Gear Ratio Lookup: Formula, Tables and Common Mistakes

If you want a straight answer on gear ratio lookup, 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.

gear ratio lookup: The RPM formula

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 285/70R17 measures 830.8 mm (32.71 in) across. That is the point this page works through for gear ratio lookup.

Worked example: RPM at 30, 50 and 70 mph for gear ratio lookup

For gear ratio lookup, the point is this: Take a 285/70R17 tyre with a nominal diameter of 830.8 mm (32.71 in) and a 4.1 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 4.1 × 336) ÷ 32.71 = 2,948. At 50 mph the same sum gives 2,106 RPM, and at 30 mph it gives 1,263 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 2,034 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.

gear ratio lookup: RPM table: 30, 50 and 70 mph

RPM for a 4.1 axle ratio on a 285/70R17 tyre (32.71 in 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 gear ratio lookup.

Speed (mph) RPM in direct drive (1.00) RPM in overdrive (0.69)
30 1,263 872
50 2,106 1,453
70 2,948 2,034

Speed at fixed RPM for gear ratio lookup

Taking gear ratio lookup as the case in point: Rearranged, the formula gives speed from RPM. With the same 4.1 ratio and 32.71-inch tyre, 2,000 RPM in direct drive means 47.5 mph, and 3,000 RPM means 71.2 mph. In overdrive the same 2,000 RPM means 68.8 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 gear ratio lookup

On this page, gear ratio lookup is 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.

gear ratio lookup: Common mistakes

  • For gear ratio lookup, 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 gear ratio lookup as the case in point: 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.
  • 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. That is the point this page works through for gear ratio lookup.
  • For gear ratio lookup, the point is this: 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.

Related calculations for gear ratio lookup

Taking gear ratio lookup as the case in point: 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.

gear ratio lookup: Putting your figure to work

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. That is the point this page works through for gear ratio lookup.

Frequently asked questions

How accurate is the RPM formula?

Taking gear ratio lookup as the case in point: 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.

What is the difference between gear ratio and final drive ratio for gear ratio lookup?

Taking gear ratio lookup as the case in point: 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 a higher gear ratio mean faster acceleration?

On this page, gear ratio lookup 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.

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 gear ratio lookup.

For gear ratio lookup, do I need to recalibrate the speedometer after a gear change?

Taking gear ratio lookup as the case in point: 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.