Okay, So I've been rapt with attention to the sage advice of the ol' timers - even heeded some of it.

I've noticed in the discussion about tire size that the consensus seems to be that:
the 240D had a stock tire of 185r14
and the 300D had a stock tire of 190/70r14

If so, wouldn't that mean that the two have entirely different speedometer calibrations??

I'd like to run 195's on my 240D (as per Larry Bible's advice) but if 185 was stock on this beast, then that would put me 5% slow on my speedo - my odometer, naturally,would be off as well. No more easy MPG calculations.

What's the word? Any opines on this topic? Speedo swap??

Thanks all.

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'82 240D, 215,000 +
Classic White, Regenbogen

You can't just swap speedos because the final gear ratios are also different. The stock 240D tires are approximately 185/75-14. If you go to 195/70-14, the speedo error will be about 1%:

http://www.miata.net/garage/tirecalc.html