I am in the process of having my car re-backhalved and setting it up for a 33x10.5W tire. I had a 29.5W tire that had 93" of roll out and now the 33 has 104 " of roll out. How much do I need to increase Gear to compensate for the tire? When do you know when you have too much gear? I run a PowerGlide 1.76 trans and whatever I shift at the car goes through the lights at (within 300 rpm).

Thanks Bob Kernwein

When changing tire size, if you want to maintain the same "effective" gear ratio as you had before the tire change, you need to calculate the % change in tire size and adjust the gear ratio by the same %. In this case the tire size was increased from 93" of rollout to 104". By taking 104/93=1.118 or 11.8% increase in tire size. Therefore, you must increase the gear ratio by the same amount. Old gear ratio times the factor equals new gear ratio. For example if the old gear ratio was 4.56 then (4.56 x 1.118 = 5.098 ) the new ratio would have to be 5.098 or the closest thing to it (5.14?). Obviously the tire size will affect the contact patch size and one may want to change ratios to adjust for that as well, but the RPM through the lights will really determine what ratio you need to run. A good basic rule of thumb is that you need to exit the traps at 105% of the peak power rpm which is usually the RPM where you end up shifting. If the gear ratio is too much, the engine stops "pulling" before you get to the traps. You need to experiment with shift speeds to determine the optimum shift points. Use the ET to determine performance and not MPH.
Hope this helps!

Tom
Precision Gear