There’s a few different ways you can try to help or atleast deal with this. As far as MPG, naturally when changing ride height/tires/etc, it’s going to affect it. Even just adding a leveling kit will affect the MPG. Some ways to help, will just depend on how much you want to spend. And how long you plan on keeping your truck will determine if it will end up saving you money in the long run. You can do things like re-gear (as previously stated), add a programmer, or both.

Now for the speed, you can do a simple math formula: take the size of your new tires and divide it by your stock tire size, than multiply it by your speed on your speedometer. It’ll give you your actual current speed. Sounds like Forscan can fix the issue, or even re-gearing could get you pretty close. And with re-gearing will get you back to the stock performance as far as pick up speed and feel like less strain. But bigger tires than stock will cause more strain as it is, but your truck transmission strength shouldn’t be affected by 35’s since these trucks are able to pull trailers on a daily basis. The tires won’t cause near as much of a drag.

When you re-gear, as long as you get your tires and gears close to matching the ratio you had stock, your truck is not going to be screaming RPMs at highway speeds. When I re-geared mine and my dad’s Jeeps to 4.88s (35” tires) on mine and 5.13s (38” tires) on dad’s, they performed like stock. To calculate your appropriate gear ratio, take your stock gear ratio and divide by your stock tire size, then multiply your new tire size. This will give you a number for example 3.31/30.5=0.1085. Then 0.1085*33.8=3.6681. So if the only options are 3.55 and 3.73, you can choose your preference. I would choose 3.73 for two reasons. 1) because i personally don’t feel going from 3.31 to 3.55 is worth the money, and 2) if I’m spending the money, id rather have slightly over stock performance than slightly under stock.

Hope this helps somewhat and isn’t too confusing. Trying to write on the phone.