4x4 Clubhouse

Calculate True MPH

Speedometer ratio adjustment calculator.

Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assuming nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) X Speedometer MPH = Actual MPH

Calculate Tire Size Change

This calc can be used to find a new gear ratio.

Bigger tires are more functional, look better, and rob you of power. One way to compensate for this is to swap out your differential gears. The left side of the calculator requires all three entries. The right side will show the results. The New Ratio box will show the ideal gear ratio (getting as close to stock as possible). If you want a bit more power, for rock crawling, go with something closer to the Lo Ratio number. Also, the Actual Ratio box shows your adjusted gear ratio if you choose to keep your original gears. 

Formula used 

(New Tire Diameter / Old Tire Diameter) X Old Axle Raito = New Axle Ratio

Ring/Pinion Calculator

A simple gear ratio calculator.

Okay, so you've torn your pumpkin apart and have the ring gear and pinion gear teeth count. This calculator will give you the gear ratio, just plug in the numbers. There are many ways to guestimate what your gear ratio is. Another popular method is to mark your tire and driveshaft, then carefully roll the vehicle so the tire turns one full revolution. While you do this, count the number of turns (down to the tenth) of the drive shaft. This will get you a close idea of your gear ratio.

Formula used

Ring Gear Teeth Count / Pinion Gear Teeth = Gear Ratio

Metric Tire to Inches Converter

This calc converts a Metric tire to inches.

Most of the formulas dealing with gear ratios will want a tire diameter (measured in inches). This formula is a quick way to get the tire diameter of those metric tires that are common on just about everything stock. For example a LT265/75R16 would be around 31.6 inches tall and 10 inches wide. Enter any three of the numbers into this form to solve for the fourth. "LT" means Light Truck and "P" means Passenger tire. The bigger number (on the left) is the Section Width. The number to the right of the slash ("/") is the Aspect Ratio (percent of width). The "R" means Radial tire and the last number, far right, is the rim diameter (in inches!).

Formula used

((Section Width x Aspect Ratio x 2) / 25.4) + Rim Diameter = Tire Diameter

Width in inches = section width / 25.4

Section Height in inches = Width in inches X Aspect Ratio (%)

Gear Ratio Guide

Gear Ratio Guide for Larger Tires

When ever you change tire size from the original size to another there will be a direct effect on power and drivability. We have developed this chart to help you select the optimum gear ratio to put your performance back where you want it.

Good "rule of thumb" calculation is to mutiply .12 by your tire diameter. (.12 X 38" = 4.56)
(see more formulas at bottom of page)

Using the chart below, first determine your original gear ratio and tire size and follow to the intersection that should be your approximate RPM at 65 MPH. Next find your new tire size and follow that across to the closest RPM to your original and that will be first choice. 

If you are interested in a change that will give more power or better fuel mileage use the color coding as a guide.

                 Highway Driving, Good Fuel Economy

                 Daily Driver, Best Overall Performance

                 Most Power and Towing, Reduced Fuel Economy

This chart is based on 65 MPH and a gear ratio of 1:1, on a manual transmission in 4th gear. Please NOTE: If you have an automatic transmission your RPM will higher due to slippage in the transmission and the torque converter. With an overdrive transmission your RPM in overdrive will be 15% to 30% less than indicated.

Miles per Gallon