Gear ratio, rpm & speedometer calculator
Cruise rpm for your axle ratio and tires, and what happens when you change tire size: how far off your speedometer will read and what ratio gets your performance back.
Cruise rpm in top gear
- Tire diameter
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- Tire revolutions per mile
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- Rpm in a 1.00 gear
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- New tire diameter
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- Speedometer reads 60, you are going
- –
- New cruise rpm
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- Effective axle ratio
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- Regear to restore stock feel
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| mph | Top gear | 1.00 gear |
|---|
The rpm formula
336 combines the unit conversions: miles to inches, hours to minutes, and diameter to circumference. Metric tire sizes are converted to diameter as (width × aspect ratio ÷ 100 × 2 ÷ 25.4) + wheel diameter.
What bigger tires do
A taller tire covers more ground per turn, so the engine turns slower at the same speed. That acts like a numerically lower axle ratio: less acceleration and towing pull, and a speedometer that reads slow. The effective axle ratio is your ratio × old diameter ÷ new diameter.
To get the old feel back, regear to your ratio × new diameter ÷ old diameter. That exact number rarely matches a gear you can buy, so the calculator shows the common ratios on either side. Going to the higher one restores more acceleration and towing; the lower one keeps cruise rpm down.
Measured tire heights often run a little shorter than the size on the sidewall, especially on a loaded tire. For the most accurate rpm and speedometer numbers, measure your tire's actual height and use the diameter box.
Standard formula, as explained in Restyling & Customizing News, "Calculating mph/rpm/Gear Ratio/Tire Size".
Frequently asked questions
Why does my speedometer read wrong after a tire size change?
Speedometers are calibrated to a specific tire diameter, and a bigger tire covers more distance per revolution than the speedometer expects — so the speedometer under-reads your actual speed, and a smaller tire has it over-read.
What rpm should I aim for at highway cruise?
There's no universal number, but many builders target somewhere in the engine's comfortable, efficient range (often referenced against the torque peak) rather than either lugging at very low rpm or running needlessly high — this calculator shows you exactly what a given ratio and tire combination produces so you can judge it against your engine.
Does changing the axle ratio fix the speedometer error from new tires?
It can offset it, since a different axle ratio changes engine rpm at a given road speed in the opposite direction of a tire size change — but a mismatched speedometer gear or a recalibration may still be needed depending on how far off the speedometer itself reads.
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