here is another example. taller side wall means a slower reading. shorter side wall means faster reading...

My OEM goodyears are getting a little worn. I happen to have some 195/60/R15 Yokie ES100s on hand. I would be replacing the stock 185/60/R15s.

In relation to the new size, the tire calculator says:

"Speedo reading is 2% to slow. when speedo reads 60 mph actually traveling 61.2 mph

 

copied from miata.net tire calculator.

oem tire 245/40/19
new tire 275/30/19

Speedometer reading with non stock tire is 4.6% too fast
When your speedo reads 60mph, you are actually traveling 57.3mph

I didnt even bother doing the front since the rear clearly show my point