The Quick and Dirty:
Your have increased the size of your tire, what gears should you buy?
Old Tire Size – 31”
New Tire Size – 35”
Current Gear Ratio – 3.73
New Gear = (New Tire Size / Old Tire Size) * Current Gear Ratio
4.21 = (35/31) * 3.73
Since you cant buy 4.21 gears, it would be advised to purchase 4.56 gears. It is advised to round to the next lower ratio to account for increased rolling resistance, and reduced aerodynamics from being lifted.
Method 2:
This method is a bit more favorable as it takes into account a target RPM in your engine's power band for a given cruise speed.
New Tire Size – 35”
Target RPM – 2500 RPM
Trans. Overdrive Ratio – 0.70 (4L60E)
Highway Cruise Speed – 70 MPH
Correction Factor - 0.8
New Gear = (Target RPM * Tire Size)/(Cruise Speed * 420.38 * OD)
4.24 = (2500 RPM * 0.8 * 35”)/(70 MPH * 336.3 * 0.70)
In this instant you would use a 4.56 gear. Note that this calculation has a correction factor that provides a little insurance for passing, hills, towing, hauling, etc.. I used 0.8 which should be about right for most engines. If your engine makes A LOT of torque at your target RPM, you can use corrections factors of 0.7, 0.6, 0.5 etc... Also, this calculation assumes that a lockup stall converter is used in automatic transmission applications.
Here is a good link with some very good calculators. Bear in mind that no corrections are applied with these calculations.
http://www.bncoffroad.com/ratio/
Picking the Right Gear for Your Tire Size
Linear Mode
