Speed Warning

I have a Borg Warner Super T-10 four-speed out of a 1979 Firebird and need to change the speedometer drive gear to accommodate a larger driven gear. I have a 4.10 gear in the rear. I am using the gray 22-tooth driven gear. The internal gear is green and I believe it to be the 8-tooth large. Is there a 8-tooth small to fit the 32-spline output shaft? I have the 24-tooth yellow that is called for in the speedometer calculators, but cannot use it with the large-diameter drive gear.

Wayne Scott
Port Colborne, Ontario, Canada

Rocky Rotella responds: For street use, I consider a speedometer as important as any other instrument within a vehicle's dash panel. What good is an inaccurate fuel gauge when you venture out on a leisurely trip away from home or an inaccurate coolant-temperature gauge should your engine boil over? An inaccurate speedometer falls into this same category. It not only incorrectly displays the speed at which you travel, making you susceptible to speeding tickets or becoming a possible traffic hazard, but it also alters the rate in which your odometer racks up miles.

It sounds as if you're experiencing the frustration most hobbyists face when swapping transmissions, rear-axle gears, and even rear tires. Pontiac achieved a specific speedometer drive ratio using specific drive- and driven-gear combinations in the transmission's tailshaft. Changing a single driveline component can alter that ratio and leave you wondering just how accurate your speedometer really is!

As you noted, simply changing the driven gear—which resides in a housing that's inserted into the side of the transmission—to one with greater or lesser teeth can improve accuracy in many instances. However, in certain cases, it also requires replacing the drive gear with a unit containing a different tooth count. Since the drive gear is pressed or clipped onto the transmission's output shaft, replacing it can require some transmission disassembly, and that's not always convenient.

Transmission speedometer gear calculators readily available on the Internet can tell you exactly how many teeth are needed on the drive and driven gears to attain proper speedometer output, but actually finding the gears today can be challenging. What complicates this seemingly simple swap is that while most General Motors speedometer gears appear identical, the actual gear diameters can vary with transmission application. In other words, automatic and manual transmission gears can be different, and not all Muncie speedometer gears will fit Borg-Warner transmissions. So while they look alike, sometimes they do not fit exactly as intended, and I believe that's exactly what you're seeing with your Super T-10.

I spoke with Brian Higgins in the Manual Transmission Shop at S-K Speed in Lindenhurst, New York (www.skspeed.com), and he confirmed that the large and small drive gears you referenced in your question are common in Muncie applications, but that 32-spline Borg-Warner transmissions used a single-sized drive gear. Brian has an assortment of Super T-10 drive and driven gears available and can help you acquire a correct set to get your speedometer performing accurately at a reasonable cost. He's available at (631) 957-9427, and when you call, be sure to have the correct diameter of your rear tires handy for easy calculation.

It's also worth noting that Pontiac sometimes used a mechanical transducer positioned between the transmission and speedometer. It contained an additional set of internal gears which further altered the ratio to attain an accurate speedometer reading when using common drive- and driven-gear combinations in the transmission. While I've had good results simply finding the correct drive and driven gear combos for my applications, there are sources that sell new speedometer ratio adapters for about $70 that can adjust the output by a fixed percentage based on how far your speedometer is off. I have no experience with them, but a simple search on the web for a speedometer ratio adapter should provide a number of potential sellers.

With the compression ratio the No. 46 heads provide on your 400, you should have no problem tolerating a grind with 220-225 degrees of 0.050-inch intake duration and 10 additional degrees of exhaust duration.


Email questions to christopher.phillip@sorc.com