I have a 01 mustang i have 255 45 18 and wanna go wider seems to me a 275 would fit in there nice my 255 are way to small for the rim's that are on there.
any one know if thats to wide i got a 2.5inch lip in the rear.
What size wheels on it? A lot of guys tend to think the wider the better and most of the time it might be true but you need to get a tire that fits your needs, going any bigger just hurts your wallet. 315s might look somewhat nice but having them on a bolt on 2v is pretty useless. Get 275-285s if you want to go a little wider, but that's about as much as you'll need.
Another thing to keep in mind is the change in sidewall. The first number is the tread width in millimeters. The middle number is a percentage based on the tread width. The sidewall of a 255 45 is 114.75mm. The sidewall of a 275 45 is 123.75mm and the sidewall of a 315 45 is 141.75mm.
What's all this mean? Well going from a 255 to a 275 will increase the overall diameter of the tire by about .71 inches while going down to a 40 on the 275 will decrease your overall diameter by .37 inches. If you plan to drop the car before you run the next set of tires out I would go with a 275 40 R18. If lowering the car is still some time off I would stick with a 275 45 R18 as the wheel gap would be intense (4x4 Mustang anyone?) with the smaller sidewall.
On a lowered car with a decreased sidewall you gain a number of things. The tire is much less likely to rub, even if we are only talking about a few extra mm. Sticking with a 275 vs. the 315 your sidewall is less likely to be an issue as it pokes out less, plus you will have considerably more play room and much cheaper tires. I recently replaced my tires with Potenza RE970AS Pole Positions, my platoon leader got the same tires on his Mustang on the exact same day. My tires (215 45 R17) were $170 each installed while his (255 45 R18) where $235 each installed, that's a 65 dollar difference. For perspective, the same tire in 225 40 R18 was around 190 each...I'll get tires on my 18s some day...