Gear Ratio Calculator
Work out engine RPM at any speed, and find out what axle ratio you need after a tire size change.
Your driveline
- Assumes no torque converter slip, so an automatic will read slightly higher in the real world.
- Transmission gear ratio means the gear you are actually in. Top gear on most modern automatics is an overdrive well under 1.00.
- Nearest common gear is picked from ratios that are actually manufactured, not the exact decimal.
How the numbers connect
Tire diameter sets how far the vehicle travels per wheel revolution. The axle ratio and the transmission gear multiply engine revolutions down to wheel revolutions. Put those together and you get engine RPM at any road speed.
A numerically higher axle ratio, 4.10 against 3.55, means the engine spins faster for the same road speed. More multiplication at the wheels, more grunt off the line, more RPM on the highway, worse fuel economy.
Why bigger tires ruin the gearing
A taller tire acts exactly like a numerically lower axle ratio. Go from 31 inch tires to 35 inch tires and you have effectively swapped 3.73 gears for something near 3.30. The truck feels lazy, the transmission hunts on grades, and towing gets unpleasant.
The fix is to re-gear by the same proportion. Multiply your axle ratio by the new tire diameter divided by the old one, then pick the closest gear set that actually gets manufactured.
What cruise RPM should look like
Most gas engines are happy cruising between 1,800 and 2,600 RPM. Below about 1,500 the transmission starts hunting and lugging on any incline. Above 3,000 for hours on end is loud and expensive. Diesels generally want to be lower than gas engines.
Frequently asked questions
What gears do I need for 35 inch tires?
It depends entirely on what you started with. Multiply your current axle ratio by 35 divided by your original tire diameter. Going from 31s on 3.73 gears, you need about 4.21, so 4.10 or 4.30 are the real world choices. Enter both diameters above and it works it out.
Do I have to re-gear when I go up a tire size?
Not for a small jump. Under about 5% taller most people never bother. Past 10%, particularly on a truck that tows or a vehicle with a small engine, re-gearing is the difference between a vehicle that drives properly and one that feels broken.
Is a higher axle ratio better?
Better at what. Numerically higher gears pull harder and tow better and cost you fuel economy and highway quiet. Numerically lower gears do the reverse. There is no universally correct answer, only the right answer for what the vehicle does.
Where do I find my axle ratio?
Usually a metal tag under a differential cover bolt, or on the door jamb sticker as an axle code. Failing that, jack up both rear wheels, mark the driveshaft, and turn the wheels one full rotation while counting driveshaft turns. The number of turns is your ratio.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.