Smaller Tire Size?
 

Hey all!

I'm looking at putting run-flat tires on my RX-8, and the only size that fits it is 225/40/18 as opposed to the standard 225/45/18. I'm concerned that this will affect the speedometer and the odometer. Is it possible to recalibrate the speedometer and odometer so that they will not be off? As usual, my Mazda dealer is taking days to get back to me, so I thought one of you might have the answer.

Thanks!

P.S. I know, run-flat tires give a rougher-ride, weigh down the car, etc... But I need the trunk space taken up by the spare, and I'm not going to go on any long trips with that little bottle of glue they give you, because I'd undoubtedly be waiting for 5 hours as Mazda roadside makes their way to some of the remote locations I vacation too.

I got a mushroom tire plug kit for ~40$ in JC Whitney. The repare kit that comes with the RX-8 has an air compressor in it (unless my manual mis-informed me). Now I can patch the tire, fill it up, and not worry about it again.

From what I understand, when a run flat goes flat, you have 50 miles at 50 MPH. I seem to remember the same claims for Mazda's kit. Either way, you will end up getting the tire repared / replaced. With a plug kit, you do the initial work, and you're good to go at whatever speed for however long.

Having never used it, I can't say that it is easy to use, but the concept is pretty easy.

I don't have the spare tire kit, BTW. So, I'm putting major stock in this.

I have 225/40/18 Bridgestone Potenza RE050 runflats. These tires are about 1/2 inch shorter than stock. According to my calculations, the speedo will be off a few mph at hi speeds - no big deal.

The puncture repair kit that comes with the 8 is a cinch to use. I don't know anyone who can vouch for its performance after a long drive though. Trouble is that to put a normal 'bung' type plug in the tire you need to take the bead off and remove all the white gunk in the tire - yuk!

I woudn't recommend going to a smaller tire. If you are that worried, buy a 5th OEM wheel/tire and keep it at home. If you get a flat, have your wife bring it to you.
I keep a tire pump in my trunk, incase anything happens.


Troy J.


Those are nice tires though, they are on the BMW Z4 3.0i Sport package's 18" wheel. Not much more rough than our stock tires.

you can change the aspect and keep the same height. a 255/40/18 might be an option and will only be .06" off. Any reputable dealer should have no problem mounting, and they are well within specs for this rim and wheel well.

.9 inch actually. The speedo will show 60 but you will be doing 58. Not a big deal.

BTW, our Miata friends have this great Tire Size Calculator .