Tires: Tire size, diameter tire, better mileage

Question
I have a 2008 Ford 350 that came with 275/65R20 tires and I use it to pull a fifth wheel. I am told that if I install a smaller diameter tire I would get better mileage when towing.  Is this fact or fiction?

Thanks John

Answer
John,

Fiction,

Smaller diameter tires make the engine rev more for the same speed.  If anything, you'll want a larger diameter tire.  The same would be true if you were to change rear gear ratios.  A larger numerical gear results in more revs.

However, you'll lose the torque needed to pull the trailer effectively.  So it's probably best to stay with the same size (and gearing).  For practical purposes, the biggest fuel economy issues with towing are the aerodynamic drag and the weight - and there's not a lot you can do with either.