Inputs
Results
Overall ratio = transmission × transfer × axle. Tire revs per mile = 20168 / tire diameter (in).
What this calculator actually solves
Most gear calculators do overall ratio × tire diameter and stop there. They don't tell you which gear you should be cruising in at highway speed, or what your engine RPM is at 60 mph in 3rd. This one gives you a per-gear breakdown so you can see — for example — whether 4th gear with your current rear-end is right for highway cruising, or whether you'll be at 3500 RPM at 65 mph and burning fuel.
It also uses measured tire diameter, not the marketing size. A 33" tire is rarely 33" once mounted; we let you enter the real number and the result reflects it.
Who this is for
Off-roaders picking a tire and a gear change. Knowing whether your new 35s need re-gearing before you spend the money.
Hot-rodders deciding between 3.42 and 3.73 gears for a street-driven combination and trying to balance acceleration against cruise RPM.
Restoration shops quoting a job and wanting to confirm the gear ratio on the diff tag matches what the customer thinks.
How to read the results
If your engine RPM at 60 mph in top gear is 1800 or below, you've over-geared — you'll have to downshift to merge. If it's above 2500, you've under-geared — cruise fuel economy will suffer.
The redline speed readout tells you what your speedometer will read at redline in each gear. If it's 180 mph in 1st gear, your speedometer isn't lying, the speed is real.
Limitations
Tire diameter changes with load, pressure, and wear. We use your number as a constant — if you tow, expect slightly different RPM.
Transmission ratio doesn't account for torque converter slip on automatics. The actual ratio at the driveshaft is roughly 90% of the listed ratio until the converter locks.
Frequently Asked Questions
How do I measure tire diameter?
Two ways: jack up the corner, mark the tire and the floor, rotate one full revolution, and measure the distance traveled in inches. Or use the formula: (section width in mm × aspect ratio × 2 / 25.4) + wheel diameter in inches. The two numbers usually agree within 0.5".
Does an overdrive transmission change the math?
No — just enter the overdrive ratio (e.g. 0.70 for a 30% overdrive) as one of the gear ratios. The calculator multiplies them all the same way.
Why is my speedometer wrong after a tire change?
Because it's geared to the original tire size. With a 33" tire and a speedometer geared for 28", you read 6 mph slower than you're going. Many ECU calibrations can correct this; aftermarket speedometer calibrators exist for cable-driven units.
About this tool
What this tool is for: Gear Ratio & Vehicle Speed Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Compound multi-stage gear (or transmission) ratio solver with tire-diameter output. The trajectory read-out assumes hard pavement and slip-free tires — off-road use will read slower.
How to use the body of this page
The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
Known limitations of every MechCalc Pro tool
All calculators on MechCalc Pro run the math in your browser and assume standard conditions (sea-level pressure, ambient temperature, dry friction, textbook geometry). They do not capture manufacturing tolerances, the wear state of your specific parts, environmental extremes, regulatory codes (ASME, DOT, AWS D1.1, etc.), or the load spectrum of your job. They are not a substitute for a stamped engineering review on safety-critical work.
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Disclaimer
All content on this page, including formulas and calculator outputs, is provided for general informational and educational purposes only. It is not professional engineering advice, it is not a substitute for manufacturer specifications, and it is not a warranty of fitness for any particular purpose.
Mechanical, hydraulic, and welding work can cause serious injury, death, or property damage if performed incorrectly. Numbers produced by this calculator reflect simplified textbook models; they do not capture manufacturing tolerances, material defects, environmental conditions, wear, fatigue, regulatory codes (such as ASME, DOT, or AWS), or the specific service conditions of your job. Always cross-check critical values against the original equipment manufacturer's published specifications and a qualified engineer's review before committing to the job.
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