OK, I just put on some 245/40f 275/40r for my stock 18" GTR wheels. I didn't just pick this from thin air, as I searched the forum and had a few people said that they ran this setup and it was fine.

Of course, after I put the tires on, I learned that this setup was a +3.7% change on the rear tires and that it might affect VDC.

The other thing I noticed is that the tire is bulging a bit on the rears. So I think the wheels might be a bit too small for these sizes.

So my question is this, besides ordering an entire new set of tires, what are my options?

I was looking at some replacement wheels for the stocks, but if the 275/40 tires are really going to intefere with the VDC, then I don't want to keep them.

Any help would be greatly appreciated...

 

What are you talking about?
245/40/18 275/40/18 is optimal.

225/45 to 245/40 is 1% change
245/45 to 275/40 is .08% change.

275 on Stocks?? That is not very good..

 

^^ you are absolutely right, I retried the calculator and realized I had compared 245/40 r18 to 275/40 r18 instead of 245/45 to 275/40.

Thanks for the slap in the head.

I am still curious about the stock wheels being wide enough for this size (8 & 8.5) for 245/40 r18 and 275/40 r18 as it seems that they bulge a bit on the sidewalls. Could this be from lack of air pressure? I currently have them at 35 PSI, should I run them with a bit more pressure (I do drive rather fast on the "track" usually around 90-95mph) and do some agressive cornering as well.

Again, thanks for the help...

 

It bulges because you are trying to install a tire that has a ~10.8" width onto a wheel that is only 8.5" wide. That means the tire has to bulge ~1" on each side of the wheel to fit

 

^Yup. I run a 275 on a 9" and 10".

 

Yea buddy, I think you might have misunderstood why people were running 275's in the rear. I run 245's on my 8.5s and 275s on my 9.5s. That tire's way too wide for such a "not wide?" wheel. I think your only option would be to purchase 2 new rear tires

 

I would not put a 275 mm tire on a 8.5 inch wheel. Maybe you should forget “upsizing” and stick with the OEM sizes.

 

You'll be fine with the 275/40s on the 18x8.5 rear rims. Heck, I've mounted 275/40s on stock 17x8 rims on all four corners for autocross without issues.

 

They seem to be holding up fine as I did a speed test with them today.

I will put some corner work in this weekend and see how they handle. I think i have convinced my wife that we need a wheel upgrade : ) so I will be looking for a larger size wheel to fit my tires now.

 

I'll bet the tires you use for that have one hell of a lot stiffer sidewall. Not to mention autocross doesnt involve cornering at higher speeds.

 

The tires are Hankook Z211s and they are road course tires. I used these instead of softer compounds for autocross since they last longer. The sidewalls are very stiff, and as much as people make a big deal out of it, mounting 275/40s on 8" wide rims is not an issue. I've actually driven my setup on the road and have taken high speed corners without issues.