I've done levels with no tire changes on several trucks and it does make a difference. It's been minimal on my DT though. I installed my current wheels/tires in July, and drove with stock rake for about 2 months. I installed a 2.5" kit in September and mileage was slightly affected. It's not a lot, but there is a difference. It's only around 1 mpg or so. I don't really drive on the freeway much (maybe twice since I bought the truck) so I can't give a fair calculation on how bad, but it's a little worse. I don't trust 1 trip as a fair assessment though because it could be any number of variables that affected the mileage on that trip. Around my rural roads though, where the speeds are slow and wind resistance isn't as much of a factor, I'm a bit under 1 mpg difference. So I'd say you're probably looking around 1 on the freeway without changing tires.