A Pontiac’s speedometer is as important as any other gauge found within its instrument panel. It was relatively accurate when new, but any deviation from original tire size and/or rear-axle ratio negatively impacts its accuracy. TCI Automotive in Ashland, Mississippi, offers an affordable solution that can help keep your driving record clean of speeding infractions while cruising in your vintage Pontiac. Follow along as we quickly and easily restore the accuracy of our Second-Gen Firebird’s speedometer in an hour.
A speedometer is a mechanical gauge that measures vehicle speed. It uses a drive gear mounted on the transmission output shaft and a driven gear fastened to the transmission case to generate the gear ratio required for the proper rate of needle sweep. Generally speaking, Pontiac used a drive gear containing 16 or 18 teeth in its Turbo 350 and 400 transmissions, while the driven-gear tooth count ranged from 36 to 45 depending upon the application. By adjusting the number of drive and/or driven gear teeth, Pontiac compensated for such factors as optional tire sizes and rear-axle gear ratios on any given model.
Since purchasing our ’72 Trans Am several years ago, its speedometer has always been slightly inaccurate, but we weren’t exactly sure by how much. After verifying that its Turbo 400 was equipped with the correct drive and driven gears for the 3.08 rear axle ratio, we surmised that the inaccuracy was the result of the modern 255/60 radial tires we use in place of the original F60-15 bias-ply units, which measure more than 1 inch different.
TCI Automotive has been delivering hobbyists with high-quality components for automatic transmissions of all makes for decades. It offers a wide array of speedometer gears to restore a vehicle’s speedometer accuracy. Using the Speedometer Gear Reference calculator found on the Tech Support page of its website, we determined the number of driven gear teeth our particular Firebird required. It retailed for less than $12 from TCI and was on our doorstep in a few days. The installation took less than an hour to perform, and the results were exactly as predicted.
Depending upon the speedometer gears that were originally installed in your Pontiac’s Turbo 400 and how far your current tire size and rear axle ratio varies from the original application, your example may simply require a driven gear change like ours, or it may be slightly more involved. No matter the case, rest assured that TCI Automotive can help improve your Pontiac’s speedometer accuracy, quickly, and easily!
