not always..i pull'd 12-13 this winter and 13-14 last spring summer fall..but i only drive 10-12 miles a day gym/work/home < rise and repeat so my engine barley get a chance to heat up but i pulled 20mpg on a road trip last year so i know its attainable.

**just a point of reference

 

My tires have a max PSI rating of 80 psi, I simply looked at the weight I'm towing and aired them up appropriately. When towing I'm between 55-65psi, unloaded usually drop it down into the 40s.

if 100% of your psi has a certain rating, find out what percentage of that PSI you need to safely tow the weight and then go for it.