Gears Unlimited

Professional Automative Gear and Transmission Analysis Tool

Complete the fields below, above the tabs, with your vehicle's current configuration, including tire size, MPH to calculate for, Transmission and Differential type, current differential ratio and what transmission gear you wish to use to calculate (typically select high gear, the numerically smallest number, for highway speed calculations).

If you're not certain what gear ratio you have now, or what transmission is in your vehicle, you can search using your VIN# at: https://www.northamericanmotoring.com/forums/vindecoder.php

Current Vehicle Configuration

inches
See the Tire Calculator Tab for help figuring tire diameter and to calculate for tire growth factor if desired.
MPH
%
Tire growth factor due to speed/heat expansion. See Tire Calculator tab for details.
%
Torque converter slip percentage. Positive = slip, negative = lockup efficiency.

Available Gear Ratios

Identification Tips: See the Differential ID Guide Tab for additional help identifying and selecting the correct differential type in your vehicle.
Ford 10.5" modern heavy duty

Current Analysis

Speed 75 MPH
Current RPM 2500 RPM

New Vehicle Configuration

inches
3.45

Available Differential Gear Ratios

3.42
2480
-20

RPM/MPH Across All Shift Points

Transmission Selected in "Gear_Ratio Calc": TH350

Note: Shift points will be displayed based on the selected transmission type.

Gear Ratio Shift Shift RPM RPM Drop New RPM Shift MPH

Input desired RPM for each gear change in the light gray, center column.

The RPM@Shift calculator uses all available gears / ratios in the transmission you select in the "Gear_Ratio Calc" form. You enter the RPM for each gear change. In the green "Shift RPM" column. As you enter each shift RPM point for each gear, the form will calculate the MPH at the shift point, along with the RPM Drop and the new RPM after the shift. The attending graph illustrates this in an easy to understand, visual format.

Tire Size Calculator

Enter tire size (P225/60R16 format):

P / R
%
Account for tire expansion at high speeds. Affects the final calculated diameter.
Calculated Tire Diameter
Enter tire dimensions above

About Tire Growth: Tire growth can occur as vehicle speed increases, influencing running diameter and engine RPM at speed, but is a variable that can be difficult to determine. Low profile and/or stiff sidewall tires may grow little, if at all, where taller profile (taller sidewall) tires, and/or tires with a soft sidewall (like slicks for example) can grow more substantially. A standard value for tire growth is said to be 3%, but this is a rough estimate. If you see where the calculated engine RPM is higher than observed engine RPM at a given speed in your vehicle, you can play with this value to match what you're seeing and get a better idea of what your tire may be doing.

Common Tire Sizes

Tire Size Diameter (in) Action
P225/60R1626.6
P235/75R1528.9
P275/60R1528.0
P315/75R1634.6
33x12.50R1533.0
35x12.50R1735.0

Note: Virtually all automobile tires are spec'd in metric and finding the diameter is easy using the Tire Size Calculator included here. However, many truck tires are still spec'd in inch or "Standard" dimensions, for example the above 35x12.50R17, which mean the first number, 35, is the diameter in inches, the second number, 12.50 (after the x) is the section width in inches, the R is for Radial, and the last number is the rim diameter in inches. Here, simply enter the first number of the spec'd tire size in the Current Vehicle Configuration section above, in the "Tire Diameter" field.

Manual Gear Ratios (1-10 Speed)

Gears 1-5

Gears 6-10

Tips: Leave fields empty for unused gears. Ratios should generally decrease from 1st to highest gear (e.g., 4.70 → 2.99 → 1.00 → 0.64).

Preset Transmissions

Popular Transmission Ratios

Select a preset or enter custom ratios manually. All calculations will update automatically.

Transmission 1st 2nd 3rd 4th Action
PowerGlide1.821.00--
TH3502.521.521.00-
TH4002.481.481.00-
C42.461.461.00-
700R43.061.631.000.70
4L60E3.061.631.000.70

Differential Ratio Selection

Differential Ratio Guide

Lower ratios (2.73-3.23): Better fuel economy, higher top speed

Mid ratios (3.42-3.90): Balanced performance and economy

Higher ratios (4.10-4.56): Better acceleration, lower top speed

Common Differential Ratios

Ratio Best For RPM @ 60 MPH* Action
2.73:1Highway cruising1820
3.08:1Balanced daily driver2053
3.42:1Performance street2279
3.73:1Street/strip balanced2486
4.10:1Drag racing2732
4.56:1Heavy acceleration3039

Comprehensive Differential Database

Take note of your differential cover shape and bolt pattern and compare to the image provided here to help confirm you have selected the correct option.

Note: The list of available ratios is unique to each type of differential, which important to choose a ratio that is actually able to be installed.