Noize

iTrader: (24)

I currently am running 265/35/18 Michelin Pilot Super Sports on a 9.5" wide wheels that have a +20 offset. With -2.0 degrees camber front and -1.2 degree camber rear, zero toe all around, I didn't have to roll my fenders, and don't have contact in the rear, ever.

While the fitment and width on the 265/35/18 is fantastic, it makes for a tire that is shorter than stock, and thus speedometer reads faster than the car actually is going.

For fun, let's look at a drag racing application with the stock 7600rpm rev limiter in 4th gear:
Stock tire- 245/40/18: 113.8mph
Popular tire- 265/35/18: 112mph
Considered tire #1- 265/40/18: 116.4mph
Considered tire #2- 255/40/18: 115.1mph

7780rpm rev limiter in 4th gear:
Stock tire- 245/40/18: 116.5mph
Popular tire- 265/35/18: 114.6mph
Considered tire #1- 265/40/18: 119.2mph
Considered tire #2- 255/40/18: 117.8mph

For practicality, let's look at the RPM at the typical interstate speed of 70mph:
Stock tire 245/40/18: 3250rpm
Popular tire 265/35/18: 3300rpm
Considered tire #1: 265/40/18: 3175rpm
Considered tire #2: 255/40/18: 3210rpm

While the fuel economy improvement and decreased odometer mileage from 265/40/18 would be pretty insignificant, the 4.5 extra mph potential in the quarter mile at the top of fourth gear over 265/35/18 is substantial.

Comparing these tires on tirerack.com, we find the following (265-35/265-40):
Tire weight: 26 lbs. / 27 lbs.
Section width: 10.7" / 10.7"
Tread width: 10.1" / 10.2"
Overall diameter: 25.3" / 26.3"
Revs per mile: 822 / 790

With the same section width, I can't see any fender rubbing.
The 265/40 are overall 0.6 inch taller than stock; the 265/30 are 0.4" shorter, making for an inch difference between the two. Half that inch will reduce wheel well gap, while the other half will increase total car height.

There are other tradeoffs such as more compliant ride due to extra isolation from the road, which will give more sidewall flex.

All this crap aside, is this a worthwhile tire to run since our final drive sucks? Has anyone done this? If not, what about part of the way (255/40/18)? Thanks in advance for info!

Iowa999

iTrader: (1)

I switched to a taller tire and the lower RPMs are nice, as is the extra compliance, but the tire is now closer to the fender liner (at lock) than I'd like. I have mixed feelings about the experiment.

Noize

iTrader: (24)

Thanks so much for the reply! What percentage size increase?

Iowa999

iTrader: (1)

A bit over 1%. 26", instead of 25.7".

tkromer

iTrader: (6)

I run 255/40 PSS on 9.5+25 rims and I had to "push" the front inner fender liner a tiny bit. I like the few lost RPM cruising on the highway. Whether to go 265/40 next time or not, I haven't decided. If you do, let us know!

Bmore_EvoX

Great write up. I was also looking into this. Thanks for doing the research for me Noize!

Noize

iTrader: (24)

Thanks! One thing I am concerned about with the 265/40/18 after more thinking is the front to back clearance. While we've established that top to bottom will be okay, I think the fender liner contact will increase dramatically. I also wonder if contact with mud flaps/body panels could be an issue. It would be great to be able to run this size, but it might not be in the cards.

Edit: Thanks to Iowa's post for making me think about this. 26.3" might not be possible.

ddawg1130

iTrader: (1)

You can heat gun the fender liner out of the way. When I first lowered my car on Ohlins I had to cut rubber from the factory mudflaps in order to make clearance for the tire, but you can do this to make room without compromising the mudflap's integrity.

As to running the tire to get the extra top end, well you're simply playing with the final gearing of the car. If at your power level you find yourself hitting 112mph right before the end of the quarter mile, then maybe running the 265/40 tire will help. But the larger diameter tire and added weight will hurt performance so the car will accelerate slower. You might find after putting a 265/40 on that you don't even get to 112 anymore in the quarter mile...

lastly, you will have a lot more trouble clearing the fenders in the rear. You almost certainly won't be able to get by without rolling them. Although rolling is soo stupidly easy I don't like that it's a deterrent for people from doing things.

For a hardcore drag race car, I'd think lighter stickier narrower race tires on some dedicated race wheels would make a far bigger difference then trying to push a 265/40 tire. 265/35 will yield an overall more enjoyable and performing setup for everyday driving with better tire feed back and turn in.

05OrangeSS

iTrader: (1)

I don't use an 18 inch rim but I do use a 26 inch tall slick and it allows me to trap over 120 in the 1/4 but I also use a significantly higher redline than stock

Iowa999

iTrader: (1)

If we were talking about a thin tire, such as my 215s, then you might be able to get away with it because you could keep the offset high and, therefore, the scrub radius low. (Remember: it's the scrub radius that determines how much the tire moves fore and aft when you steer.) But when you run a tire that forces you to run an offset as low as +20, then 26" is pretty much the limit before you're all over the mud-flap. I have an offset of +20, as well, and I have light contact with the mud-flap at lock with 26" tires. But, again, my tires are only 215s. 265s would be chewing the mud-flap to bits.

Note: you need to think about offset and tire-width, as well as tire diameter.

MrAWD

One thing there is flawed with your math...

Running shorter tire and making 114 mph is partially possible because shorter tire put more torque down on the road. Once you get to the taller tire, you do gain top speed at the same RPMs, but most likely loss of the torque that you will be experiencing at the end, might make you to never reach the same RPMs you were getting with shorter tires.

Now, I am sure one of the setups has advantages for a particular application. If it was autoX, I would go to the shortest one I can, because SST can utilize extra shifting that might be necessary. Of course, 5 speed trainy might get in the way of this, so you have to find the place where MAX speed in 2nd gear is optimal for a given course - tricky to say the least. For drag racing and street driving, I have no idea which is better between the two. It will be harder to spin larger tire, so your engine will have to work more than with shorter tire that might not bring you as much speed. Harder engine work will drop your RPMs at the end of 1/4 mile and might use more gas to do the same thing. It might not and you could ended up quicker time wise at the 1/4 and you might even need less gas to do the same for daily driving. The only thing I do know is that this will have effect on the final results.

Iowa999

iTrader: (1)

Here's the piece that's missing, dude: Setting the car up to avoid another shift often more than makes up for a little lost torque.

MrAWD

So glad you were not talking to me on this one, since I said the same thing above...close one though. ..

Noize

iTrader: (24)

It's just stated a little differently. I agree with Iowa's take, especially for drag racing. With a stock appearing frame turbo upgrade, it's effortless to be all over the rev limiter before the 1320 foot line, even on pump gas. If you go to 5th gear, you lose.

MrAWD

True in what was said except the part where it points I was saying something about the drag racing and effect of tire diameter change for the final results, which is not correct. Ony part about the gearing I was referring to was about the autoX application.