over 10, I run at 64/65 mph. This drive ratio gets my engine up to
about 2900 RPM which is right in the power portion of the torque curve.
With the larger tires that your using your rpm is probably down near
2250 to 2300 rpm. At these rpm range the engine does not work as
efficiently to burn the fuel that it is being fed. As for tire pressure
you can get all different opinions on what to run depending on the type
of tire and ride that you want. I run 70# without any problems.

If you are still using the stock temp sender on your GMC 1/4 reading on
the gauge (is 235 deg F). The Robertshaw 190-330 is the recommended
unit, it is a high flow low restrictive unit and AutoZone has it for
under $5. If you want to get the reading up into the middle of the gauge
a lot of GMC owners have installed the NAPA TS 6469 sender which at
middle of the gauge reading is approx. 215 Deg F. If you if you are
getting to almost the middle of you gauge reading with the stock temp
sender you are close to doing damage to your engine as at 50% of gauge
is 250 deg f.

J.R.Wright
GMC GreatLaker
77 Eleganza II
Michigan

>
> thanks for questions
>
> I just checked speedometer on a 100 mile stretch - with new 9.50 tires
> on front it is 5.5 % low - prior to 9.50's I had 8.75's and speedometer
> was right on - I am running 60lbs - tire wear has been good on all 6
> wheels (I have driven 14k miles with it) - I had to replace one front
> tire due to blowout on right front so went to larger 9.50's on both and
> moved left front back to spare
>
> thermostat was replaced because it was running TOO cool - never over 1/4
> meter - new thermostat is only slightly warmer than old one - I have
> never seen meter even close to half scale
>
> I have never checked timing but plugs removed before last trip were
> clean and worn evenly - power is what I would expect from timing that is
> in the ballpark - no problem with any octane gas
>
> I am not complaining about 7.5 mpg - that is what I expected from past
> experience - like you I question 9+ numbers
>
> --
>
> ron keener near Austin, Texas http://MidAmericaBreedingTech.com
> **************************************************************
 
"If you are still using the stock temp sender on your GMC 1/4 reading on
the gauge (is 235 deg F). The Robertshaw 190-330 is the recommended
unit, it is a high flow low restrictive unit and AutoZone has it for
under $5. If you want to get the reading up into the middle of the gauge
a lot of GMC owners have installed the NAPA TS 6469 sender which at
middle of the gauge reading is approx. 215 Deg F. If you if you are
getting to almost the middle of you gauge reading with the stock temp
sender you are close to doing damage to your engine as at 50% of gauge
is 250 deg f."

Now this is the kind of information that is really USEFUL!!

everyone note that when answering a question we all benefit from answers
like this - thanks J.R.Wright

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
 
Hi guys,

Just thought I'd throw this out there. Are you guys figuring in the speedo
error caused by the final drive when you are calculating your mileages? It
seems like a 3.55 ratio getting 9 mpg is really only getting (3.07/3.55)*9
mpg = 7.78 mpg. It also seems like 7.5 mpg with larger tires making a 5.5%
difference would give 7.5 mpg * 1.055 = 7.91 mpg. This would make the two
cases pretty darn close. I'm thinking that this might be some of the cause
of the differing mpg figures given by some who have not made other
modifications which would explain it. Now if you guys are all including
this error, then it throws my theory right out the window. I'll bet,
however, there are at least a couple of people out there that aren't
correcting their figures back to the stock final drive/tire ratio to get
true mpg.

Zak
 
OK, here's some baseline data re: gas mileage:

Our '76 Royale has a 455 with about 30,000 miles since its last rebuild, an
original HEI distributor, stock 3.07 final drive and 8.75 x 16.5 tires. The
odometer registers 99.5 miles for each 100 miles actually traveled
(repeatedly tested against mile markers on various interstates), and we
travel with the cruise control set at 58 mph. The plugs and wires were
replaced prior the following trips.

Last fall we traveled from Santa Barbara to Boston and back, crossing
11,992' Loveland Pass in Colorado twice. We traveled a total of 7,189.8
miles and used 840.945 gallons of 87 octane regular unleaded gas.
Individual tank averages ranged from 5.830 to 11.359 due to wind,
mountains, difficulty filling completely etc., but the "corrected"
overall average worked out to 8.59 mpg.

This spring, a trip of 2,513 miles thru the southwest (much less mountain
climbing) yielded an overall average of 9.10 mpg. Combining the two, we
traveled 9,702.8 miles (9750.8 corrected) and used 1118.487 gallons, thus
averaging 8.718 mpg.

Hope this gives some of you a basis for judging whether your mileage is up
to par considering your specifics (fuel injection, tire sizes, towing,
driving speeds, final drive ratios, etc, etc).

Regards,

___________
Dave (& Dege), '76 Royale /_][__] [_] | "SR JAMES"
Santa Barbara, CA *0-------OO--* (our hobby)
 
Hi Zac,
In my comment earlier this weekend I said that I was get 8 to 8.5 towing
and a little over 10 without the toad, When I had the 3.55 gear
installed the speedo correction gearbox was also installed to make the
speedo correct.

