Gear Ratio Chart: Formula, Tables and Common Mistakes

If you want a straight answer on gear ratio chart, 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 gear ratio chart

Taking gear ratio chart as 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 285/70R17 measures 830.8 mm (32.71 in) in overall diameter.

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

Taking gear ratio chart as the case in point: Take a 285/70R17 tyre with a nominal diameter of 830.8 mm (32.71 in) and a 3.55 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.55 × 336) ÷ 32.71 = 2,553. At 50 mph the same sum gives 1,823 RPM, and at 30 mph it gives 1,094 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,761 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 gear ratio chart

RPM for a 3.55 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 chart.

Speed (mph) RPM in direct drive (1.00) RPM in overdrive (0.69)
30 1,094 755
50 1,823 1,258
70 2,553 1,761

Speed at fixed RPM for gear ratio chart

For gear ratio chart, the point is this: Rearranged, the formula gives speed from RPM. With the same 3.55 ratio and 32.71-inch tyre, 2,000 RPM in direct drive means 54.8 mph, and 3,000 RPM means 82.3 mph. In overdrive the same 2,000 RPM means 79.5 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 chart

Taking gear ratio chart as the case in point: Think of the axle ratio as a torque multiplier. A numerically higher ratio — 4.10 instead of 3.21 — multiplies engine torque more before it reaches the wheels, so the vehicle pulls harder and tows more easily, but the engine spins faster at any road speed. A numerically lower ratio trades that pull for relaxed cruising and lower RPM on the motorway. Fuel use splits the same way in general terms: higher cruising RPM usually costs more fuel, while a ratio too low for the tyre size can also waste fuel, because the engine labours below its efficient range and the gearbox downshifts all the time. Bigger tyres act like a lower ratio, lowering RPM at a given speed, which is why a ratio change is the standard fix after fitting larger tyres. The right ratio is the one that keeps the engine in its useful rev range for the driving you actually do.

gear ratio chart: Common mistakes

  • Taking gear ratio chart as the case in point: Treating the calculation as a safety approval. The formula tells you the RPM; it does not tell you the gears will fit, the axle will cope, or the vehicle is safe. Confirm fitment with a drivetrain specialist.
  • On this page, gear ratio chart is 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.
  • Taking gear ratio chart 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.
  • On this page, gear ratio chart 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 265/70R17 is 31.61 inches across, and using 17 inches instead would give nonsense.

Related calculations for gear ratio chart

Taking gear ratio chart 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.

Putting your figure to work for gear ratio chart

For gear ratio chart, 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

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

Taking gear ratio chart 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, gear ratio chart 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.

Do I need to recalibrate the speedometer after a gear change for gear ratio chart?

For gear ratio chart, the point is this: 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.

What happens if my gear ratio is too low?

For gear ratio chart, 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.

How do I find my gear ratio?

Check the axle tag on the differential cover, the door-jamb sticker, the build sheet, or ask a dealer to decode the VIN. You can also count driveshaft turns against wheel turns, or check the ratio against the RPM formula at a steady speed. That is the point this page works through for gear ratio chart.