MotorCrunch
🧰 Car Tools & Specs

Gear Ratio & RPM Calculator

Turn your axle (final drive) ratio, transmission gear and tire diameter into an effective ratio and the engine RPM you'll actually see at any cruising speed.

What RPM will my engine turn at a given speed?

Multiply your speed, transmission gear and axle ratio by 336, then divide by tire diameter in inches. At 65 mph with a 3.73 axle, a 1.00 gear and a 28-inch tire that's about 2,910 RPM. The same tool gives your effective final ratio (axle × gear) and how many MPH each 1,000 RPM buys you.

Your numbers

Engine RPM at 65 mph

2,910.57

3.73:1 effective

Effective final ratio

3.73

axle × gear

MPH per 1,000 RPM

22.33

Tire revolutions per mile720.29
Wheel RPM at speed780.31

Insight — A numerically higher axle ratio (e.g. 4.10 vs 3.55) multiplies torque for quicker acceleration and towing, but raises cruising RPM — which usually costs fuel economy. Bigger tires effectively lower your ratio; that's why a lift-and-tires build often wants a matching gear change.

What if Speed changes?

SpeedEngine RPM at 65 mph
40 mph 1,791.12
55 mph 2,462.79
65 mph · now2,910.57
75 mph 3,358.35
85 mph 3,806.13

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For context: at 65 mph with a 3.73 axle, a 1.00 top gear and a 28-inch tire, the engine turns about 2,910 RPM. Drop to a 3.08 axle and it falls to roughly 2,400 RPM — the classic overdrive-cruise trade-off. Enter your own driveline above.

How this calculator works

The calculator uses the standard driveline relationship between road speed, gearing and tire size. Wheel revolutions per mile come from the tire's circumference; multiplying wheel speed by the transmission gear and the axle ratio gives engine speed. It reports engine RPM at your chosen speed, the effective final ratio, MPH per 1,000 RPM and tire revolutions per mile so you can compare setups.

Formula

Engine RPM = (MPH × gear × axle × 336.135) ÷ tire diameter(in). Effective ratio = axle × gear. Tire revs/mile = 63,360 ÷ (π × diameter).

Worked example

At 65 mph, gear 1.00, axle 3.73, tire 28 in: RPM = (65 × 1.00 × 3.73 × 336.135) ÷ 28 ≈ 2,910. Effective ratio = 3.73 × 1.00 = 3.73:1. MPH per 1,000 RPM = (1,000 × 28) ÷ (1.00 × 3.73 × 336.135) ≈ 22.3.

What affects your result

  • →Axle / final-drive ratio
  • →Transmission gear ratio (which gear you're in)
  • →Overall tire diameter
  • →Road speed
  • →Overdrive vs direct-drive top gears

What this estimate includes

  • ✓Engine RPM at your chosen speed
  • ✓Effective final ratio (axle × gear)
  • ✓MPH per 1,000 RPM
  • ✓Tire revolutions per mile and wheel RPM

What it does not include

  • ×Torque-converter slip and lock-up on automatics
  • ×Tire growth at speed and real rolling radius
  • ×Driveline and tire wear tolerances
  • ×Actual fuel economy (RPM is only one input)

Good to know

Clear, practical answers about the gear ratio calculator.

How do I calculate engine RPM from gear ratio?＋

RPM = (MPH × transmission gear × axle ratio × 336) ÷ tire diameter in inches. The 336 constant converts miles per hour and tire circumference into engine revolutions per minute. Lower the tire diameter or raise either ratio and RPM goes up.

What's the difference between axle ratio and gear ratio?＋

The axle (final-drive) ratio is fixed in your differential — how many times the driveshaft turns per wheel revolution. The transmission gear ratio changes with each gear you select. Multiplying them gives the effective ratio at the wheels in that gear.

Do bigger tires change my gearing?＋

Effectively, yes. A taller tire travels farther per revolution, which acts like a numerically lower (taller) gear — slower acceleration, lower cruising RPM and an optimistic speedometer. Many people re-gear the axle to restore the original feel after a tire upsize.

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How MotorCrunch estimates are made

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