Anyone has or remembers the formula to calculate equivalent gear ratios?
Have rear 275/40-18's with 2.73 OEM and A4, looking for equivalent gear ratio.

Rgrds

 

Not exactly "advanced", but to answer the question - and assuming you want to compare the new tire size to stock 4th Gen tires (hard to tell.... since you didn't say).....

The "equivalent" gear ratio is revised by the ratio of the stock tire diameter divided by the new tire diameter.

If you are starting with a stock 4th Gen V8, your stock tire was probably a 245/50-16, with a diameter of 25.65"

Your new tires are 18 + 2x((275/25.4)x(40/100)) = 26.66"

The effect of installing these 18" tires with a 2.73 rear axle ratio would be the same as swapping to a 2.63 rear axle ratio with stock tires..... is this a "land speed record" application?

25.65" / 26.66" = 0.962

0.962 x 2.73 = 2.63

Your speedo will also read 57.7mph, when you are actually doing 60mph, and your car will sit about 1/2" higher than it used to.

You would obviously be closer to the stock tire diameter with a 285/35-18 or a 275/35-18.

 

Here's a bunch of formulas I find useful:

http://murray.faithweb.com/formulas.htm#find_rear

 

Thanks you Thank you Thank you

Rgrds...all