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Gear Selection Guide: Choosing a Rear Gear by Trap RPM

Choosing a rear gear by trap RPM, tire growth and converter behavior.

Rear gear selection looks like a parts choice. It is really a question: at what RPM do you want the engine when the car crosses the finish line? Answer that first, and the ratio mostly picks itself. Answer it wrong and no amount of tuning will give the RPM back.

Start at the stripe, not the starting line

A drag engine makes its best power in a band, and it should be at or just past its horsepower peak as it crosses the stripe in high gear. If the car crosses at 5,800 RPM but the engine peaks at 6,800, you left power unused on every run — the car spent the whole pass below where the engine works best. If it crosses at 7,500 on an engine that peaked at 6,500 and is laying over, you have geared it into wheezing.

So the workflow runs backwards:

1. Know your engine's useful top end — where power peaks and how far past peak it still pulls acceptably. A chassis dyno sheet helps; honest timeslip history helps more.
2. Know your trap MPH, or a realistic target for it.
3. Know your rear tire's true diameter at speed (see growth, below).
4. Use the relationship between MPH, tire size, gear ratio, and RPM to solve for the ratio that puts you at the target RPM at the stripe. A gear-ratio or gear-speed calculator does this arithmetic instantly; run several ratios and look at the whole table, not just top gear.

Do not forget transmission ratios: the rear gear affects every gear, and a ratio perfect at the stripe may make first gear uncontrollable off the line. Gear selection is a system decision.

Tire growth changes your diameter

A bias-ply slick grows. Centrifugal force stretches it as speed rises, and a typical slick can gain roughly an inch or more in diameter — several percent in circumference — by the top end. That growth is effectively a gear change happening during the run: the tire gets taller, so the car goes faster per engine revolution than the static numbers say.

Radials grow very little. That is one reason a ratio that worked on bias slicks can feel too steep on drag radials of the same marked size.

Practical consequences: use the manufacturer's diameter for your actual tire — a change from a 28-inch to a 30-inch tire with the same ratio is like removing several percent of gear, so recheck trap RPM after any tire-size change. Use tire pressure for traction, not as a gearing tool.

The converter is part of the gearing

In an automatic car, the engine and the rear wheels are not locked together (unless you have a lockup converter that is locked). The converter slips — by design at the hit, and by a few percent even at the top end.

That means your trap RPM will be higher than the clean math predicts. If a calculator says 6,400 RPM at your trap speed and your tach and data say 6,800, the difference is largely converter slip, commonly in the 4 to 8 percent range for a race converter at full load, sometimes more. Our converter slip calculator will give you the percentage from your own numbers.

When selecting a gear, leave room for your real slip. If you gear for a theoretical redline-crossing and your loose converter adds 6 percent, you will be over it. Conversely, tightening a converter later — a common winter change — will drop trap RPM and may want a little more gear to compensate. Gear and converter are a matched pair; change one and recheck the other.

Clutch cars have no slip at the stripe, so the clean math holds — but clutch hit and first-gear ratio do the converter's job at the launch.

The trade-offs every ratio forces

More gear (numerically higher — 4.56 vs 4.10):

• Stronger launch multiplication; the engine reaches its powerband sooner.
• Better ET potential for a car that was under-geared, particularly in the front half.
• Higher trap RPM — the risk above.
• More RPM everywhere: more wear, more heat, and on a street-driven car, worse cruising manners and mileage.
• Can overwhelm traction. If the car already spins, more gear usually makes it worse.

Less gear (numerically lower):

• Calmer launch, sometimes the cure for a car that will not hook on a marginal track.
• Lower trap RPM — power left unused if taken too far.
• Often more MPH per ET potential in very powerful cars that are traction-limited early; this is why some very fast cars run surprisingly tall gears.

Eighth-mile cars need a different answer: they only reach eighth-mile trap speed, so they can use considerably more gear and cross their shorter stripe at the same target RPM. Gearing a quarter-mile car like an eighth-mile car is a classic mistake — check which stripe you are racing to.

A worked approach

Say your engine peaks around 6,800 RPM, you trap about 125 MPH in the quarter, you run a 29-inch tire, and your converter slips about 6 percent at the top end.

Work the gear-speed math for candidate ratios, then allow for slip: you want the calculated RPM a little below 6,800 so the real, slip-included RPM lands near your target. Run the table for, say, 4.10, 4.30, and 4.56. One of them will put calculated RPM in the low-to-mid 6,000s at 125 MPH, which with your slip lands you near peak at the stripe. That is your candidate — and you then sanity-check it against the launch: does first gear times rear gear give you a starting-line ratio your tires can hold?

Change one thing, verify on the slip

Gearing changes are not cheap or quick, so make them deliberately: record trap RPM, trap MPH, sixty-foot, and eighth-mile numbers before and after, in similar air. The right gear shows itself as a car that leaves well, crosses at the intended RPM, and gains MPH for the power it makes. If trap RPM is right and the car still underperforms, the gear was never the problem — and now you know that for certain, which is worth something too.