How to Check Gear Ratio: Worked Example With the RPM Formula

If you want a straight answer on how to check gear ratio, start with the formula: RPM equals speed times gear ratio times final drive times 336, divided by tyre diameter in inches. That one formula answers every gear-ratio question on this page — what RPM you will see at a given speed, what speed a given RPM means, and what ratio you need when the tyre size changes. This page works the formula step by step, gives RPM tables at 30, 50 and 70 mph, and explains what the numbers mean for acceleration, towing and cruising.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

The RPM formula for how to check gear ratio

On this page, how to check gear ratio is the case in point: The formula is RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter (inches). Speed is in mph, the gear ratio is the transmission gear you are in (1.00 in direct drive, less than 1.00 in overdrive — for example 0.69 for a typical overdrive gear), the final drive is the axle ratio, and 336 is the constant that makes the units work. Rearranged, it gives speed: speed = (RPM × tyre diameter) ÷ (gear ratio × final drive × 336). The formula uses the tyre's nominal diameter, and all dimensions here are nominal (new-tyre) figures — real tyres vary slightly by brand, model, wear and inflation pressure. Use the diameter from the tyre-size chart for your size rather than guessing it from the sidewall: for example, a 275/55R20 measures 810.5 mm (31.91 in) in overall diameter.

Worked example: RPM at 30, 50 and 70 mph for how to check gear ratio

Take a 275/55R20 tyre with a nominal diameter of 810.5 mm (31.91 in) and a 4.1 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 4.1 × 336) ÷ 31.91 = 3,022. At 50 mph the same sum gives 2,159 RPM, and at 30 mph it gives 1,295 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 2,085 RPM, which is why motorway cruising feels relaxed in top gear. Change any one input and the answer moves in proportion: a bigger tyre lowers RPM, a higher ratio raises it, and a taller overdrive gear lowers it again.

how to check gear ratio: RPM table: 30, 50 and 70 mph

RPM for a 4.1 axle ratio on a 275/55R20 tyre (31.91 in diameter). The direct-drive column uses a 1.00 gear ratio; the overdrive column uses 0.69 as an example.

Speed (mph) RPM in direct drive (1.00) RPM in overdrive (0.69)
30 1,295 894
50 2,159 1,489
70 3,022 2,085

Speed at fixed RPM for how to check gear ratio

Rearranged, the formula gives speed from RPM. With the same 4.1 ratio and 31.91-inch tyre, 2,000 RPM in direct drive means 46.3 mph, and 3,000 RPM means 69.5 mph. In overdrive the same 2,000 RPM means 67.1 mph. That is the quickest way to sanity-check a ratio you have found on an axle tag: drive at a steady speed in a known gear, note the RPM, and see whether the formula's answer matches the speedometer. If it does not, either the ratio is not what you thought, the tyre diameter is not what you assumed, or the speedometer itself is off — all three are worth checking before spending money on gears.

how to check gear ratio: What the gear ratio changes

The ratio sets where the engine lives. With a numerically higher ratio, such as 4.10 against 3.21, the engine turns faster at the same road speed and delivers more torque to the wheels — stronger acceleration, easier towing, more noise and usually more fuel on a motorway run. With a numerically lower ratio the engine turns slower, cruising is quieter and more economical, but the vehicle is lazier off the line and downshifts more on hills. A ratio that is too low for the tyre size is its own problem: the engine labours, the transmission hunts, and fuel use can rise rather than fall. Larger tyres have the same effect as a lower ratio, which is why the ratio is the usual correction after a tyre-size change. Judge the choice by the RPM at the speeds you actually drive, not by the size of the number. That is the point this page works through for how to check gear ratio.

Common mistakes for how to check gear ratio

  • For how to check gear ratio, the point is this: Using the rim size as the tyre diameter. A 17-inch rim is not a 17-inch tyre: the overall diameter includes both sidewalls, so a 265/70R17 is about 31.6 inches across, and using 17 inches in the formula gives nonsense.
  • On this page, how to check gear ratio is the case in point: Changing ratios without recalibrating the speedometer. The speedometer, odometer and shift points all depend on the ratio and tyre size, so a gear change without recalibration leaves every reading wrong.
  • Taking how to check gear ratio as the case in point: Forgetting the transmission gear. The formula needs the gear you are actually in: 1.00 in direct drive, less than 1.00 in overdrive. Using 1.00 for a top-gear cruise overstates the RPM by roughly a third.
  • For how to check gear ratio, the point is this: Assuming the ratio is the same across a model range. Axle ratios vary with engine, tow package and trim, so the ratio on one vehicle is not the ratio on another with the same badge.

Related calculations for how to check gear ratio

For the next step, these pages apply the same method to a closely related question — or hand the arithmetic to the calculator. That is the point this page works through for how to check gear ratio.

how to check gear ratio: Putting your figure to work

On this page, how to check gear ratio is the case in point: Putting the figure to work means using the formula with your own numbers before spending money. Measure the tyre diameter from the chart for your size, find the axle ratio from the tag or the build sheet, and work out the RPM at the speeds you actually drive. If the cruising RPM is far from where the engine is happy, a ratio change is worth pricing — but price the whole job, including the ring and pinion, the installation and the speedometer recalibration, and confirm the choice with a drivetrain specialist. The formula is the cheap part; the gears are the expensive part, and the calculation exists so you only buy them once.

Frequently asked questions

What does 3.73 mean on an axle tag?

It means the driveshaft turns 3.73 times for one turn of the wheels. Higher numbers mean more torque multiplication and higher engine RPM at a given speed; lower numbers mean more relaxed cruising and less pull. That is the point this page works through for how to check gear ratio.

What is the difference between gear ratio and final drive ratio for how to check gear ratio?

For how to check gear ratio, the point is this: They are the same idea at different places. The gear ratio is the transmission gear you are in; the final drive ratio is the axle ratio. The formula multiplies them together, so the total reduction from engine to wheels is the product of the two.

Does gear ratio affect fuel economy?

Taking how to check gear ratio as the case in point: Yes, in both directions. A ratio that is too high raises cruising RPM and usually uses more fuel on the motorway; a ratio that is too low for the tyre size makes the engine labour and the transmission downshift, which can also waste fuel. The efficient ratio is the one that keeps the engine in its useful rev range for how you drive.

What is a gear ratio?

It is the number of times the driveshaft turns for one turn of the wheels. A 3.73 axle ratio means the driveshaft turns 3.73 times for each wheel revolution. A numerically higher ratio multiplies engine torque more and raises engine RPM at a given road speed. That is the point this page works through for how to check gear ratio.

Does a higher gear ratio mean faster acceleration?

Taking how to check gear ratio as the case in point: In general, yes: a numerically higher ratio gives more torque at the wheels and quicker acceleration, at the cost of higher cruising RPM and usually more fuel use on the motorway. A numerically lower ratio gives relaxed cruising but less pull off the line.