JasZ

I posted a while back that I ordered my Volk GT-7's for my roadster. Now I've seen some cars with the same wheels that seem to have a very large gap.

I personally chose to get 285/35x19 tires for my rear hoping that the added width would reduce any stretched look and the thicker sidewall would help fill in the gap.

Does the sidewall height really help fill in the gap? I really would like to avoid lowering my car for the time being. I was planning on getting an S-Tune suspension later on anyway, but does anyone know what springs can be used with the stock roadster suspension without causing any problems? I assume they would be linear springs.

Also, I know that my speedometer will be a little off after increasing the overall diameter of my tire/wheel combo so does anyone know if it is possible to have someone reprogram whatever electronics are responsible for that to compensate for the difference? How does this affect the warranty of the car overall?

Sorry to ask so many quesitons about all this stuff, but I am a total NOOB and an engineer and it seems that too many people change things out on their car without thinking it through or having all the information.

I plan on keeping this car forever so I want to be sure that I don't do anything stupid to it.

Thanks for the help.

-JasZ

JasZ

bumperooni

ares

iTrader: (2)

285/35 is pretty close, only about 2mm taller. wouldnt worry about the speedometer, when your doing 60, youll actually be doing like 60.1.

as for gap, larger sidewall reduces gap by half of the increased diameter. the other half lifts the car...

but as I said, your changing it by 2mm, so that means youll have 1mm less gap(than stock), and 1mm taller ride height. no big deal.

but as you can see, you wouldnt want to use this method to get the same effect as, for example, lowering the car 1.5". cause then youd have to increase your diameter by 3" and youd look like your driving a 4x4 with all that clearance.

2003z

iTrader: (7)

I know you said you don't want to lower, but....

We used 245/40-19 and 255/40-19 on our G35c, then lowered it with eibachs, which improved the ride. The tire difference raised the car .35" and the eibachs then lowered it 1" for a very mild .65" drop. There is no wheelwell gap now, and it looks very sharp, yet close to stock height.

sorry, this isn't the best picture, because its a black car.

JasZ

Thanks for all the great feedback. How do you know that half the extra overall diameter raises the car and the other half fills the gap? Is there some way to calculate that? Also, since I am going from 17" to 19" in both the front and the rear, which one determines the effect on the speedometer?

slaponte

iTrader: (2)

jasz, because of the added diameter, half is on the lower part of the tire, which lifts the car up, the other half is in the upper part of the tire, which fills the gap between tire and fender... get it?

Same with sidewall. Every time it grows, half is in the "bottom" lifting the car, half is on top filling the distance to the fender...

So a 285/45 is taller than a 285/35...
a 285/45 is taller than a 265/45...

in either case, 1/2 lift, 1/2 fill...

2003z

iTrader: (7)

http://www.miata.net/garage/tirecalc.html
you can use this link to calculate tire size differences.

JasZ

oh yeah. Sorry. That should have been obvious to me. Thanks for the help.

JasZ

According to that tire calc. my speed will be out of whack by 2.3% since I'm upgrading from 17" wheels. Would you guys recommend correcting for that?