Its easy, get the stock tire dimension from when the truck was factory new say 27" also you'll need the factory gear ratio say 4.30:1.
Take the 27" and divide by the 35" you wish to run and you get .77. The reciprocal (subtract from 1.0) is .23 or 23% bigger.
Now take the stock gearing 4.30 and divide by .77 and you get 5.58 and to double check your figures do it the same way, divide your current gear ratio by the new one 4.30/5.58 and you get .77 or 23%. This will maintain you existing RPM at a given speed.
Of course you're not going to get 5.58 gears so you need to take the gears that are close to what you want and do the divide thing again to see what the figure is. 4.30/5.29=.81 or 19%, since the percentage is lower the RPM will be lower at a given speed and the motor will have to work harder to accelerate.
Now something to keep in mind, if you raise up the truck you create a bigger brick to push through the air and you might want to go 5 or 10% bigger ratio to maintain performance at the cost of fuel mileage.
If you want to calculate the rpm at a given speed you can also take your MPH say 70mph at 3500 rpm and multiply the rpm by the bigger increase above 3500 x .77= 2695 with the bigger tires.
Gears are the cheapest horsepower ever bought, you can take a fuel efficient slug and make a rocket out of it if your willing to pay the price at the pump.
I just did a search and if your not interested in the math here's the various calculators your looking for <A target="_blank" HREF=http://www.4lo.com/4LoCalc.htm>http://www.4lo.com/4LoCalc.htm</A>
Curt
VORRA Class 7
#769
