Originally Posted by c4cruiser
But according to that page, the 4+3 trans and 3.07 ratio got the same speedo gear (Green) regardless of the wheel/tire size.
And the table shows 16" wheels listed up through 1990
The GM engineering formula for odometer/speedometer calibration and the catalog applications through the years have had some "conflicts" - that's the first time that particular one has been mentioned that I've noticed. I believe I've got the formula (or so I've been told). It's likely that in the GM engineering formula when using the 3.07 ratio there was a marginal difference so one tire size or the other was rounded up or down. There's no way of confirming without "rolling radius" or "rolling circumference". The side wall support/stiffness may have effected those dimensions.
The "chart/table" is a compilation of the individual and there's no GM validation.
Originally Posted by Keystring
Here is a calculator to help you determine tire/wheel size in relation to speed..
Just put in the old size on the left, and the new size on the right.. speed difference is below.
That calculator and "MOST" used advertised RPM & diameters. Neither of those are used when calibrating/calculating speedometer/odometer.
Speedometer/odometer is done using a "rolling radius" - or a "rolling circumference".
OP - Your only concern is the tire dimensions. If you're using a "factory spec" tire size then you should be able to rely on the information in GM Charts. They need to be confirmed by you. If it's a "non-spec" tire size then to accurately do the math you need "rolling radius" or the "rolling circumference"!
rolling radius - center of rear axle to ground dimension
rolling circumference - chalk tire at 6 o'clock and mark floor "roll" 1 revolution back to 6 o'clock mark floor and measure.