I posted this sometime ago it seems to be close enough for some and way too much for others...

Stock tire on the 914-4 seems to have been 165-82-15.

Assuming this is correct then the stock tire was as follows:

25.65” diameter
80.541” circumference
801.37 revolutions / mile

Because variances in tire pressure, tread height, diameter from different tire manufacturers and especially when the actual accuracy of the speedometer is questioned – this is not an exact science.

However I have extrapolated the following information from http://www.miata.net/garage/tirecalc.html

Tire Size (TS)
Diameter (D)
Speedometer Reading % ERROR (%E)
Actual speed when Speedometer reads 60 mph (AS)


(TS) (D) (%E) (AS)
165-82-15 25.65” 0 % 60 mph
165-60-15 22.8” 12.3 % fast 52.6 mph
165-65-15 23.4” 9.8 % fast 54.1 mph
185-60-15 23.7” 8.7 % fast 54.8 mph
185-65-15 24.5” 5.9 % fast 56.5 mph
195-50-15 22.7” 9.9 % fast 54.1 mph
195-60-15 24.21” 6.9 % fast 55.9 mph
195-65-15 24.98” 3.9 % fast 57.6 mph
205-60-15 24.69” 5.1 % fast 57 mph
205-65-15 25.49” 2 % fast 58.8 mph
205-50-15 23.1” 11.3 % fast 53.2 mph

Cheers,

Rob

Remember that the speedometers in most of these cars were pretty inaccurate when they started. I wouldn't get too hung up over a couple of percent, frankly...

--DD

I was told by a BMW mechanic that it is mandated buy the TUV (German DMV) that speedos read a little high so as to err on the side of caution.

I've heard the same thing about car manufacturers.

Just out of curiosity, I tested the odometers on a couple of my cars, including my RX-7, that read tenths of a mile. On the 5-mile-long odometer test sections of Washington state, the odometers of both cars read 4% high. So logically, it seems that the speedometers also read 4% high.

I wonder why the car manufacturers do that.