How hard to determine final drive ratio (Revcon)

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dave silva

New member
Oct 2, 2009
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The information regarding the final drive ratio on the Revcon is sketchy.

In one place i read that it is stock Toronado. But when i look into what that number might be i get different answers.

Also, mine is a 72 which makes it unclear what generation Tornado they used. It may have been sourced much earlier.

My calculation watching the tach and measuring the tires makes it close to 4.10 but that seems doubtful.

More likely is my tach and/or speedometer are not accurate.

Do i have the math right?

engine RPM at 60 equals 5280/tire circumference X final drive ratio ??

So, how much trouble is it to unbolt something and count the teeth?

Thanks

--
Dave & Ellen Silva

1972 Revcon Olds 455, toro drive train. All Stock
 
Depending on the tire size, you’ll have 670-690 revolutions per mile (5280
/ tire circumference in feet).

670 is close the original GMC tire size of 16.5x8.75R78. 690 is closer to
the 225/75R16.

At 60 mph, RPM = tire rotations per mile X final drive ratio, but there
will be some slippage in the fluid coupling and some difficulty precisely
messing tire rolling circumference.

But it is unlikely that an electronic tach, if it reads at all, will read
only slightly off.

I run 70 mph down the highway at 2600 RPMs with a 3.21 final drive. That’s
with 225/75R16’s. That calculates to 694 tire rotations/mile.

What is your tach reading?

But from the factory, GM never had a gear shorter than 3.21 (used in
late-60’s Cadillacs). The GMC MH used a 3.07 from the factory, and I think
the standard Toro gear was 2.78 or similar.

Rick “not quite understanding your math notation” Denney

On Thu, May 9, 2019 at 12:12 PM dave silva via Gmclist <

> The information regarding the final drive ratio on the Revcon is sketchy.
>
> In one place i read that it is stock Toronado. But when i look into what
> that number might be i get different answers.
>
> Also, mine is a 72 which makes it unclear what generation Tornado they
> used. It may have been sourced much earlier.
>
> My calculation watching the tach and measuring the tires makes it close to
> 4.10 but that seems doubtful.
>
> More likely is my tach and/or speedometer are not accurate.
>
> Do i have the math right?
>
> engine RPM at 60 equals 5280/tire circumference X final drive ratio ??
>
>
>
>
> So, how much trouble is it to unbolt something and count the teeth?
>
> Thanks
>
>
> --
> Dave & Ellen Silva
>
> 1972 Revcon Olds 455, toro drive train. All Stock
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
 
> The information regarding the final drive ratio on the Revcon is sketchy.
>
> In one place i read that it is stock Toronado. But when i look into what that number might be i get different answers.
>
> Also, mine is a 72 which makes it unclear what generation Tornado they used. It may have been sourced much earlier.
>
> My calculation watching the tach and measuring the tires makes it close to 4.10 but that seems doubtful.
>
> More likely is my tach and/or speedometer are not accurate.
>
> Do i have the math right?
>
> engine RPM at 60 equals 5280/tire circumference X final drive ratio ??
>
> So, how much trouble is it to unbolt something and count the teeth?
>
> Thanks

Dave,

I do not have the time to resolve your equation right now, but will remind you that the tire circumference has to be in feet.
Here is a link that will take you to the final drive page of one of the GMC sites.
http://bdub.net/gmcmotorhome.info/front.html

Yes, 4.10 is realistic as that is a very available ratio. The only issue would be that it is only common in a hypoid gear set. Revcon had to have
figured out that none of the common Toro gears would be right and maybe they couldn't get the tow-package set that GMT&C elected to use in the GMCMH.
Or, maybe they knew it was a wrong as it is.....

In our coaches, opening the final to count teeth is a major PITA, but it can be done.

Matt - With a coach up in the air for a while
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
Someone may jump in on figuring out the ratio other than using the RPM/MPH/tire size but I'm not familiar with any other likely method. The problem is
using this method is you MUST have accurate data. The RPM is the hardest to get perfect without a good instrument. Tire size is pretty easy and MPH
using GPS is a snap. That all being said, GM only had a few choices in gear ratios, 3.07 being the probable choice. Of course it could have been
changed to a 3.21 or even a 2.73 or higher (lower number) by sourcing through salvage yards, etc. Aftermarket sets have been available through our
vendors in later years but that would probably be an unlikely guess.
As for checking the teeth, I don't think I'd go that way-way to much work.
Good luck, Hal
--
1977 Royale 101348,

1977 Royale 101586, Diesel powered,

1975 Eleganza II, 101230,

1974 Eagle Bus 45',w/slideout,

Rio Rancho, NM
 
remove the diff cover there will be 2 # on the edge of the ring gear one larger than the other the larger one is ring gear teeth the other is pinion
ear teeth eg 41 / 10 divide to get ratio then change oil while you are at it
 
I'm sorry, i think i put the question o=in the wrong order.

The key parts was more mechanical than mathematical:

"how much trouble is it to unbolt something and count the teeth? "

I only included the math part to confirm that I was not missing something.



--
Dave & Ellen Silva

1972 Revcon Olds 455, toro drive train. All Stock
 
One can always go to our web site under trans and final as we show number
of teeth on rig that can be a good indicator.

On Thu, May 9, 2019 at 12:04 PM Eelko Byker via Gmclist <

> remove cover the nos are stamped in all diff gear sets
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
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