So I want some new rims for my tC, big ones, like 18-19s. Now, in all logic, the speedometer is calibrated based on having the same diameter of the stock 17s with the stock tires. I wanted bigger wheels without a sidewall that is less than 45% of the width of the contact patch.
So, logically, that would throw the speedometer off in my car, since now the diameter of the wheels is bigger than before.
My question is, does the tC auto calibrate its speedo using sensors, and if not, how does one go about re-calibrating the speedo to match the new wheel size?

 

Good question. I wanted to know what the techs do when they put on the 18's at the dealership. Do they reflash the ECU with the different tire data? Maybe that is a possibility.

 

I should turn this into a poll of how many people never even thought of this, haha

 

I was wondering what the deal was too. I've got plus zero sized tires that are something like 4.5 mm larger in diameter than the stock tires. I don't really care because it's so close to stock, but I've been wondering how hard it is to get this fixed. I'd like to know exactly how fast I'm going, not something close to how fast, you know? Someone has to know how to do this...

 

Give me some numbers to plug and I can tell you.

For example... Going from a 215/450/17 to a 225/35/18, your speedometer will read 1.6% too fast. Meaning when it says you're going 101.6 mph, you're really going 100. That's not too big of a deal to me. If you went to a 225/40/18 instead for ride comfort, your speedometer will read 1.92% too slow. Meaning at when it says 100mph, you're really going 98.08. That's really not a big deal to me. With the money you plan on spending for a reflash/recalibration, buy yourself a nice lil inexpoensive mod and keep on going!

 

Sounds comforting... so where are you getting your calculations? Can you post either a link or the formula you used? Thanks!

 

I have special access to a site I'm a premium member of. So if I try linking it, it'll be dead.

I did a search though...

Here's what I pulled off of a Mazda site...
http://www.miata.net/garage/tirecalc.html

That will help you realize that it's no sweat! Mod away!

 

Perfect, thanks!

 

The guys at tire rack showed me the difference when I went to 225/45-17s. At 65, I'll be going 65.9. The point wasn't that we planned on spending a lot of money fixing this, it's that we wanted to know how hard it was. If it was easy and cheap (or free), it would be good to get done. If there is something flashable in the ECU, it seems like it should be free.

 

I don't think its that easily adjustable because the odometer runs off the same thing yes? If it was that easily you would just program it to think that it was running on 44 inchers and your odo wouldn't roll as many miles.

unless my thinking here is flawed.

 

Possibly, but it's got to be calibratable, or else as soon as the dealer put on 18"s or 19"s, it would be incorrect. Right now my speedo is recording 1.4% too few miles. I know you can use recalibration for bad things, but bad things happen if you can't recalibrate too. I have a feeling that the dealer plugs something into the computer and sends it a new value. Maybe when I'm in to get my oil changed I'll ask.

 

You should do that then post up your findings!

 

it is calibratable... I had a friend who was caught for speeding, went to the dealer, got a recalibration, and used it in court to prove his innocense :D
But yes, the question is, how is easy is it, and if its easy, is it free

 

He did this on a tC all in the time since it came out?

 

I think he meant: Meaning at when it says 98.08mph, you're really going 100.

 

I was thinking about this. If you speedometer is off... your odometer will be off in reporting the correct mileage. So if you have a bigger wheel/tire comibined diameter from stock, your speedometer readling will be less than your actual speed --> you will travel a great distance per unit of time than when your odometer will report. And also in this case, where the tire/wheel combined diameter is greater than stock, your engine will be working a little bit harder since the bigger wheels/tires setup will probably require a little big more energy to rotate them (greater diamter).

Conclusion: if your wheel/tire combined diameter is greater than stock's, your odometer will report less miles (just minor) and your engine worked harder. Thus when you sell you car, the buyer is a little bit at a disadvantage by just reading the odometer. is this right?

 

Yeah lol... I'm tired dammit

 

Yes... it will "have less miles on it" than it should. But from Plus Zero sizing, it's almost insignificant. For plus one or plus two sizing, it's a real difference. As for the "engine working harder" part, I prefer to think of it as changing all the gear ratios a bit. I'm ok with sacrificing just a teeny bit of power off the line to have a slightly lower RPM when cruising on the freeway - again, with Plus Zero sizing, it's almost nothing, but I don't mind it.

 

As long as it's the same height you dont need to change a thing... as long as there is a +10% -10% change in the tire.

Example- 215/45/R17 would have to go to a 225/35/R18... otherwise you have to
change the speed sensor gear ratio.