Rear end gear ratio in a 64 LeSabre Sedan


I'm not sure if this belongs here or in the engine swap forum. I'm trying to figure out the rear gear ratio on my 1964 LeSabre Sedan. The car originally had a a Wildcat 310 (2 bbl 300 cid v8) with a three speed manual. I've swapped in a Cadillac 425 with a Turbo 400 from a 1979 DeVille. I believe the DeVille had 2.70 rear gears, but I'm not certain. My speedometer is reading 25% too fast. Example, at 60 mph it's reading 75 mph. I confirmed this with a cellphone with navigator using GPS. Hunting around, I need fewer teeth in the speedometer driven gear, but I need to know what the rear gear ratio is so I know what driven gear to use. The shop manual said there were numbers stamped on the bottom of the differential housing, but I can't find them. There is an adapter that reduces the reading by 26%, but I'd rather use the proper driven gear if possible. Thanks for any insight,
Jack up the rear of the car and spin a tire with both wheels off the ground. Count how many times the driveshaft goes around to one roration of the tire. This will be the rear ratio.

If the driveshaft goes around less then twice to a tire rotation you have an open differential (non-positraction). Multiply the number of driveshaft rotations in a wheel rotation by two to determine the rear ratio of an open differential.
The standard axle ratio for a manual transmission '64 LeSabre was 3.42. A 2.73 ratio was one of three available on the '79 Deville. The difference in driveshaft speeds (assuming the same tire size) is 3.42 / 2.73 = 1.253. So that agrees exaxctly with your driving tests showing the speedometer is running 25% high.

The replacement driven gear in the THM400 will need 25% more teeth than the existing gear to slow down the speedometer cable enough. That is a big change, maybe outside the range of available driven gears.

The adapter gearbox might be the way to go. A lot better than changing the driving gear inside the transmission.

As a footnote, the factory in the early '60s sometimes supplied these external adapters to obtain correct speedometer calibration. The design standard is 1000 rev per mile on the odometer and 1000 rpm = 60 mph on the speedo.