Speedometer Reading Correction

by George Lyle


> A while ago there was a thread about speedometers and wheel sizes.
> question 1, what is the correction factor that I should apply with
> 135*15 front wheels?

One could calculate a correction factor, but it probably wouldn't be correct. Like all instruments, speedometers tend to lose accuracy with age. They generally start out reading a bit more than the actual speed (no manufacturer wants to be sued for speeding tickets) and _usually_ tend to read even higher with age.

A good speedo shop can recalibrate your speedometer for your "way kewl dood" tire size, but some find it more fun to do the corrections in their heads and impress their passengers.

Get a stopwatch and find a long straight road with mile markers (most states mark each road every mile either with clear mile markers or with some sort of cryptic monument. Get the bus up to an indicated 60MPH and measure the time it takes to go one mile (by the mile markers) with the stop watch. It will probably take a bit more than a minute. For every second past 60, subtract 1MPH from your indicated speed. If it took you 64 seconds to go a mile, you were going 60-(64-60)=56MPH. (Yes, I know this isn't exact, but it works fine for minor corrections).

From this you can figure corrections for other speeds. Since you're off by 4 MPH at 60, you can figure on 30MPH being actually 28, 15 being actually 14, and, of course, 90MPH being actually 84MPH :

> Question 2, what is the RPM/mph that I amdoing with 165*15's on the
> back and a standard 63 transmission?

Two ways to figure this out. You can figure the RPM of the wheels using their rolling circumference and the road speed, then multiply by the reduction box ratio, multiply by the ring and pinion ratio, multiply by the 4th gear ratio, and you have your engine RPM.

The other way is to hook up a tune-up tachometer to the engine, run the wire out through the engine lid and in through the hatch, drive 60MPH and have someone read the RPM.



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