i have just gotten a 1999 1 ton dullie E350 power stroke van with a unibuilt sleeper and flat bed mod.i am wondering about the tires on this unit. the front are 245/75r16 uniroyals, and the rear are 225/75r16 firestone. i need to know why some one would put differant size tires on the front and what the differance will be and how it will affect the mileage reading and if any differance in handling and towing will be affected. i plan on putting a 5th wheel on this unit for some towing. any help?.  

It's probably not going to be a huge difference one way or the other. In terms of the different brand tires, as long as they're the same brand on the same axle, you should probably be OK. Main thing you want to keep an eye on is inflation pressures and tread wear.  

I WOULD GO WITH 265/75/16 LOAD RANGE E  10-PLY, THOSE ARE THE SAME TIRES THAT CAME STOCK ON THE 1999 SUPER-DUTY.