Tire Size effect on gear ratio

G

gene

New member
Sep 29, 1999
2,215
0
0
so should we buy real small tires for the front and not change the final
drive ??

gene

>Do you really have the final drive ratio that you think you have?
>
>Some GMCnetters have reported that they are using wider tires for their
Alcoa
>wheels. The tires may be wider but they are usually also larger in
diameter.
> This has the effect of increasing the gear ratio and can negate some of the
>benefits of going to a lower ratio final drive (higher numerically).
>
>Goodyear has their specs for the G159LT tires at
>http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html
>
>This shows that the 8.75R16.5LT tire has 719 revolutions per mile. The
>LT225/75R16 has 703 rev/mile and the LT235/85R16 has 677 revs/mile.
>
>The use of the 16 inch tires on Alcoa wheels will give the following effect
>on the final drive ratio:
>
>3.07 with the 16.5" tires will become 3.00 with the 225 tire and 2.89 with
>the 235 tire.
>
>3.42 with the 16.5" tires will become 3.34 with the 225 and 3.22 with the
235.
>
>3.55 with the 16.5" tires will become 3.47 with the 225 and 3.34 with the
235.
>
>3.7 with the 16.5" tires will become 3.62 with the 225 and 3.48 with the 235.
>
>The speedometer and odometer can also be affected by a tire change. Going
>from the 16.5 tires to the 225 would make the speedometer/odometer off by
>2.2% and the 235 tire would be off by over 6%. The easiest way to correct
>this is with an accessory gear box attached to the speedometer connection at
>the transmission. There might also be appropriate sized gears that could be
>installed into the transmission.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/
 
great information. I'll archive that data.
Bob Mcl

- -----Original Message-----
From: owner-gmcmotorhome
[mailto:owner-gmcmotorhome]On Behalf Of
EMERYSTORA
Sent: Tuesday, January 25, 2000 11:34 PM
To: gmcmotorhome
Subject: GMC: Tire Size effect on gear ratio

Do you really have the final drive ratio that you think you have?

Some GMCnetters have reported that they are using wider tires for their
Alcoa
wheels. The tires may be wider but they are usually also larger in
diameter.
This has the effect of increasing the gear ratio and can negate some of the
benefits of going to a lower ratio final drive (higher numerically).

Goodyear has their specs for the G159LT tires at
http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html

This shows that the 8.75R16.5LT tire has 719 revolutions per mile. The
LT225/75R16 has 703 rev/mile and the LT235/85R16 has 677 revs/mile.

The use of the 16 inch tires on Alcoa wheels will give the following effect
on the final drive ratio:

3.07 with the 16.5" tires will become 3.00 with the 225 tire and 2.89 with
the 235 tire.

3.42 with the 16.5" tires will become 3.34 with the 225 and 3.22 with the
235.

3.55 with the 16.5" tires will become 3.47 with the 225 and 3.34 with the
235.

3.7 with the 16.5" tires will become 3.62 with the 225 and 3.48 with the
235.

The speedometer and odometer can also be affected by a tire change. Going
from the 16.5 tires to the 225 would make the speedometer/odometer off by
2.2% and the 235 tire would be off by over 6%. The easiest way to correct
this is with an accessory gear box attached to the speedometer connection at
the transmission. There might also be appropriate sized gears that could be
installed into the transmission.

Emery Stora
77 Kingsley
Santa Fe, NM
 
Emery,

I hear what you're saying - bigger tires equate to a lower effective ratio -
but your analysis assumes that the G159 was original equipment. The
Bridgestone R187LT in 225/75R16 has 711 RPM and in 225/85R16 has 689 RPM.
The R260(closest comparison to the 187) in 8.75R16.5 has 713 RPM.

I was going to compare Michelin tires also, but couldn't find the pertinent
data on their site.

Patrick

>
> Goodyear has their specs for the G159LT tires at
> http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html
>
> This shows that the 8.75R16.5LT tire has 719 revolutions per
> mile. The LT225/75R16 has 703 rev/mile and the LT235/85R16
> has 677 revs/mile.
 
How low and we go?

Wayne Newland F9300 75 Palm Beach (835's for the time being)

>
> drive ??
>
> gene
> >>
>
> Hey -- you might have something there, maybe we could go for the "low rider"
> look and save a bundle on the final drive! :)
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
 
Emery

My very un-scientific check of my 235 Goodyear G-159's odometer against road
mileposts, indicate about a 10% error, and your chart says that it should be 6%.
Interesting.

Wayne Newland F9300 75 Palm Beach Columbia, Md

> Do you really have the final drive ratio that you think you have?
>
> Some GMCnetters have reported that they are using wider tires for their Alcoa
> wheels. The tires may be wider but they are usually also larger in diameter.
> This has the effect of increasing the gear ratio and can negate some of the
> benefits of going to a lower ratio final drive (higher numerically).
>
> Goodyear has their specs for the G159LT tires at
> http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html
>
> This shows that the 8.75R16.5LT tire has 719 revolutions per mile. The
> LT225/75R16 has 703 rev/mile and the LT235/85R16 has 677 revs/mile.
>
> The use of the 16 inch tires on Alcoa wheels will give the following effect
> on the final drive ratio:
>
> 3.07 with the 16.5" tires will become 3.00 with the 225 tire and 2.89 with
> the 235 tire.
>
> 3.42 with the 16.5" tires will become 3.34 with the 225 and 3.22 with the 235.
>
> 3.55 with the 16.5" tires will become 3.47 with the 225 and 3.34 with the 235.
>
> 3.7 with the 16.5" tires will become 3.62 with the 225 and 3.48 with the 235.
>
> The speedometer and odometer can also be affected by a tire change. Going
> from the 16.5 tires to the 225 would make the speedometer/odometer off by
> 2.2% and the 235 tire would be off by over 6%. The easiest way to correct
> this is with an accessory gear box attached to the speedometer connection at
> the transmission. There might also be appropriate sized gears that could be
> installed into the transmission.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
 
Emery

As far as I know, I have a stock 3.07 final. I had a long conversation with the
PPO and nothing led me to believe that any work had been done in that area. Even
with a 3.07(?) and 235 tires, the coach runs at 65 to 70 easy.

Wayne

>
> My very unscientific check of my 235 Goodyear G-159's odometer against road
> mileposts, indicate about a 10% error, and your chart says that it should be
> 6%.
> Interesting.
>
> Wayne Newland >>
>
> Well, - I said that it would be over 6%. This was computed from the chart on
> the Goodyear site. Tires can vary in size a little from batch to batch or
> mold to mold. Are you using the stock 3.07 final drive?
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
 
Emery

Thanks for the info. Since my coach is knee deep in snow right now, and
another big storm due Sunday, it may be a while until I can even get close
enough to look. Besides, it's cold out there.

Wayne

>
> >
>
> Wayne - if you have the 3.07 you will have a "T" stamped near the spreader
> hole on the passenger side when you look at the final drive from the front.
> The spreader hole is what they use to spread the case in order to assemble
> the gears into it. It will be a hole in the flange just left of the cover.
> If I remember, the "T: will be about 3/16" in height. There are also some
> other numbers there but they tell the build date.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
 
You must log in or register to reply here.
Applied GMC
Get your shine on at www.cruzingear.co

Share this page

Share this page
Top Bottom
Back