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If you'll pardon a suggestion, and with all due respect, I think that you're obsessing too much on your measurements. Yes, it's true that there is some variation in sizes among different makers, but is it important? The answer is almost always no. Permit me to explain....
First, I'm really confident that your speedometer isn't accurate (it never really is), even with the original tires that are long gone. And with the tires you have on it now, do you have a GPS to verify speed and distance? Or have you carefully timed your speed against highway 1/4-mile stripes? I think you'll be surprised at the results.
That said, how much accuracy do you really require (to, e.g., avoid a speeding ticket :-)? A 2% inaccuracy is only 1 mph at 50 mph, and can you read your 240's speedometer to one-fifth of a division (assuming your speedo has 5mph divisions)?
Moreover, that same 2% variation (i.e., plus or minus 2%) allows you to have a tire diameter of between 602 and 627 mm, or 23.7 and 24.7 inches, given that the OEM tire was a nominal 614.6 mm. That's quite a substantial range, one full inch, to be within 1 mph accuracy at 50 mph.
And of course, if your speed is over 75 mph, this 2% error rounds up to a +/- 2 mph deviation, but if you're driving that fast, I doubt you should be studying the speedometer needle that closely to discern that level of accuracy.
What do you think?
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