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Driveline Tech Tips

SELECTING THE BEST GEAR RATIO

One of the primary reasons why the 9" Ford rear end has come to dominate drag racing is the ease with which thirdmembers with various ring & pinion gear ratios can be installed. The aftermarket has responded to this by offering a plethora of ratios ranging from 2.91 to 6.50:1. At last glance there are nearly 40 ratios available in standard 9" size, with many of the more popular ratios offered with a 9-1/2" ring gear, which can be accommodated in a 9" housing.


What is the optimum gear ratio for your application? To help you determine this Mark Williams Enterprises offers a handy plastic gear ratio calculator. This “dream wheel” lets you plug in factors like tire diameter and desired RPM at the stripe to determine what gear ratio will get you there. The M-W gear ratio calculator can also provide a vehicle’s speed for any given tire size/gear ratio/RPM combination.


You can purchase one (p/n CAL) online for $3.00, or for $5 Mark Williams Enterprises will send you the calculator plus a printed copy of the firm’s 92-page catalog. Remit to 765 S. Pierce Ave., Louisville, CO 80027.

Although gear ratios will vary with each application and the altitude of the race track, the key to optimizing the ring and pinion ratio is knowing what your engine is capable of. Probably the most important of the three variables required to calculate the gear ratio is engine RPM. Ideally you want your engine RPM at the finish line to be somewhere close to the point of peak power. If the RPM is too low, power is wasted. If the RPM is too high and goes past peak power, acceleration will drop off so knowing your peak horsepower and the RPM where it happens is critical. With this info in hand along with the tire size being used and the speed you are looking to run, MW can pinpoint the gear ratio.


As the rear end gear is a multiplier of torque relative to what’s delivered to the rear wheels, some interesting calculations can come into play. Let’s take a hypothetical engine with 500 ft. lbs. torque. At the hit, you’ve got a transmission gear as a multiplier and also the ring & pinion. For example, a 1.82 low gear Powerglide mated to a 4:11 rear end would multiply the base torque to 3,740 ft. lbs. Change the rear end gears to a 4.29 and the torque ramps up to3,903 ft. lbs. Will the track hold it? Can it take more?


When you’re in high gear (assuming it is 1:1) approaching the lights the lower gear will obviously be eating up more RPM. And how does that relate to your engine’s power curve? There are many important questions that can be answered by the M-W calculator.


Mark Williams Enterprises endeavors to make things easier for racers by offering compete ready-to-use thirdmembers that have been set up by experienced technicians. There are a dozen or so different combinations of carriers and spools (or Posi units). Multiply that by the available gear ratios and there’s quite a selection.
As a final touch, M-W includes a specially formulated gear lube with each Thirdmember. This is made to Mark Williams’ specs by Torco (the highly respected lubricant manufacturer that’s been in business since 1950) and has proven to work great under the most rigorous racing conditions.


You can up your chance of wining by having the ability to change gear ratios. Take it to the bank!