You'll need to use something like the miata garage tire calculator if you have changed tire size to see what percentage your speedo is off and then you can set the ScanGauge to take that percentage into it's calculations. I use MPH display of the SG on the dash of the '55 since I've gone to a taller tire. Keep the air pressure at max in the tires and drive conservatively but it is what it is and if you need MPG, this the WRONG vehicle.
BTW, stock tires are almost identical height/diameter on the 16" and 18". Come on guys, don't post INCORRECT information, it just confuses the newbies. Now the 18" rims could be heavier but I would not expect more than a few pounds - anybody have those specs? And if so, that would be more of an onroad handling issue when pushed.
1999 245/70/16 Diameter 29.5", 6.8" sidewall height (comfortable)
2000/01 245/60/18 Dia. 29.6", 5.8" sidewall height (less flex)
And yes, the wheels are interchangeable between ALL years.
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