IRON MAIDEN

I'm about to swap my OEM 17/18 wheel/tire package with an aftermarket 19/20 setup. Will my speedometer/odometer read correctly or will it have to be adjusted in some way?

IRON MAIDEN

Anyone?

Allzwell

Depends on tire sizes ... what sizes are you running?

silverbullit99

have a look at this link.. you put your original size on the left, and the size you want to get on the right and you see how it sizes up http://www.miata.net/garage/tirecalc.html

IRON MAIDEN

Ttt

futuretech

My understanding is that the speedometer is controlled by the trans not the tire size.

IRON MAIDEN

305/30R20 in the rear
245/35R19 in the front

pcbunn

Go to the Miata link previously posted. It will tell you the percentage change in the speedo with any size tire. I have no clue how to compensate for the different tires or if it can be done. But would like to know if possible?

MilleniumVette

If you go with a bigger rim, you would have to go with a correspondingly lower profile tire that. If not, you would have to have your computer re-programmed with your specific combo.

IRON MAIDEN

So a mail order tune could correct this as well?

hollyt

When I moved to 18 9 1/2 front 19 10 1/2 in the back
(I do not remember my tire size at the moment)

ECS (East Coast Supercharging) updated my computer to compensate for the change. It was a 5 minute update. I think it was somewhere in the 3% range on the speedometer. But let's face it 3% is the difference between getting a ticket or not getting a ticket.

Allzwell

The sizes that I am running without effect on the speedo ar
285/30-20
275/30-19

Tire rack also has a calculator that you can use to calculate tire heights

silverbullit99

No, it goes by the tire size 100% sure ,, you can see this in that link I posted. By changing the tire size you see the difference .. http://www.miata.net/garage/tirecalc.html

Vetteman Jack

As long as you put the proper size tires on the car, your speedometer should read accurately. I ran a 19/20 setup on mine:

19x8.5 front with 245/35ZR19 tires and 20x10 rear with 285/30ZR20 tires.

8VETTE7

The rear tire size is all that matters with respect to the speedometer. Your 305/30/20 is about 1/2 inch taller than the stock rear tire size (on a non Z06) so yes both the odeometer and MPH will be incorrect. There are settings in the PCM that can compensate for the different tire sizes so a knowledgeible tuner can make a correction for the tire you want to use.

IRON MAIDEN

So with the taller tire, am I right to think that it will read slower than accurate?

Allzwell

Yes, fewer RPM's

8VETTE7

Stated another way, when the speedo reads 70mph you are actually going faster than 70mph.