Gear Ratio Calculator Fuel Economy: The RPM Formula, Tables and What Changes

If you want a straight answer on gear ratio calculator fuel economy, 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 calculator fuel economy

On this page, gear ratio calculator fuel economy is 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 245/75R16 measures 773.9 mm (30.47 in) in overall diameter.

Worked example: RPM at 30, 50 and 70 mph for gear ratio calculator fuel economy

Take a 245/75R16 tyre with a nominal diameter of 773.9 mm (30.47 in) and a 3.21 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.21 × 336) ÷ 30.47 = 2,478. At 50 mph the same sum gives 1,770 RPM, and at 30 mph it gives 1,062 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,710 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. That is the point this page works through for gear ratio calculator fuel economy.

RPM table: 30, 50 and 70 mph for gear ratio calculator fuel economy

For gear ratio calculator fuel economy, the point is this: RPM for a 3.21 axle ratio on a 245/75R16 tyre (30.47 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 1,062 733
50 1,770 1,221
70 2,478 1,710

gear ratio calculator fuel economy: Speed at fixed RPM

Taking gear ratio calculator fuel economy as the case in point: Rearranged, the formula gives speed from RPM. With the same 3.21 ratio and 30.47-inch tyre, 2,000 RPM in direct drive means 56.5 mph, and 3,000 RPM means 84.8 mph. In overdrive the same 2,000 RPM means 81.9 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 gear ratio calculator fuel economy

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 gear ratio calculator fuel economy.

Common mistakes for gear ratio calculator fuel economy

  • For gear ratio calculator fuel economy, the point is this: 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 calculator fuel economy 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.
  • 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 gear ratio calculator fuel economy.
  • 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. That is the point this page works through for gear ratio calculator fuel economy.

gear ratio calculator fuel economy: Related calculations

On this page, gear ratio calculator fuel economy is the case in point: For the next step, these pages apply the same method to a closely related question — or hand the arithmetic to the calculator.

gear ratio calculator fuel economy: Putting your figure to work

The formula is the cheap part of the job, so use it first. Work out your RPM at real driving speeds with your tyre diameter from the chart and your axle ratio from the tag, and compare it with where the engine feels comfortable. If a change is needed, get a full quote — gears, labour and recalibration — and have a drivetrain specialist confirm the ratio before ordering. A ratio picked from a table without the RPM check is a guess, and guesses in gearing are expensive to undo. That is the point this page works through for gear ratio calculator fuel economy.

Frequently asked questions

What is a gear ratio?

On this page, gear ratio calculator fuel economy is the case in point: 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.

What does 3.73 mean on an axle tag?

Taking gear ratio calculator fuel economy 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.

How accurate is the RPM formula?

On this page, gear ratio calculator fuel economy is 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.

For gear ratio calculator fuel economy, what is the difference between gear ratio and final drive ratio?

For gear ratio calculator fuel economy, the point is this: 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.

How do I find my gear ratio?

Taking gear ratio calculator fuel economy as the case in point: 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.