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Weird rpm/speed readings...input?

nosajwp

NAXJA Forum User
I've always thought my 2001 XJ's speedo was a little off. This weekend, I drove by one of those speed trailers cops put out, and it said I was doing 51, and my speedo said 55 dead on.

I also figured I would check my speedo based on rpm's. The numbers are as follows:

speedo reading/expected rpm/actual rpm
40/1680/1750
50/1575/1580
55/1733/1750
60/1890/1920
65/2048/2080
70/2205/2250
75/2363/2375

I got the expected rpm numbers from the calculator on Randy's R&P website, for a 3.55 R&P ratio, 28.4" tire diameter, and 0.75 transmission ratio (1.00 for 40 mph). Do you guys think my speedo is really that far off, or could it be something else?
 
My 2000's semes to be spot on running AW-4, 3.55's, and BF KO 31's. I checked to 100mi on a trip. For a referance the ZJ says 75 when I got 70. Most later model owners seem to report 4/5mph slower than true speed on stock tires.

Oh, BTW lift it, and you'll never seem to find one of those trailers again.:laugh3:
 
The wife's 2000 Classic is off by about the same amount. I think the factory does this intentionally, both to cut down complaints about getting a speeding ticket because the speedo indicated SLOWER than actual speed, and also to allow for the reality that many people upgrade one or two sizes when replacing tires.
 
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