J.R. Wright

>
> Hi guys,
>
> Just thought I'd throw this out there. Are you guys figuring in the speedo
> error caused by the final drive when you are calculating your mileages? It
> seems like a 3.55 ratio getting 9 mpg is really only getting (3.07/3.55)*9
> mpg = 7.78 mpg. It also seems like 7.5 mpg with larger tires making a 5.5%
> difference would give 7.5 mpg * 1.055 = 7.91 mpg. This would make the two
> cases pretty darn close. I'm thinking that this might be some of the cause
> of the differing mpg figures given by some who have not made other
> modifications which would explain it. Now if you guys are all including
> this error, then it throws my theory right out the window. I'll bet,
> however, there are at least a couple of people out there that aren't
> correcting their figures back to the stock final drive/tire ratio to get
> true mpg.
>
> Zak
 
thanks for info on setting timing emery - I have not checked it with a
timing light but accelerator response and power indicate that timing is
not to slow and ability to use 87 octane indicate it is not too
advanced - and the electronic distributor has very little to wear out
internally - so I would guess that I am within 1 degree on the timing -
i have not had the distributor cap off to see what the contacts look
like

I am off to columbia MO tomorrow for a 10 day horse show so will have
another trip to see if anything has changed - I did have to bypass the
battery isolator (both sides were shorted) so now I have the alternator
tied directly to the engine battery post on the firewall and throw the
boost switch when I am on the road to tie in the cabin batteries for
charging - I might have a stronger voltage on the distributor now so
that might make for a stronger spark

I used to have a chev 400 with a RV cam in a van I used for trailer
pulling - 11 mpg loaded or empty but that was considerably less gross
weight than the GMC so I was not surprised at 7.5 mpg but another mpg
would not hurt the pocket book
- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
**************************************************************
 
> Ron - you might want to check your timing and also the mechanical and vacuum
> advances on the distributor. They can make a big difference in the fuel
> economy. The pins and weights on the mechanical advance wear and the springs
> loose their tension.

Emery,

What gas mileage do you get? Ron's story is mighty familiar. I'm getting about
the same.

Waldo
 
> I actually hate discussions of gas mileage. Since we all drive differently
> there can be a wide variation even with two drivers on the same vehicle.
> When I bought my first motorhome in 1970 I asked what the mileage was. The
> salesman put his arm on my shoulder and said "son, if you have to ask that
> you can't afford it".

Emery,

I believe the salesman was correct. Thank you for an enlightening response.

I think I'll stay "stock" on my engine. Seems you have a very large investment
in modifications to achieve an additional 1 or 2 mpg. However, a tune-up and
general check-up "under the hood" seems in order.

Waldo
 
Emery - thanks for the details on your gas mileage

I think you hit on several key points

1 - the 455 will have a better chance of good mileage over the 403 due
to higher torque at lower rpms

2 - fine tuning of the wires, plugs, etc pays off

3 - computer controlled injection will be better than plain carburators

4 - adding spark advance options adds a bit more fine tuning

5 - higher tire pressure should help

summing it all up I would speculate that I can gain a mpg by going over
each item in my system - gain a 1/2 mpg here, a 1/4 mpg there, etc -
which leads me back to my logic and past experience tells me I should be
getting between 8 and 9 mpg - this range just rings true for the torque
to weight ratio of a 403 with a carburator

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
 
Ron:

I'm not convinced that your first item below is valid?? What supports =
that
theory??

The reason I say this is, one of the reasons GM went to the 403 engine =
was
for fuel economy during the 70s oil crises. If you look at the =
distributor
advance characteristics in the shop manual for instance, you'll see =
there is
a big difference between the 403 vs. 455 cid engines in when it comes =
on and
when it maximizes.

Paul Bartz

From: Ron Keener [mailto:rkeener]
Sent: Friday, July 30, 1999 8:03 AM
Subject: Re: GMC: gas mileage

Emery - thanks for the details on your gas mileage
I think you hit on several key points
1 - the 455 will have a better chance of good mileage over the 403 due =
to
higher torque at lower rpms
2 - fine tuning of the wires, plugs, etc pays off
3 - computer controlled injection will be better than plain carburators
4 - adding spark advance options adds a bit more fine tuning
5 - higher tire pressure should help

summing it all up I would speculate that I can gain a mpg by going over
each item in my system - gain a =BD mpg here, a =BC mpg there, etc -
which leads me back to my logic and past experience tells me I should =
be
getting between 8 and 9 mpg - this range just rings true for the torque
to weight ratio of a 403 with a carburator=20
 
"The reason I say this is, one of the reasons GM went to the 403 engine
was for fuel economy during the 70s oil crises. If you look at the
distributor advance characteristics in the shop manual for instance,
you'll see there is a big difference between the 403 vs. 455 cid engines
in when it comes on and when it maximizes.

