[Graymist]

Can anyone explain to me the relationship between rim-size, and gear ratios ? I was talking to a truck dealer yesterday, and I was told that apparently running 24.5 rubbers with a 4.33 ratios would be the same as running 22.5 rubbers with a 4.10 ratio. Might I say, I didn't quite ubderstand !!

[allan5oh]

He wasn't being specific enough, is he talking 11r's?

There isn't a "correlation" so to speak, but rather changing tire size is similar to changing gear ratios, in a way.

The taller the rubber, the further you'll travel for each engine revolution, enabling the truck to move faster in each gear. Shorter rubber does the opposite, and gives you more "gearing".

[BIG JEEP on 44's]

He's talking final gearing and the difference tire diameter plays in it ...not rim diameter ...unless you drive on your rims sparky...if you have 40 inch tire and have 4.10 gears [ring/piion} and drop to a 38 it's like going deeper in gears to a 4.30 r/p set ...and if going up to 42 from 40 with 4.11 r/p it's like dropping to a taller 3.90 r/p...


basicly every 1inch +or- in tire diameter changes final gearing 1 tenth ...

[tracer]

here's a useful formula. Your RPM at 60 mph equals REVS PER MILE (drive tires) times REAR AXLE GEAR ratio times TOP GEAR ratio. The lower your RPM is the better your truck will be on fuel.

Example (my own truck). I have a 13 speed tranny with 0.73 top gear ratio, and my rear axle is a 3.73. When I go to my tire manufacturer's web site I learn that my 295 low profile YOKOHAMA tire rotates 512 times per mile. So, at 60 miles per hour, my RPM should be

512 x 3.73 x 0.73 = 1394

For some reason it's never quite that high in my case because I'll probably get 512 revs/mile in a year when the tires that I put on in January 2008 will be more worn out.

But the idea is solid: there's a concrete tie between your tire size, axle ratio and top gear ratio. To compute your RPM at another speed, just divide the result by 60 and then multiply again by the new speed.

Example, if I want to know my RPM at 65 mph, I take 1394 from the above, divide it by 60 and multiply by 65. The result is 1510 RPM (too high). I have a CAT and they say your best RPM is below or around 1350 rpm. That's why I mostly cruise at 58 mph (1300 rpm).

[fdmax]

this is what mine would look like.... Thank You for the formula.

Super 18 18-Speed Models
RTLO-18918B

Gear Ratio
Eighth H 0.73
Eighth L 0.86
Seventh H 1.00
Seventh L 1.17
Sixth H 1.38
Sixth L 1.62
Fifth H 1.95
Fifth L 2.29
Fourth H 2.73
Fourth L 3.20
Third H 3.74
Third L 4.38
Second H 5.16
Second L 6.05
First H 7.30
First L 8.56
Lo H 12.29
Lo L 14.40

rear’s Dyna DSP40 3.55
Michilin xda3 11r24.5 rpm’s 473
Top gear ratio 18th gear 0.73


473 * 3.55 * 0.73 = 1225 rpm’s on engine at 60 miles per hour
Mile/feet 5280 / 473 = 11.162790697674418604651162790698
xda3 11r24.5