Speedometer calibration for larger rim size.
 

Will upgrading to a 19" rim size require a speedometer calibration? If so, how is it done?

Size matters....
 

The rim by itself has very little to do with the equation, a 19" rim may actually be smaller than your stock 18", depending on the size of the RUBBER you buy.

Usually you would lose some tire sidewall height, (and comfort/protection) and if you stay with the same cross-section as with the 18" your wheels would be HUGE!

Miata tire sizer....

Try the Miata guys calculator - if you pick the right sized rubber, the speedo will be accurate, if you go to a way different profile, and don't want to just do the math in your head, there are little circuitboxes available that you can install on the gearbox to adjust your speedo rate.

S

what kind of rims are you getting blue?

As long as you stay within 3% difference in overall circumference of the wheels, it will be ok. Visit www.1010tires.com and use the tire size calculator.

If you end up with a + or - in the tire revs/mile you'll have the speedo error. I have 235/40R18 and my speedo read ~ 2 to 3 mph fast at 60-70mph. My GPS I have gives me exact speed to check against.

There are little boxes that change the speedo signal up or down I know, can't think of one now... anyone? They will solve the problem if it is wayout of line, otherwise may not be worth the $$ and the bother.

If the 'handheld ECU reflasher' talked about in another thread actually gets made, there is an ECU parmeter that's called "TIRE REVs/MILE' that can be set. That would fix the issue at the core and IMO is best solution & what I'm waiting for.

You know, I would like to know the answer to that myself...haha. There are so many rims out there, it is hard to figure out what is the best. I was, however, directed to look at the Enkei RP03 since it fits my budget.

I don't want to have to spend anymore than $2000 including mounting & balancing.

Any ideas??

Call Tire Rack

go to autornd in milpitas they will find something within that budget, and you can actually see the wheel in person.

Also it depends what you're looking for, lightweight performance or the bling factor.
2k with tires would be some nice enkeis or ever some work wheels.

It's "ok"? What do you mean? I'm looking into some wheels for my X3 and I'm sweating a 1.6% difference (larger). I'm also concerned about the impact this will have on my odometer.

If you go to a 19" rim, you'll want to use a 245/35/19 tire, that will fit and have the same dimentions as the stock setup while giving you the larger rim. You can also go a little less wide (instead of 245) and it will still work.

that tire sizer thing is awesome! lol

Getting the speedometer recalibrated is not doable at this time. Only Mazda engineers at the factory can do that and they aren't offering that service. I tried and tried to find a way to get it done and apparently the speed is calculated by the rotation speed of the rear wheels; it's a constant calculation being done by the computer, there's no mechanical link of any kind.

Any deviation from the standard size will cause an inaccurate odometer; either too many miles will be recorded than what you've actually driven or not enough. Since car value, warranty and service schedules are based on the odometer reading, I decided to stick with the stock 18" rim size and stock tire size. I got the BFGoodrich KDW2's when it came time to replace the stock tires and I'm happy with them.

according to the miata tire sizer thing, my current set up of 275/30/19 as opposed to the stock 225/45/18 is "1.8% too fast, and traveling at 58.9MPH". i'd have to get a 275/35/19 set up to travel at 61.4MPH to break even for another 30k miles.

Here's another tire size calculator...

http://www.bigcustomwheels.com/rt_specs.jsp