Gear ratio calculator
Enter your tire size, gear ratios, final drive and redline. The calculator uses mph = rpm × tire diameter ÷ (gear × final drive × 336.13) to show engine speed at any road speed and the speed you'll hit at redline in each gear. A shorter (numerically higher) final drive raises RPM at every speed.
Result
Fix the highlighted box to update the results. 1st gear is required; leave unused higher gears blank.
| Gear | Ratio | RPM at 60 mph | Speed at redline |
|---|
How to use the gear ratio calculator
Pick a preset or choose Custom and enter your own numbers. You need four things: the tire size, the gear ratios, the final drive (axle) ratio and the redline. Gear and final drive ratios are in the owner's manual or the maker's spec sheet. Redline is on the tachometer.
Type a tire size like 205/45R17 and the calculator fills in the diameter. If you've measured the real diameter, type that in instead. The RPM check speed sets which road speed the RPM column uses.
The table gives engine RPM at your check speed and the road speed at redline for each gear. The chart draws one line per gear, so you can see where each gear runs out and where the next one picks up.
How the math works
One formula covers it. Quadratec, the Jeep parts retailer, gives it as:
rpm = mph × gear ratio × final drive × 336 ÷ tire diameter
The 336 isn't magic. It's the unit conversion from inches per minute to miles per hour for a wheel of a given diameter: 63,360 inches in a mile, divided by 60 minutes and by π. That works out to 336.135. This calculator uses 336.13.
Here's the default, a 2019 Mazda MX-5 with the 6-speed manual on 205/45R17 tires. Mazda lists the ratios as 5.087, 2.991, 2.035, 1.594, 1.286 and 1.000, with a 2.866 final drive and a 7,500 rpm redline (Mazda USA). The tire is 17 + 2 × 92.25 mm ÷ 25.4 = 24.264 inches tall.
| Gear | Ratio | RPM at 60 mph | Speed at 7,500 rpm |
|---|---|---|---|
| 1st | 5.087 | 12,118 | 37.1 mph |
| 2nd | 2.991 | 7,125 | 63.2 mph |
| 3rd | 2.035 | 4,848 | 92.8 mph |
| 4th | 1.594 | 3,797 | 118.5 mph |
| 5th | 1.286 | 3,063 | 146.9 mph |
| 6th | 1.000 | 2,382 | 188.9 mph |
Take 6th at 60 mph: 60 × 1.000 × 2.866 × 336.13 ÷ 24.264 = 2,382 rpm. And 1st at redline: 7,500 × 24.264 ÷ (5.087 × 2.866 × 336.13) = 37.1 mph. The 1st-gear figure at 60 mph is above redline, which just means you can't do 60 in 1st.
What the numbers mean
The overall ratio is gear times final drive. A higher number means more torque at the wheels and more RPM at a given speed. That's why a "shorter" final drive, say going from 3.73 to 4.10, makes a car quicker but busier on the highway.
Speed at redline tells you where each gear runs out. In the MX-5, 2nd tops out at about 63 mph, so a 0–60 run fits in two gears.
RPM at your cruising speed is what you'll hear on the highway. Taller tires lower it. Our tire size calculator shows how much a new size changes the diameter, and you can paste the new size here to see the RPM effect.
What this calculator can't tell you
It's pure gearing. Top gear at redline is a theoretical number. Most cars run out of power against drag long before then, so 189 mph in an MX-5 isn't happening.
It ignores tire growth at speed, tire squat under load and clutch or converter slip. An automatic with an unlocked torque converter will turn more RPM than this shows. Expect real numbers to differ by a few percent.
It also uses the nominal tire size. Measure the real rolling diameter, or use the tire maker's revs per mile, if you need it exact.
Sources are linked in the text. Main references: Quadratec, Mazda USA and Toyota UK.