Starting Line Ratio Calculator

What the starting line ratio is

Starting line ratio is first gear times axle ratio: the total gear reduction between the crankshaft and the tires at the moment the car leaves. A Powerglide's 1.76 behind a 4.56 axle is 8.03, a ten-speed's 4.70 first behind a 2.77 axle is 13.02, and the number says how hard the engine's torque is multiplied at the hit and how soon first gear is used up. It is the quickest way to compare two combinations that look nothing alike on paper, which is why racers quote it as a single figure and why the 9 to 11 band Wallace Racing calls desirable gets repeated. That band is a rule of thumb Wallace gives for clutch cars, quoted here as his, not a law: a car that hooks can carry more, a car that spins wants less, and a high-stall converter multiplies torque on its own so a converter car can live with a smaller mechanical number.

Why the tire is part of it

The ratio alone does not say what the tires feel, because a taller tire lengthens every gear. Wallace's tool scales the ratio by the old tire over the new, so the same gears on a 28 inch tire instead of a 31 count as more ratio, exactly as if the axle had been changed, and this page keeps that adjustment. The other way to see the tire is the speed first gear runs out at for a shift rpm, which is engine speed times tire diameter over the ratio times 336. Racers ask a first gear to carry the car to roughly 40 to 55 mph before the shift: a first gear done before 40 mph is too short and a first gear still pulling at 60 is too long, and the rpm at those two speeds in the table reads the same thing from the engine's side.

Gear for the finish line first

Circle D Specialties, who build torque converters, run it the other way round. Pick the axle for the stripe: the speed the car will run, the rpm the engine should be turning there, the tire and the gear ratio at the finish give the axle directly, with converter slip added because a slipping converter lets the engine turn faster than the driveshaft. The gear ratio calculator holds the same arithmetic for every gear. Only then choose the transmission, and if the first gear that comes with it makes the starting line ratio wrong, change the first gear, not the axle: in Circle D's words, compromising the final drive to manage the starting line ratio immediately sacrifices the maximum performance potential of the combination, since the axle then no longer puts the engine at the rpm it wants at the stripe. The finish line card is that method: it gives the axle for the stripe and what your current axle turns there, lists the ring and pinion ratios Strange Engineering sells for your rear end with both ends of each worked out, and for a converter car tables the slip, so the range of axles a real converter allows is visible rather than a single number.

Where the numbers come from

First gear ratios are the published figures for each transmission, including the common aftermarket Powerglide planetaries, and the tool lets you type any other. Ring and pinion ratios are the sets Strange Engineering lists for each rear end, cut by US Gear, Richmond and Motive; other makers cut a few more, and Other takes your own list. The 1.8 to 2.5 range for a car converter's torque ratio at stall is Wikipedia's. The finish line equation is the one Circle D Specialties prints, with 336.136 as the constant; Wallace Racing's version rounds it to 336 and has no slip term. The examples in the explainer are the published ones, and the quarter mile calculator and drag race calculator are where the time slip itself gets predicted and scored.

What this tool will not do

It will not predict a 60-foot or an ET, because the ratio is only one input to those and traction is the rest. It does not turn a stall speed into an equivalent gear ratio, because no published formula does. Typed in, the converter's torque ratio gives the peak multiplication at stall, but how quickly that fades to 1:1 depends on the converter, so the page leaves the fade to the converter builder and quotes Wallace's note that a high-stall car can run a smaller ratio. It also does not account for tire growth at speed, which lowers the rpm at the stripe a little below what the equation says, so the data logger's rpm beats the sum. For the top speed the gearing allows in the last gear, the top speed calculator is the neighbour to use.