Gear and Speed Calculator
This gear and speed calculator connects engine RPM, gear ratio, final drive, and tyre diameter to road speed in one sweep. Use it as a speed by gear calculator to see MPH per gear at any RPM, flip to RPM-from-speed to plan a cruise, or solve for the gear ratio that will give you a target top speed at redline. SI and imperial units are both supported.
The gear and speed formula
Road speed from engine RPM is one equation with three inputs. The gear ratio (transmission) tells you how many engine turns per driveshaft turn. The final drive ratio tells you how many driveshaft turns per axle turn. Multiplying gives the overall drive ratio = gear ratio x final drive. Combine with tyre diameter:
MPH = (RPM x tyre diameter x pi) / (overall ratio x 1056)
The 1056 constant is 12 in/ft x 5280 ft/mi / 60 min/hr, so the units all cancel to miles per hour when tyre diameter is in inches. Divide the result by 0.621371 to get km/h. Invert the equation to back-solve RPM from road speed: RPM = (MPH x overall x 1056) / (tyre x pi). Invert again to solve for the required gear ratio: gear ratio = (RPM x tyre x pi) / (MPH x 1056 x final drive).
Speed by gear calculator reference values
| Gear | Trans ratio | Final drive | RPM | Tyre in | MPH |
|---|---|---|---|---|---|
| 1st | 3.58 | 3.73 | 6500 | 26 | 37.8 |
| 2nd | 2.02 | 3.73 | 6500 | 26 | 67.0 |
| 3rd | 1.35 | 3.73 | 6500 | 26 | 100.2 |
| 4th | 1.00 | 3.73 | 6500 | 26 | 135.3 |
| 5th overdrive | 0.78 | 3.73 | 3000 | 26 | 80.0 |
| Highway cruise | 0.78 | 3.73 | 2250 | 26 | 60.0 |
You can read this as a speed by gear calculator: pick the row that matches your gear and RPM, check the MPH. The calculator below does the same math with exact precision for any combination you enter.
Picking the right gear and final drive
The gear and speed calculator becomes most useful when you are comparing builds. A common trade is a shorter final drive (numerically higher, say 4.10 instead of 3.73) to improve acceleration at the cost of highway cruise RPM. With a 4.10 instead of 3.73 at 70 MPH in top gear on 26 inch tyres, engine RPM jumps from roughly 2626 to 2887 RPM - about 10 percent higher. That hurts fuel economy on long drives but drops 0 to 60 by half a second or more.
The mirror trade is a taller tyre. Up-size from 26 in to 31 in and effective gearing drops by 31 / 26 = 1.19x. Acceleration softens, top speed grows, and highway cruise RPM drops. This is why off-road trucks often regear to 4.88 or 5.13 after a tyre upsize: the axle change restores the original effective ratio.
How this relates to the bigger transmission picture
A speed by gear calculator is really a single-gear slice of a full gear-ratio chart. If you want to see all six gears at once (e.g. comparing a TR-6060 manual to a ZF 8HP automatic), use the transmission gear ratios calculator. If you just need one answer (am I cruising at 2200 RPM or 2800 RPM at 70 MPH?), this gear and speed calculator is faster.