What Gear Ratio for 37 Inch Tyres: Worked Example With the RPM Formula

If you want a straight answer on what gear ratio for 37 inch tyres, 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.

what gear ratio for 37 inch tyres: The RPM formula

Everything on this page comes from one formula: RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter in inches. The speed is in mph, the gear ratio is the transmission gear you are in (1.00 direct drive, or about 0.69 in a typical overdrive), the final drive is the axle ratio, and 336 is the unit constant. The same formula rearranged gives speed from RPM: speed = (RPM × tyre diameter) ÷ (gear ratio × final drive × 336). Use the nominal diameter for your size rather than a guess from the sidewall — a 37-inch tyre measures 939.8 mm in nominal diameter (37 × 25.4); the site's size chart covers up to 33.46 in, so this example uses the nominal figure for a 37-inch tyre — and remember these are nominal new-tyre figures: real tyres vary slightly by brand, model, wear and inflation pressure. That is the point this page works through for what gear ratio for 37 inch tyres.

Worked example: RPM at 30, 50 and 70 mph for what gear ratio for 37 inch tyres

Taking what gear ratio for 37 inch tyres as the case in point: Take a 37-inch tyre with a nominal diameter of 939.8 mm and a 3.21 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.21 × 336) ÷ 37 = 2,041. At 50 mph the same sum gives 1,458 RPM, and at 30 mph it gives 875 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,408 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.

what gear ratio for 37 inch tyres: RPM table: 30, 50 and 70 mph

RPM for a 3.21 axle ratio on a 37-inch tyre (939.8 mm nominal diameter). The direct-drive column uses a 1.00 gear ratio; the overdrive column uses 0.69 as an example. That is the point this page works through for what gear ratio for 37 inch tyres.

Speed (mph) RPM in direct drive (1.00) RPM in overdrive (0.69)
30 875 603
50 1,458 1,006
70 2,041 1,408

Speed at fixed RPM for what gear ratio for 37 inch tyres

On this page, what gear ratio for 37 inch tyres is the case in point: Rearranged, the formula gives speed from RPM. With the same 3.21 ratio and 37-inch tyre, 2,000 RPM in direct drive means 68.6 mph, and 3,000 RPM means 102.9 mph. In overdrive the same 2,000 RPM means 99.4 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 gear ratio for 37 inch tyres: What the gear ratio changes

Taking what gear ratio for 37 inch tyres as the case in point: 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.

Common mistakes for what gear ratio for 37 inch tyres

  • On this page, what gear ratio for 37 inch tyres is the case in point: Ignoring the transmission's gear spacing. The axle ratio is only half the story: the transmission's own ratios decide how the gears step, and a wide-ratio gearbox can make a tall axle work where a close-ratio one would struggle.
  • For what gear ratio for 37 inch tyres, 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 37-inch tyre is 37 inches across, and using 17 inches in the formula gives nonsense.
  • For what gear ratio for 37 inch tyres, 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.
  • Taking what gear ratio for 37 inch tyres as the case in point: Mixing up high and low ratios. A numerically higher ratio (4.10) is a 'lower' gear in driving terms — more torque, higher RPM — while a numerically lower ratio (3.21) is a 'higher' gear. The wording trips people up every time.

what gear ratio for 37 inch tyres: Related calculations

For what gear ratio for 37 inch tyres, the point is this: If this page answered your question, these pages take the next step — the same method applied to a closely related question, or the calculator that does the arithmetic for you.

what gear ratio for 37 inch tyres: Putting your figure to work

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. That is the point this page works through for what gear ratio for 37 inch tyres.

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 what gear ratio for 37 inch tyres.

Do I need to recalibrate the speedometer after a gear change for what gear ratio for 37 inch tyres?

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 what gear ratio for 37 inch tyres.

What happens if my gear ratio is too low?

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. That is the point this page works through for what gear ratio for 37 inch tyres.

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

On this page, what gear ratio for 37 inch tyres is the case in point: 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.

How do I calculate RPM from gear ratio?

For what gear ratio for 37 inch tyres, the point is this: Use RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter in inches. Speed is in mph, the gear ratio is the transmission gear (1.00 in direct drive), the final drive is the axle ratio, and the tyre diameter is the nominal figure for your size — for a 37-inch tyre it is 939.8 mm (37 in).