Speedline Corse Challenge Rims - Tyre sizes

Tire math:

225/40ZR18s go with 265/40ZR18s (355 original) <- and yes I know this is for a 348

(225*0.40*2)/25.4+18 = 25.08 rolling radius 225/25.4 = 8.85"
(265*0.40*2)/25.4+18 = 26.34 rolling raidus 264/25.4 = 10.43"
265/225 = 1.177

(255*0.35*2)/25.4+18 = 25.02 :: 255/25.4 = 10.0"
255*1.177 = 300 -> so your choice is between 295 and 305.
(295*0.35*2)/25.4+18 = 26.13 :: 295/25.4 = 11.6"
(305*0.35*2)/25.4+18 = 26.40 :: 305/25.4 = 12.0"

So;
255/35ZR18s go with 305/35ZR18s
245/40ZR18 are too tall
295/35ZR18 are too short

At this point anyone with access to a calculator can determine the required tire sizes.

There are a number of combinations that work on the rims, however, the tires comming off the car are tall at the front and short at the back. This SHOULD have caused an oversteer issue (improper rake on the chassis) unless the nose was lowered (or rear raised) to deal with the insufficient rake. Since we don't know, and it is likely you don't know, you will want to find this out--and should consider putting the car back at its specified ride height.

Another sublty is that lower aspect ratio tires will want to run slightly less camber front and rear in the ratio of:

(WSM spec camber)*(original footprint)/(new footprint)

So with 2.0 degrees of negative camber in the back, and switchinig from 265/40 tires to 305/35 tires one would want:

2.0*265/295 = 1.8 degrees of negative camber.
0.5*225/255 = 0.44 degrees "

This puts the same amount if inward thrust from the tires onto the chassis with the new tires as the old tires did.

 

please can you post some pics of car with the 295s on the rear

I am running 275s on 360 modena wheels on my 348 and was thinking to go up to 295s on the rear.

 

the best idea would be to send those challenge wheels over to me, and choose some nice new shiny ones for yourself

 

oh brother
that is super HOT
i love 348 spiders

 

thats the hottest car in the universe

 

looks fantastic

 

Challenge wheels are 18X8 F and 18X10 R.

 

The tire size that will maintain your speedometer accuracy is 225/40/18 front and 285/35/18 rear. Giving you respective revolutions of 25.1 and 25.9.

Your stock rear tires meant to work with the speedometer are 255/45/17 with a 26.

That gets you the closest and that's what I have as well.

Kai

 

Love it!

 

Hi Stig,

Long time since you have been over here?

Stick with the sizes you have now, they are the nearest to the original 17" sizes!

Here is a tyre size calculator to help (thanks to Mouser57), saves the head hurting!!

Regards,
Phil.

http://www.trailtiredistributors.com/TireCalculator.aspx

 

Don't let the speedo variance get you down on tire selection...here is an easy cure

http://www.healtech-electronics.com/

Always remember there is a hugh difference between the "how bad it looks bling thing" and setting the car up for performance...bigger is not always better.

 

That's was the part about ::

245/215 = 1.139
versus
295/235 = 1.255
whereas you want:
295/1.139 = 258.9 fronts to go with 295 rears
or
235*1.139 = 267.6 rears to go wth 235 fronts

 

Doesn't the 348 speedo read high from the factory? The slightly taller tires may be dead on.

 

A narrower tire should heat up quicker so if you're doing short stints (Auto-X) or just some spirited street driving that may be an advantage in colder situations. They will also weigh less which will help all other aspects of performance aside from outright grip.


Rob

 

If you get the footprints matched, the oversteer/understeer relationship will be unchanged.

Wider tires will always have a heavier feel at the steering wheel.

Anyway, if you get close to those equations, the tires will be close enough to balance with air pressure.