I ran 13" tires on 10" rims. Tires were MT Sportsmans, and the car was a '71 Camaro. If I ran the air down low enough to keep the tread flat on the pavement, then the sidewall flex made the car handle poorly when cornering. If I ran them up at rated levels, the tires wore out in the center and the outter edge was almost new. The traction was also poor as the car ran on about 50" of the 13", and that was not nearly enough for a healthy 427.
Both my present hotrod setups are less than 2" difference between tire and wheel widths, and fit great. They keep all the rubber in contact, without having to run odd pressures.
10" slick with 8.5" rim on my Falcon: