I had the same question last winter when I was building my new car. I have always run a 29.5X9 slick. With my new car weighing in almost 1,000 lbs. lighter than the truck I wanted to keep the same tire size just wider because I didn't want to get into the problem of too much tire for not enough motor. When choosing tire and rim size you must be careful no to go too wide on the rim or you will stretch the contact patch out and cause the tire to cup in the center loosing grip as the tire grows under acceleration. This goes the same for a rim that is too narrow.