What Gear Ratio for 33 Inch Tyres: Worked Example With the RPM Formula

If you want a straight answer on what gear ratio for 33 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 33 inch tyres: The RPM formula

For what gear ratio for 33 inch tyres, the point is this: 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 285/60R20 measures 850.0 mm (33.46 in) in overall diameter.

what gear ratio for 33 inch tyres: Worked example: RPM at 30, 50 and 70 mph

For what gear ratio for 33 inch tyres, the point is this: Take a 285/60R20 tyre with a nominal diameter of 850.0 mm (33.46 in) and a 3.55 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.55 × 336) ÷ 33.46 = 2,495. At 50 mph the same sum gives 1,782 RPM, and at 30 mph it gives 1,069 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,722 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.

RPM table: 30, 50 and 70 mph for what gear ratio for 33 inch tyres

For what gear ratio for 33 inch tyres, the point is this: RPM for a 3.55 axle ratio on a 285/60R20 tyre (33.46 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 1,069 738
50 1,782 1,230
70 2,495 1,722

what gear ratio for 33 inch tyres: Speed at fixed RPM

On this page, what gear ratio for 33 inch tyres is the case in point: Rearranged, the formula gives speed from RPM. With the same 3.55 ratio and 33.46-inch tyre, 2,000 RPM in direct drive means 56.1 mph, and 3,000 RPM means 84.2 mph. In overdrive the same 2,000 RPM means 81.3 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 what gear ratio for 33 inch tyres

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 what gear ratio for 33 inch tyres.

Common mistakes for what gear ratio for 33 inch tyres

  • 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. That is the point this page works through for what gear ratio for 33 inch tyres.
  • On this page, what gear ratio for 33 inch tyres is 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 285/60R20 is 33.46 inches across, and using 20 inches instead would give nonsense.
  • 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. That is the point this page works through for what gear ratio for 33 inch tyres.
  • 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. That is the point this page works through for what gear ratio for 33 inch tyres.

what gear ratio for 33 inch tyres: Related calculations

Taking what gear ratio for 33 inch tyres as the case in point: These pages continue from here: the same method on a neighbouring question, or the calculator that runs the arithmetic for you.

what gear ratio for 33 inch tyres: Putting your figure to work

For what gear ratio for 33 inch tyres, the point is this: 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

How accurate is the RPM formula?

It is exact for the inputs you give it: speed, gear ratio, final drive and tyre diameter. The uncertainty comes from the inputs, mainly the tyre diameter, which is a nominal new-tyre figure that varies slightly with brand, wear and pressure. A few percent either way is normal. That is the point this page works through for what gear ratio for 33 inch tyres.

What happens if my gear ratio is too low?

For what gear ratio for 33 inch tyres, the point is this: 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?

Taking what gear ratio for 33 inch tyres as 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.

What does 3.73 mean on an axle tag?

On this page, what gear ratio for 33 inch tyres is 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.

Does a higher gear ratio mean faster acceleration?

On this page, what gear ratio for 33 inch tyres is 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.