I'm running 275/35's on the 18x9 MB Motorsports Weapons with a 45 offset.

 

It's a bit off topic, but what about 235/40 for the stock wheel?

 

That would work fine. Not as wide as a 245x40, but a bit better sidewall support on the stock 18x8 rim.

 

In my opinion, LionZoo knows what he's talking about and better still, knows why.

Why do I say that? I've researched this topic too and also bought and used at HPDE the different approaches mentioned. I've then taken HPDE track data and compared performance and formed my own opinions. My emperical results are telling (and my basement is full of wheels and tires - 4 sets wheels and 5 sets tires and counting!!)

After wearing out the stock 8" w/225s, I tried 275s and (now) 245s on lightweight RPF1 9.5" wheels. I will never get 275s again. Why? On track you'll accelerate slower and your top end will be lower, and worse I saw no appreciable cornering improvement...did I mention the expense? On a car with too much HP it would be fine, on our low power car the tires drag the car down.

Beyond that the 245s are braced VERY well on 9.5" width, maybe hurting the ride a bit, but giving big dividends in crispness and handling precision. The 275s were not un- stable by any means, but the wheel is really overtired for best performance in our cars as described above.


Further, as time progressed I've finally settled on 17" x 9" rims (45mm offset) and have tried two tire widths: 245mm and 255mm on them. There is actually not that much discernable difference in performance, but for HPDE use I've settled on the 255 as my favorite. Again the 255 is the max I would go without gaining the offsetting downsides of being overtired on our car and under braced on a 9" wheel.

Botton line, I believe the 9" wheel w/245 or 255 width tire to be optimum from all perspectives...suitably lightweight, best performance per $$, fits the optimal size tires best for max HPDE use performance....not to big..not to small...just right

PS Oh and as to weight, don't go the heavy wheel route to save a few $$ or for cosmetic reasons. You'll regret it bigtime. My 17" x9 are 15 lbs and cost $249 each, 20lbs would be the absolute max I'd go on any 9" wheel.

 

i have 245/40s right now, which is tad bit shorter than the stock 225/45s, but i wouldn't want anything taller! a couple mentioned 255/40...that seems awfully tall to me!! i would 255/35 would be better - i haven't worked out the math or researched it though...

 

spin9k what rims do you have?

 

Another question for Spin9K: What make/model of tires did you use? When you say the 275s did not gain any appreciable cornering speed over the 245s, were you comparing the same make/model of tire, just in a different size?

 

stock 18"x8" ......... ~23lbs
RPF1s 18"x9.5".......~18lbs
Hot Wheels 17"x9"..~20lbs
RPF1 17"x9"...........~15lbs

all ~-45mm offset

 

First, I had 235/40/18s RT-615s on the stock wheels then 'upgraded' to RPF1 275/25/18s RT-615s. My feeling (and track data backed it up) was that the car was slower and had lower top end on track. Places like at the end of a straight where I used to go ~112mph I was only going ~107mph, things like that. Cornering really was about the same plus or minus. Talking to people I got some different possibles like...

1) wider tire has more frontal wind drag
2) wider contact patch has more rolling friction
3) even thou tire/wheel was couple lbs lighter, more tread mass was concentrated further out so using more HP to get up to speed

....probably a combination of all these.... so I went in another direction...

Second, I had 245/45/17s Nitto NT-01s on Discount Tire's house "Hot Wheel" 17x9 @20lbs. I liked this so combo much I decided to go lightweight & upgrade to RPF1s 17x9 @ 15lbs and went with 255/40/17s NT-01s to get a bit lower profile and slightly lower gear ratio equivalence.

With that experience I can say that on track the car as I have it setup (which is fairly tightly sprung, nominally lowered, w/stiff set sway bars) has a sweet spot with tires that are somewhat wider then stock, e.g., 245/255, but not too large like 275s. It seems to accelerate better, handle extremely well and is tossable and predictable. I can therefore trust the car explicitly to do what I want, and therefore push it near its limits regularly.

 

Great to see some real world data Spin9k. So it would seem on 18x9, as OP requested, the 245's are the way to go.

Just brings me back to the re-realization that the Mazda engineers knew what they were doing with 225x45 on the stock 18x8 rims. Go figger.

 

Thanks, Spin, that's some good information you have there. I do know that autocrossers like as much tire as they can get under the car, but they are not terribly concerned with top speed. Sounds like for track days, 275s are best left for folks with forced induction.