Final Drive Ratio Explained: Worked Example With the RPM Formula

If you want a straight answer on final drive ratio explained, 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.

final drive ratio explained: The RPM formula

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. That is the point this page works through for final drive ratio explained.

final drive ratio explained: Worked example: RPM at 30, 50 and 70 mph

For final drive ratio explained, the point is this: Take a 245/75R16 tyre with a nominal diameter of 773.9 mm (30.47 in) and a 3.92 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.92 × 336) ÷ 30.47 = 3,026. At 50 mph the same sum gives 2,161 RPM, and at 30 mph it gives 1,297 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 2,088 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 final drive ratio explained

On this page, final drive ratio explained is the case in point: RPM for a 3.92 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 1,297 895
50 2,161 1,491
70 3,026 2,088

Speed at fixed RPM for final drive ratio explained

On this page, final drive ratio explained is the case in point: Rearranged, the formula gives speed from RPM. With the same 3.92 ratio and 30.47-inch tyre, 2,000 RPM in direct drive means 46.3 mph, and 3,000 RPM means 69.4 mph. In overdrive the same 2,000 RPM means 67.1 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 final drive ratio explained

On this page, final drive ratio explained is the case in point: The ratio sets where the engine lives. With a numerically higher ratio, such as 4.10 against 3.21, the engine turns faster at the same road speed and delivers more torque to the wheels — stronger acceleration, easier towing, more noise and usually more fuel on a motorway run. With a numerically lower ratio the engine turns slower, cruising is quieter and more economical, but the vehicle is lazier off the line and downshifts more on hills. A ratio that is too low for the tyre size is its own problem: the engine labours, the transmission hunts, and fuel use can rise rather than fall. Larger tyres have the same effect as a lower ratio, which is why the ratio is the usual correction after a tyre-size change. Judge the choice by the RPM at the speeds you actually drive, not by the size of the number.

final drive ratio explained: Common mistakes

  • Taking final drive ratio explained as 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.
  • 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 final drive ratio explained.
  • 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 final drive ratio explained.
  • Taking final drive ratio explained as 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 final drive ratio explained

For final drive ratio explained, the point is this: 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 final drive ratio explained

For final drive ratio explained, the point is this: 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.

Frequently asked questions

What does 3.73 mean on an axle tag?

Taking final drive ratio explained 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.

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 final drive ratio explained.

How do I calculate RPM from gear ratio?

Taking final drive ratio explained as the case in point: 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 comes from the size chart — for a 245/75R16 it is 30.47 inches.

Does gear ratio affect fuel economy?

For final drive ratio explained, the point is this: Yes, in both directions. A ratio that is too high raises cruising RPM and usually uses more fuel on the motorway; a ratio that is too low for the tyre size makes the engine labour and the transmission downshift, which can also waste fuel. The efficient ratio is the one that keeps the engine in its useful rev range for how you drive.

How do I find my gear ratio?

On this page, final drive ratio explained is 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.