Calculator Inputs
Example Data Table
| Setup | Tire Diameter | Axle Ratio | Top Gear | Engine RPM | Approx Speed |
|---|---|---|---|---|---|
| Street cruiser | 28 in | 3.73 | 0.70 | 2500 | 79.5 mph |
| Towing setup | 31 in | 4.10 | 0.75 | 2500 | 75.0 mph |
| Off-road low range | 35 in | 4.56 | 3.00 | 3000 | 22.8 mph |
Formula Used
Tire diameter from a metric tire code is found with: diameter = wheel diameter + 2 × width × aspect ratio ÷ 25.4. Effective diameter adds tire growth.
Overall ratio = transmission gear ratio × axle ratio × transfer ratio. Wheel rpm = engine rpm ÷ overall ratio.
Vehicle speed in mph = wheel rpm × tire circumference × 60 ÷ 63360 × slip factor. Slip factor = 1 − slip percent ÷ 100.
RPM at target speed = target mph × overall ratio × 63360 ÷ tire circumference ÷ 60 ÷ slip factor.
Speedometer factor = new axle ratio × original tire diameter ÷ original axle ratio ÷ new effective tire diameter. Error percent = speedometer factor − 1, then multiplied by 100.
How to Use This Calculator
Choose the tire input method first. Use known diameter when the measured tire height is available. Use metric tire code when you only know width, aspect ratio, and wheel diameter.
Enter engine rpm, target speed, axle ratio, transfer ratio, and slip. Add each transmission gear ratio that you want to compare. Leave unused gear fields blank.
Press calculate. The result appears below the header and above the form. Review road speed, cruise rpm, speedometer error, and wheel torque factor. Export the result with the CSV or PDF buttons.
Engineering Article
Why Gear and Tire Speed Matters
Gear, tire, and speed planning is a core part of drivetrain setup. A small tire change can alter road speed, cruise rpm, launch feel, and speedometer accuracy. This calculator helps compare those effects before parts are bought. It works for street cars, trucks, race cars, and off-road builds. It also helps when checking a transmission swap or axle ratio change.
Tire Diameter Changes the Final Result
Tire diameter acts like a final drive modifier. A taller tire travels farther during each wheel revolution. That lowers engine rpm at a fixed road speed. It can also make acceleration feel softer. A shorter tire does the opposite. It raises engine rpm and usually improves mechanical leverage. Real tire height can differ from catalog size. Loaded radius, pressure, tread wear, and tire growth all matter. For accurate planning, measure the tire under vehicle load.
Ratios Control Engine Speed
Transmission ratio, axle ratio, and transfer ratio combine into one overall ratio. A high overall ratio gives more wheel torque. It is useful for towing, crawling, and quick launches. A low overall ratio reduces rpm during cruising. It can improve comfort and fuel use. The best setup depends on engine torque curve, vehicle weight, road use, and tire size. Overdrive gears often look small, but they have a large cruise effect.
Reading the Output
The table shows speed at the entered rpm for every gear. It also shows required rpm at the target speed. This makes shift planning easier. Drag racers can estimate trap rpm. Off-road drivers can compare crawl behavior. Tow vehicles can check highway rpm before changing tires. The wheel torque factor is an estimate. It includes drivetrain efficiency, but it does not replace dyno testing.
Speedometer and Setup Checks
Speedometer error is important after tire or axle changes. The calculator compares the original calibration with the new setup. A positive error means the indicated speed is higher than actual speed. A negative error means the vehicle is moving faster than shown. Always confirm final calibration with a trusted road test or scan tool. Use the exported files to save each setup. They are useful for comparing several combinations during a build.
FAQs
What does this gear tire speed calculator do?
It estimates road speed, engine rpm, tire diameter, gear behavior, and speedometer error from common drivetrain inputs.
Can I use metric tire sizes?
Yes. Select the metric tire code option. Then enter width, aspect ratio, and wheel diameter.
What is tire growth?
Tire growth is the diameter increase that may occur at speed. It is common with soft racing tires.
What does converter slip mean?
Slip is the percentage loss between engine rotation and effective wheel speed. Automatic transmissions often have some slip.
Why is my speedometer wrong after new tires?
The vehicle calibration expects a certain tire size and axle ratio. Changing either one changes wheel pulses per mile.
Can this help choose axle gears?
Yes. Change the axle ratio and compare rpm, speed, and wheel torque factor across every gear.
Is the wheel torque factor exact?
No. It is a planning estimate. Real torque depends on engine output, traction, converter behavior, and drivetrain losses.
Should I use measured tire diameter?
Yes. Measured loaded diameter usually gives better results than a sidewall rating or catalog size.