I need one of you "young" engineers whose brain still functions to answer this. Ken and I have had a dialog going and are in agreement with the problem but opposite in the solution. I was using the tire rack calculator and the Miata Tire calculator.
The problem: If your speedo reads 60 mph
when you are actually only doing say 55 mph
how do you correct it, taller tires or shorter tires.
My hypothisis, A taller tire/wheel combo will travel more distance per revolution, so if your odometer is calibrated to count revolutions to convert to miles then the only way to get the correction to get the reading closer (increasing reading on odo) is to put a smaller tire/wheel combo on so that you will record more revolutions.
This is where it's confusing. Maybe I'm making this more confusing than it is but you have to think "backwards" on this to get to the solution and if I'm thinking right on this (and I very well may not be) then below is my conclusion.
The calculator shows if you use a shorter tire (P225/50R16) it would show 57 MPH
(5% LOWER) compared to your stock P215/60R16 tires that were reading 60 mph
with the P215/60R16. However if you car by "GPS" or radar is actually only going 57 mph
when your odo is reading 60 with the stock tires then you need to "make up" the extra. The Miata calculator says the non-stock tire reading is 5% too high assuming the actual reading is correct. Thus what you really what to do is put a tire / wheel combo on that will make up the error and actually read too high compared to the stock tire and the current odo reading. Therefore, as complicated as it is to my feeble mind it takes a shorter tire combo to correct the error.
If that is true then I can see why Toyota didn't correct the problem with tires because few people will want a TCH with 50 series wide tires. They should have corrected it with calibration of the spedo itself.
I have to admit when I first looked at this I thought it took taller tires to correct it but what you really need is a tire combo that reads faster than the current tires
if you want your actual 57 at a spedo reading of 60 to actually be corrected to 60.
Help me so I can sleep tonight (I will admit that the right side of my brain says put on taller tires so that the odo reads slower)
Hey, I'm an old engineer that has had chemo treatments and my mind just doesn't do logic like it used to.