Paul Bartz"

exactly Paul - the 403 was ideally suited for the automobile where the
torque to weight ratio is much lower - they were able to run at lower
rpms and still have plenty of response as the engine rpms moved up into
the ideal torque range

but with the heavier motor home it is necessary to keep the rpms up to
get the torque to fight the wind and weight resistance - higher rpm
means lower fuel economy - the 455 has lots of torque at lower rpms but
needs more fuel because of the larger displacement BUT it is running at
much lower rpms so when pulling high loads the larger engine should
actually have better economy because it is working less for the same
amount of torque

this is my experience from a farm background where heavy towing is a
daily necessity

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
 
Ron:

Not sure I agree with "..the 455 has lots of torque at lower rpm's...".
Have you ever seen the torque curve for the 455?? It has nearly a vertical
slope, which means that the higher the rpm's the more torque it makes. The
torque curve peaks at ~ 2750 rpm, which for the original drive train gearing
equates to ~ 70 - 75 mph. Therefore, at lower rpm's, the torque is much
lower.

I also don't see a larger engine working less than a smaller engine. All I
see is that the larger engine just has a higher capability. I would agree
though, that the larger engine is working at a lower percentage of its rated
capacity in the situation you describe.

Paul Bartz

From: Ron Keener [mailto:rkeener]
Sent: Friday, July 30, 1999 10:44 AM
Subject: Re: GMC: gas mileage

"The reason I say this is, one of the reasons GM went to the 403 engine was
for fuel economy during the 70s oil crises. If you look at the distributor
advance characteristics in the shop manual for instance, you'll see there is
a big difference between the 403 vs. 455 cid engines in when it comes on and
when it maximizes.
Paul Bartz"
exactly Paul - the 403 was ideally suited for the automobile where the
torque to weight ratio is much lower - they were able to run at lower rpms
and still have plenty of response as the engine rpms moved up into the ideal
torque range but with the heavier motor home it is necessary to keep the
rpms up to get the torque to fight the wind and weight resistance - higher
rpm means lower fuel economy - the 455 has lots of torque at lower rpms but
needs more fuel because of the larger displacement BUT it is running at much
lower rpms so when pulling high loads the larger engine should actually have
better economy because it is working less for the same amount of torque this
is my experience from a farm background where heavy towing is a daily
necessity
 
> Have you ever seen the torque curve for the 455??.... The
> torque curve peaks at ~ 2750 rpm, which for the original drive train gearing
> equates to ~ 70 - 75 mph. Therefore, at lower rpm's, the torque is much
> lower.

According to the GMC Parts Book pub. 78Z, page 7, the Olds 455 produces its
maximum torque at 2400 rpm, not 2750. In my rig this equates to ~ 62-65mph -
about where I like to cruise.

Richard
 
Richard:

Just goes to show you that first you have to define the source. The torque
curve (I. e. an actual graph and not a narrative statement as in the parts
book) I referred to was sent at my request from Chuck Stoddard of Caspro,
and as I recall originally came from GMI (General Motors Institute).

Paul Bartz

From: Richard Guthart [mailto:rguthart]
Sent: Friday, July 30, 1999 11:32 AM
To: gmcmotorhome
Subject: Re: GMC: gas mileage

> Have you ever seen the torque curve for the 455??.... The
> torque curve peaks at ~ 2750 rpm, which for the original drive train
gearing
> equates to ~ 70 - 75 mph. Therefore, at lower rpm's, the torque is much
> lower.

According to the GMC Parts Book pub. 78Z, page 7, the Olds 455 produces its
maximum torque at 2400 rpm, not 2750. In my rig this equates to ~ 62-65mph -
about where I like to cruise.

Richard
 
Interesting........................I wonder why the discrepency? Possibly the
graph represented the 455 breathing through more efficient heads and exhaust,
while the GMC Parts Book cites the engine as it is installed in the motorhomes,
with more restrictive heads, manifolds and long exhaust? (Just speculating.)

Richard

> Richard:
>
> Just goes to show you that first you have to define the source. The torque
> curve (I. e. an actual graph and not a narrative statement as in the parts
> book) I referred to was sent at my request from Chuck Stoddard of Caspro,
> and as I recall originally came from GMI (General Motors Institute).
>
> Paul Bartz
>
> From: Richard Guthart [mailto:rguthart]
> Sent: Friday, July 30, 1999 11:32 AM
> To: gmcmotorhome
> Subject: Re: GMC: gas mileage
>
> > Have you ever seen the torque curve for the 455??.... The
> > torque curve peaks at ~ 2750 rpm, which for the original drive train
> gearing
> > equates to ~ 70 - 75 mph. Therefore, at lower rpm's, the torque is much
> > lower.
>
> According to the GMC Parts Book pub. 78Z, page 7, the Olds 455 produces its
> maximum torque at 2400 rpm, not 2750. In my rig this equates to ~ 62-65mph -
> about where I like to cruise.
>
> Richard
>
 
Recent discussion on mileage got me to thinking of
what mine was. First and only trip so far was Tulsa to
Fort Lauderdale. Drove 1381 miles, used 119.8 gallons
of fuel. My math says 11.53 MPG. I thought that should
be about right but from the recent postings it now
seems high. Anybody else getting 10-11+. Mine is a
23', crusing speeds ranged from 50-65, average
probably close to 60. Only other fluid used on the
trip was 1/4 quart of oil. Coach has 73k miles and
appears to have been well maintained.

Just curious,

Keith
'73, Tulsa

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