What's the down side to mounting a 245/40-18 on a 7" rim or a 275/35-18 on a 8" rim? Before the lecturing starts, I know they should be on 8 and 9" rims, but what is the down side to mounting them on a narrower rim? Do they wear more in the middle? Is it dangerous? I know I can mount them up, but want to know what are the associated problems. I have 295/50-15's mounted on 8" rims and have never had any problems.

Sidewalls are shorter and stiffer then the 50 series. Takes a bit to get them on. Trust me............I saw the tire guy struggling a bit when I got my 275/40/17's mounted to a 17x8.
I want to go with a 50 series next time.

Once there on, is there any problems like wearing funny?

Holiday weekend so not many have probably seen this so I wanted to get it to the top. If anyone knows the cons, please let me know.

Jeff. It would slightly balloon the middle of the tire causing the tire to wear in the center. However if you lower the pressure slightly then it should wear even.

Thanks Ron! Outside of being difficult to mount and the wearing issue, there apparently is no safety problem is what I'm hearing??

A 245 will mount on a 7" wide wheel just fine as will a 275 on a 8" wide wheel. They are both within the recommended range.