Quote:
Originally Posted by Peter Serio
1001 RPM at 60 MPH yes for the odometer to count over 1 mile. We are not dealing with something calibrated to land on Mars.
None of the passenger car speedometers were ever that close to start with. In fact if you look into any of those old Road & track or Autoweekly magazine road tests you will see that GM delivered all new cars with the speedometer's reading greater than actual. Over the life of the car the magnetic strength looses energy so each year the speedometer would read a little less MPH. Often they were magnetized up to strength to read 10 or 11 MPH greater than actual. During the "life" of the car each year the magnet (actually a piece of grey iron) would loose a tiny bit of it's strength so you can in all of those old road tests where they put a Weston Electric speedometer in the car ( a 10 speed bicycle wheel & tire) clamped to the rear bumper. All GM cars & trucks, their speedometers were calibrated to read higher than actual in the beginning and then over the normal life of the car they settle down bit by bit.
Back then it was expected the car would run for about 10 or 12 years ( driven every day) and in that amount of time the car would accrue at least 100,000 and perhaps more miles.
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Quote:
Originally Posted by Peter Serio
1001 RPM at 60 MPH yes for the odometer to count over 1 mile. We are not dealing with something calibrated to land on Mars.
None of the passenger car speedometers were ever that close to start with. In fact if you look into any of those old Road & track or Autoweekly magazine road tests you will see that GM delivered all new cars with the speedometer's reading greater than actual. Over the life of the car the magnetic strength looses energy so each year the speedometer would read a little less MPH. Often they were magnetized up to strength to read 10 or 11 MPH greater than actual. During the "life" of the car each year the magnet (actually a piece of grey iron) would loose a tiny bit of it's strength so you can in all of those old road tests where they put a Weston Electric speedometer in the car ( a 10 speed bicycle wheel & tire) clamped to the rear bumper. All GM cars & trucks, their speedometers were calibrated to read higher than actual in the beginning and then over the normal life of the car they settle down bit by bit.
Back then it was expected the car would run for about 10 or 12 years ( driven every day) and in that amount of time the car would accrue at least 100,000 and perhaps more miles.
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And look at the (lack) of precision for these GM speedo adjustments: change the trans driven gear by one tooth. The range of most gear teeth choices is around 20 or so, so you are making approx 5% changes when you correct there. No way the math of 1000/1001 comes into play. Tire brand height variations can push you quite a few % one way or the other too.
So if your dash is anywhere _near_ your GPS, celebrate!
I just swapped from 3.55 open to 3.23 posi, did the math and formula said swap existing 22 with a 20 tooth gear. But since my speedo was already reading a bit high with my tires (Coker redline G70’s) I put in a 21 tooth and now it is within approx 1 mph at cruising speeds.
Victory!
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