How to Calculate Gear Ratio From Tire Size: How the Ratio Changes RPM and Speed

If you want a straight answer on how to calculate gear ratio from tire size, 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 calculate gear ratio from tire size

For how to calculate gear ratio from tire size, the point is this: RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter (inches) is the whole method in one line. Speed is in mph, the gear ratio is the transmission gear in use — 1.00 in direct drive, under 1.00 in overdrive, 0.69 for a typical overdrive gear — the final drive is the axle ratio, and 336 is the constant that makes the units work. Turn it round for speed: speed = (RPM × tyre diameter) ÷ (gear ratio × final drive × 336). Diameters here are nominal (new-tyre) figures, and real tyres vary slightly by brand, model, wear and inflation pressure, so take the diameter from the tyre-size chart for your size: a 265/75R16 measures 803.9 mm (31.65 in) across.

how to calculate gear ratio from tire size: Worked example: RPM at 30, 50 and 70 mph

For how to calculate gear ratio from tire size, the point is this: Take a 265/75R16 tyre with a nominal diameter of 803.9 mm (31.65 in) and a 3.55 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.55 × 336) ÷ 31.65 = 2,638. At 50 mph the same sum gives 1,884 RPM, and at 30 mph it gives 1,131 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,820 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 calculate gear ratio from tire size: RPM table: 30, 50 and 70 mph

Taking how to calculate gear ratio from tire size as the case in point: RPM for a 3.55 axle ratio on a 265/75R16 tyre (31.65 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 1,131 780
50 1,884 1,300
70 2,638 1,820

Speed at fixed RPM for how to calculate gear ratio from tire size

For how to calculate gear ratio from tire size, the point is this: Rearranged, the formula gives speed from RPM. With the same 3.55 ratio and 31.65-inch tyre, 2,000 RPM in direct drive means 53.1 mph, and 3,000 RPM means 79.6 mph. In overdrive the same 2,000 RPM means 76.9 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.

What the gear ratio changes for how to calculate gear ratio from tire size

A numerically higher ratio — 4.10 rather than 3.21, for example — multiplies the engine's torque more before it reaches the wheels, so the vehicle accelerates harder and pulls a trailer more easily, but the engine turns faster at any given road speed. A numerically lower ratio does the opposite: relaxed cruising and lower RPM on the motorway, but less pull off the line and more downshifting on hills or when towing. Fuel use follows the same split in general terms: higher cruising RPM usually means more fuel on the motorway, while a ratio that is too low for the tyre size can also waste fuel, because the engine labours below its efficient range and the transmission downshifts constantly. Bigger tyres have the same effect as a lower ratio — they lower the RPM at a given speed — which is why a ratio change is the standard correction after fitting larger tyres. The right ratio is the one that puts the engine in its useful rev range for the way the vehicle is actually driven, not the one with the biggest number. That is the point this page works through for how to calculate gear ratio from tire size.

how to calculate gear ratio from tire size: Common mistakes

  • Taking how to calculate gear ratio from tire size as the case in point: Judging a ratio by one speed only. A ratio that looks right at 70 mph can be wrong at 30 mph, so check the RPM at all three speeds in the table before deciding.
  • Taking how to calculate gear ratio from tire size as the case in point: Using the tyre's sidewall size instead of its diameter. The formula needs the overall diameter in inches, not the width or the rim size — a 265/70R17 is 31.61 inches across, and using 17 inches instead would give nonsense.
  • For how to calculate gear ratio from tire size, the point is this: Forgetting that front and rear ratios must match on a four-wheel-drive vehicle. Different ratios front and rear will fight each other in four-wheel drive and damage the transfer case, so both axles have to be changed together.
  • On this page, how to calculate gear ratio from tire size is the case in point: 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 calculate gear ratio from tire size

These pages continue from here: the same method on a neighbouring question, or the calculator that runs the arithmetic for you. That is the point this page works through for how to calculate gear ratio from tire size.

how to calculate gear ratio from tire size: Putting your figure to work

On this page, how to calculate gear ratio from tire size is the case in point: Before you spend money, run the formula on your own numbers. Take the tyre diameter from the chart for your size, find the axle ratio from the tag or build sheet, and work out the RPM at the speeds you drive most. If the cruising RPM is a long way from where the engine is happy, price a ratio change — ring and pinion, fitting and speedometer recalibration together — and confirm the choice with a drivetrain specialist. The arithmetic is free; the gears are not, and the calculation exists so you buy them once.

Frequently asked questions

What does 3.73 mean on an axle tag?

Taking how to calculate gear ratio from tire size as the case in point: 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.

What happens if my gear ratio is too low?

On this page, how to calculate gear ratio from tire size is the case in point: The engine feels gutless, the transmission hunts between gears on hills, and acceleration from a standstill is sluggish — especially with bigger tyres. The fix is usually a numerically higher ratio, fitted by a drivetrain specialist with the speedometer recalibrated afterwards.

Can I change just the rear axle ratio on a 4x4?

For how to calculate gear ratio from tire size, the point is this: No — on a four-wheel-drive vehicle the front and rear ratios must match, or the axles will fight each other in four-wheel drive and damage the transfer case. Both axles have to be changed together, which is why a 4x4 gear change costs more than a two-wheel-drive one.

Do I need to recalibrate the speedometer after a gear change for how to calculate gear ratio from tire size?

Yes. The speedometer, odometer and shift points depend on the ratio and tyre size, so changing gears without recalibration leaves every reading wrong. Recalibration is usually done with a programmer or by a dealer, depending on the vehicle. That is the point this page works through for how to calculate gear ratio from tire size.

What is a gear ratio?

For how to calculate gear ratio from tire size, the point is this: 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.