I just bought a used 2007 3500 dually which has 235/85/16 tires on it, why? It needs new tires, should I go back to stock, or stay with the 235s?

Dont go back. I hated mine. Thats way most of them is in front of my house melted to the pavement.

I have 235's and love them !!! I like 85's because they're skinnier, which means less heat !!!!!!!!!!!

I have 235's and love them !!! I like 85's because they're skinnier, which means less heat !!!!!!!!!!!

the 215/85/16 is skinnier and not as tall, a 235/85 is almost 32" tall vs the 215/85 being 30.4" tall
the 235's would put another 3" worth of rubber on the road on a dually which may or may not be a bad thing

the 215 has less rolling mass and would be better for out of the hole power and better fuel mileage

Actually, i looked it up, the sidewall is .67" taller which makes the truck sit up .67" taller, even though it adds 1.34" in total tire diameter, half of that diameter is sitting up in your wheel well. I don't know if I quite agree with you on the MPG, the rotating mass of the tire is not that much more, however, the tire turns 28 revolutions less per mile than the smaller tire. When your speedo reads 65 mph, you are actually going 67.5 mph, which means your going a further distance on the same amount of fuel as the engine rpms are the same, better gas mileage.