Gear Ratio Changes & RPM Calculators
About Differential Gear Ratio Changes
Here’s an example: Your new truck came from the factory with 31″ tall tires. You’ve since installed a suspension lift, some new wheels and a set of 37″ tall tires. The result is a huge loss in acceleration, hills have become a challenge and your fuel economy has also taken a huge hit. The easiest way to regain lost performance and economy is to change the gear ratio in the differential! The goal when changing the ratio is to get the engine back into a more efficient RPM range and to compensate for the added weight of larger tires.
Gear ratio changes are also very effective for street rods, drag racers and lots of performance applications where a specific RPM range at a particular speed is desired.
There are a wide range of ratios available for most vehicles and axle assemblies. The calculators provided below are a simple way to compare different gear ratios and their effect on RPM and different speeds.
The differential gear experts at Ring and Pinion Service are also a great resource and would be happy to recommend and install the ideal ratio for your needs.
Tire Diameter Calculator
Metric tire specifications follow a three number format, for example: 275/75R16. 275 is the WIDTH of the tire tread in millimeters, 75 is the ASPECT (or percentage of tread width to yield the sidewall height), and 16 is the WHEEL diameter. Entering the numbers in this example in the Tire Diameter Calculator below, you’ll get a tire height of 32.24″.
Truck and off-road tires may have a specification like 30×9.5R15, where 30 is the Tire Diameter, 9.50 is the tread width and 15 is the wheel diameter. You don’t need the tire diameter calculator in this case.
The calculated value should be used only as a guideline for RPM calculations. If you have access to the actual tire in question the simplest and most accurate way to determine diameter is to simply measure the distance from the ground to the top of the tire. (Keep in mind that the tire height while in motion might be slightly taller).
One more consideration is that tires that are made by different manufacturers often have different actual diameters despite being labelled as the same size.
Gear Ratio Calculator
RECOMMENDED ENGINE RPM @ HIGHWAY SPEED
- 4 cylinder: 2200 – 3200
- V6 cylinder: 2000 – 3200
- Small block: 1800 – 2800
- Big block: 1800 – 2600
- Diesel: 1600-2800