I'm trying to figure out how much wrong my speedometer is showing.
I know there are other ways around this, but I like the mathematical approach :)
So, what was the dimensions (circumference is all I need) for the original
-75 el Camino SS ?

Thanks / hillbilly

27" They had some tall ass skinny tires origonally. Like 215R75x15 tires

I think the 75 SS came with GR70-15 tires. They are the same as 225/70/15 which has a height of 27.4"

:mrgreen:

The mathematical approach is a real challenge because the speedometer reading is directly related to the rolling radius of the tire on the car. The varying diameters listed don't help because the rolling radius is smaller than half of the diameter. You can't get a rolling radius from the manufacturers any more, but can calculate it from the revs per mile stat that they list in their tire specs. The best way to do this, even though it costs a couple of bucks is to go to a speedo shop and run it on their machine. Next best is to run the mileposts on the Interstate, but you'll need a friend to tell you exactly when you pass the posts as you look at the odometer. Check for either 5 or 10 miles and do at least 6 or 8 checks and then average the results. You'll be amazed at how much they vary. The posts are not exactly a 5280 feet apart and at over 100 (70 mph) feet per second, you can easily be off 5+% on any given check. I've spent a ton of time playing with this because of changing transmissions and gearing and trying to reconcile the tach readings to the speedo readings because I'm too cheap to pay the speedo shop, but I've finally decided that until it drives me crazy, I'll live with a speedo that is (I think) about 4% slow. But, if you're determined to do it mathematically, I'd get the revs per mile number on several 225/70/15 brands from my local tire guy, average them, divide the average into 5280 and you'll have the "circumference" in feet. Do your calculation from there. You should be within 3-4%. Good luck. Sorry to be so windy.