The overall height of the tire/wheel assembly will be a factor. If a 15" wheel and a 20" wheel have tires that are about the same height by comparison, the stopping ability will be similar. Most of the 20's I've seen have about an inch and a half of rubber on them. If the 20's are taller overall, there's more leverage on the brakes, and less stopping effect.

Maybe the truck is embarassed to have those ugly wheels on it, and it's trying to run away from them!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!