With Google maps adding speedometer to it's directions I noticed my 99 being 2 to 3 mph behind.

I wonder how accurate that would be.

My 99 says 70 but Google maps says 67

 

That's very common. 5% deviation (or was it 3% ?)is acceptable and manufacturers usually set up speedometers to show a little more rather than a little less to avoid getting blamed for speeding tickets.

 

My 99 is 3 mph fast at that speed, too.

 

What size tire? On ours with a 245/40-18 the rounded amount is about 3MPH high and increases fractionally more at higher revs. Google as well as the stand alone GPS should be pretty accurate.

 

Need to double check but pretty sure it's 225/60/16

It's funny at 60 it's 2mph high and 3mph at 75 lol

 

You should have a 245/45/18 for the most accurate overall diameter to stock. I've got IS350 wheels with that size tire and I believe I'm only 0.2% larger than stock. Your size is 3.4% smaller than stock which would explain more revolutions per mile.

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

 

Yes. The 245/45 was the correct choice for size matching. I desired a slightly smaller diameter for a tire combo that wasn't available in a 45 series and kept with 40 series then on.