This gear ratio and tire diameter chart meant to give you an approximation of ideal RPMs at highway speed (65 mph) for three situations: fuel economy, everyday driving, and increased towing power. This chart provides values that represent a 1:1 ratio on a manual transmission.  Gear ratio and tire diameter charts are ideal for making your car as efficient as possible. Note: Automatic transmission RPMs tend to be a little higher and transmissions with an overdrive tend to be lower. Here’s a formula you can use to calculate your vehicle’s tire diameter: (CROSS SECTION WIDTH x 0.ASPECT RATIO x 2 ÷ 25.4) + WHEEL DIAMETER = OVERALL TIRE DIAMETER For Example: 235/45ZR-17 (235 x 0.45 x 2 ÷ 25.4) + 17 = … Continue reading Gear Ratio and Tire Size Chart