Usually, the FSM (downloadable from www.mymopar.com or elsewhere) usually has the speedo drive/driven gear combinations for the particular car and its normal (when built) tire sizes.
The way I check speedometer accuracy is by doing a mileage check on the Interstate, using the mile marker posts. I would find a generally flat 10 mile stretch and check the odometer accuracy first. You can use a 5 mile run, but a 10 mile run can make up for any speed difference issues, or at least average them out more. 10 miles makes the math a bit easier, but 5 miles can work too. Steady-state cruise at 60mph or more.
The reason for the distance check is that that relates to the number of revolutions the speedo cable makes, which is a geared situation. The rear speed cup has a bar magnet in it, which turns the front speed cup, which is attached to the speedo needle for speed indication. NO mechanical link between them. So getting the mechanical components correctly calibrated is the FIRST step in speedometer accuracy. Which also relates to the speed indication accuracy.
Pre-GPS, I'd maintain a steady speed over a flat mile or two of Interstate. 60mph works well, if possible, as 60mph = 60 seconds/mile ("Mile a minute"). 5mph either way, it's a pretty linear relationship, but greater than 5 miles, the timing loses that linearity, by observation.
I believe you can also use GPS to measure distance too.
Finding the exact year of FSM might not be important, so long as the model year used reflects the tire size the vehicle came with.
Just some thoughts,
CBODY67
The way I check speedometer accuracy is by doing a mileage check on the Interstate, using the mile marker posts. I would find a generally flat 10 mile stretch and check the odometer accuracy first. You can use a 5 mile run, but a 10 mile run can make up for any speed difference issues, or at least average them out more. 10 miles makes the math a bit easier, but 5 miles can work too. Steady-state cruise at 60mph or more.
The reason for the distance check is that that relates to the number of revolutions the speedo cable makes, which is a geared situation. The rear speed cup has a bar magnet in it, which turns the front speed cup, which is attached to the speedo needle for speed indication. NO mechanical link between them. So getting the mechanical components correctly calibrated is the FIRST step in speedometer accuracy. Which also relates to the speed indication accuracy.
Pre-GPS, I'd maintain a steady speed over a flat mile or two of Interstate. 60mph works well, if possible, as 60mph = 60 seconds/mile ("Mile a minute"). 5mph either way, it's a pretty linear relationship, but greater than 5 miles, the timing loses that linearity, by observation.
I believe you can also use GPS to measure distance too.
Finding the exact year of FSM might not be important, so long as the model year used reflects the tire size the vehicle came with.
Just some thoughts,
CBODY67
