... Speedometer accuracy

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How inaccurate would getting new wheels (say 17" SSR competitions) and tires typically make the speedometer? Is there any way to correct the stock one? I'm looking for clean solutions... no ghetto Apexi Rev/Speed meters all over the car to tell me my "true" speed.

[AKADriver]

Get tires in the correct overall diameter, and your speedometer output will be fine.

Stock 240SX tire sizes are 195/60R15 (S13, S14 base), 205/60R15 (S13 w/handling pkg), and 205/55R16 (S14 SE/LE).  All came with the same speedometer gear as far as I know.

These tires have diameters:
195/60R15: 24.21"
205/60R15: 24.69"
205/55R16: 24.88"

So, for a 17" wheel, the following tire sizes would be in the right ballpark:

205/45R17: 24.26"
215/45R17: 24.61"
225/40R17: 24.09" (a little too small)
225/45R17: 24.97" (a little too big)
235/40R17: 24.40"
245/40R17: 24.72"