Rear end ratios and tire size - what the scoop?

[Levis Classic]

I am trying to remember the relationship of rearend ratios to tire size.Cant recall if taller tires help or hurt with regards to ratios?

For example if you had a 3.00 to 1 rearend, and 17" wheels vs. 15" wheels. My situation is I will be running 17" wires with 7.50 x 17" bias plys, a very tall 33" total tire with a 3.00 to 1 chunk. If that were say a 7.50 x 15" tire would that help? I may need something like a 3.50 to 1.

What do you know?

[Greezy]

The taller tire in fact will raise the ratio by a little. If its 3.00 now you might end up with 2.90 or there abouts.

[fab32]

Since there are so many combinations of tires and wheels I reference off the outside dia. of the tire being used. The taller the tire the farther down the road you go with each revolution. The smaller the gear ratio the farther down the road you go with each rev of the engine.
1. tall tire, low (numerically) rear ratio= low RPM
2. Short tire, high (numerically) rear ratio= high RPM

Frank

[Elrusto]

Check out Richmond Gear's calculator here. (http://www.richmondgear.com/101032.html)

[bulletproof1]

you can check out richmonds site but i wouldnt use there gears,something to also remember the weight of the car has alittle to do with the ratio,say if you have a real light car you may not need as low a gear,takes less leverage to move i and vise versa.

[Morrisman]

I've got a rather stupid 4.9:1 diff on my car, but I have 29" tall tyres, which means I'll be running at 4000rpm at 70mph http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif

Time to go look for a lower diff ratio before I get my baby on the road I think http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

Trouble is, I'm using some obscure axle out of a Sherpa (Leyland) van http://www.jalopyjournal.com/ubbthreads/images/graemlins/frown.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif

[Levis Classic]

OK my next question the current 8" ring and pinion setup is 39 teeth on the ring and 13 on the pinion. According to Richmonds website the 3.55 ratio setup has 39 teeth on the ring and 11 teeth on the pinion.

I have an 11 tooth pinion could this work with my existing 39 tooth ring to get me to a 3.55 ratio?

[draggin'GTO]

[ QUOTE ]
OK my next question the current 8" ring and pinion setup is 39 teeth on the ring and 13 on the pinion. According to Richmonds website the 3.55 ratio setup has 39 teeth on the ring and 11 teeth on the pinion.

I have an 11 tooth pinion could this work with my existing 39 tooth ring to get me to a 3.55 ratio?

[/ QUOTE ]
Won't even come close to working. The gear patterns are completely different between the two sets you want to mix.

The ring and pinion are ground as a matched set, swapping them even between sets that are the same ratio is not a good idea.

As far as tire size tire size is concerned, adding taller tires are like putting in a 'taller' gear (lower numerical).

I had a 28" tire on my race car with 3.25 gears, it was the equivalent of running a 2.96 gear with 26" tires.

[Levis Classic]

I thought it was too easy!

Thanks