Posted 2020-03-15 11:06 PM (#595635) Subject: Need help figuring out what I need re: Speedo Gear change
Location: Lower Mainland BC
A few months ago, I completed (with the help of a transmission shop) a conversion from Powerflite to Torqueflite in my 1956 Dodge. Subsequently, I have been trying to determine whether the speedometer is in calibration or not.
Today, I took the car out for a little drive for the first time in 4 weeks (I was a way for 3 and it was raining before that). I drove on a road with an 80 kph (50 mph) limit so I held the speedo at 50 mph (as close as I could). Looking at the tach that I had installed earlier (last year), I think that I read the RPM at 50 mph to be around 1900 (or so)
With a 3.54/1 rear end and 235/75/15 tires, my speed in gears spreadsheet says that 1900 RPM should be 46.1 mph (which is about 8% out from the indicated 50 mph). So *IF* I wanted to correct this, I think that I would have to change the speedometer drive gear in the Torqueflite.
I don't know what gear I have in there but I assume that I need to spin the cable a bit faster to correct the apparent error, so I would need a gear with fewer teeth than what I have (??) Does that make sense?
Now I need to try to relate it to the 235/75/15 tires, which according to the tire size table below George's speedometer table, is pretty much the same as old school 8.20/15 but not necessarily 9.00/14 which is about the same as "J" which is similar to 225/75/15. (??)
Posted 2020-03-16 2:11 AM (#595637 - in reply to #595636) Subject: RE: Need help figuring out what I need re: Speedo Gear change
Location: Lower Mainland BC
Looks like I might want a 1636 408 which has 19 teeth. What is in there now is probably 20 or 21 tooth (??) (for a 1957 Chrysler Windsor - but I don't know the rear axle ratio it had or the tire size)
Posted 2020-03-16 9:23 AM (#595638 - in reply to #595635) Subject: Re: Need help figuring out what I need re: Speedo Gear change
Location: Muskego, WI
Do you have a smart phone? If so, there are several free apps you can download that will display your speed by GPS. They are quite accurate, this is what I've been relying on while driving my 1960 Plymouth. You can use this to confirm your car's speedometer accuracy.
Posted 2020-03-16 1:14 PM (#595646 - in reply to #595635) Subject: RE: Need help figuring out what I need re: Speedo Gear change
The only way to know if you have the correct pinion is by the odometer reading, not the speedo. The speedo needle isn't driven directly like the odometer is and can have wide errors; and torque converter slippage makes engine rpm at road speed inconsistent.
Also, when using tire diameter in calculations, be aware that the diameter of any given tire size varies with the wheel width and maker. To get the actual diameter, measure the rollout of the rear tire(s). To do that, paint a dot on the tread of the tire and, while the paint is still wet, roll the car forward until the tire makes one revolution leaving two paint marks on the ground. Measure the distance between the two paint dots to get the tire circumference and calculate the diameter from that.
For example, the maker of the tires on my '57 says the tires are 28" in diameter, the rollout shows the actual diameter at 26.75".