Drag racing gear selector
The axle ratio that puts your engine where you want it as you cross the finish line. Use a real trap speed if you have one, or estimate it from horsepower and weight.
Ideal axle ratio
- Trap speed used
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- Closest common ratios
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- Estimated ET
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How the gear is chosen
This is the same rpm formula as the gear ratio calculator, solved for the axle ratio. Most racers aim to cross the finish slightly past the engine's peak horsepower rpm, so it's pulling hard all the way through the traps.
Estimating trap speed
This is Roger Huntington's classic estimate. For the eighth mile, trap speed is roughly the quarter-mile figure ÷ 1.26 and ET roughly ÷ 1.57, both common rules of thumb.
Frequently asked questions
Should I use a real trap speed or an estimated one?
Use a real trap speed from an actual run whenever you have one — it reflects your car's true weight, power, and traction on that day, while an estimate built from horsepower and weight alone can miss real-world losses that a timeslip already accounts for.
Why does tire diameter matter so much for gear selection?
A taller tire covers more ground per revolution, which changes the effective gearing at the wheels — the same axle ratio behaves differently on a taller tire, so tire diameter has to be part of the calculation, not an afterthought.
What if the calculated ratio doesn't match a ratio my axle actually comes in?
Round to the nearest commercially available ratio for your axle and rerun the numbers with that real ratio — the calculation gives you the ideal target, but axles only come in fixed increments, so a little compromise is normal.
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