I have a 2015 F150 Screw 4x4 with the 5.0 engine and 3.55 axles that I just leveled up and put new tires on, I went up one tire size to a 285R65/18. I am noticing a fair sized drop in fuel mileage. Had been getting around 17 mpg around town and as much as 20 mpg on the highway according to the onboard stuff in the instrument panel. Now I'm getting way less especially on my highway driving which is down to about 16 mpg. I know the larger and a bit heavier tire can cause a drop in fuel consumption but I didn't think it would be that much , 4-5 mpg lower. Truck runs fine no issues as I can tell, it has about 60,000 miles showing on the odometer. Was curious if anyone else noticed a significant drop in fuel mileage after leveling up and going to a larger tire?

 

Did you adjust your tire size with FORScan?

Tires are bigger. One revolution covers more ground, but truck doesn't know that...

 

No , I didn't even think about that. What do I need as far as tools or scanners to adjust that. Thanks

 

I went from 17's to 18's I lost 2 to 3 mpg.

 

You need to either acquire FORScan, or a tuner that adjusts tire size. FORScan thread is pinned.

There are many online calculators where you put in both tire sizes and it shows you the difference. You got an MPG drop, just not as big as you think. Using the calculator will show you the exact change.

 

Likely your speed readings are off a bit, too. FORSCAN to the rescue.
Once your truck is updated for the larger tires, there is a thread on using engineer mode in your dash to alter the bias on the MPG readings. Just doing that, I lost 2 mpg on the trip meter BUT I altered the bias just using 200 miles mostly in town driving data. Now I’m saving all my gas receipts with miles written on them to average the data better.

 

I lost 1 - 2 mpg just by going from the OEM P-rated tires to LT tires Load Range E. Same exact tire size (265/70R17). And yes, this is all hand calculated (actually, by an Excel spreadsheet). I could tell the difference as soon as I drove away from Les Schwab tires after the installation - felt like the truck had 1000 lbs in the bed. The E-rated tires just don't roll down the road as easy as the OEM and are substantially heavier.

 

I haven't adjusted my tire size in Forscan, but I only went up to a 275/60R20 with a 2" level and noticed a mpg drop similar to yours. I used to get about 17 mpg with my normal mix of city and highway driving, now it's more like 15 mpg, and I stayed with a P rated tire that actually weighs 1 pound less than the stock Hankooks I had.

 

Do expect to lose roughly 2 MPG with heavier E load tires, but it shouldn't be quite as dire a drop as the LOM in the dash shows since you didn't recalibrate for new tire size

 

Several things. You went to a WIDER tire. It isn't all that much taller, about 1/2", or 1.5% taller. That isn't really enough to make a real difference in your speedometer or odometer. Wider tires, and tires with more aggressive tread increase rolling resistance and hurt fuel mileage.

My truck came with 275/65/18's and when they wore out I replaced them with 265/70/18's. They are the same height as your tires, but significantly narrower. There was zero difference in fuel mileage and according to my GPS only about 1 mph difference off the speedometer @ 70.

My current tires are 275/70/18 E rated tires, They are more than 3" taller than stock, about 3.5%, but are the same width and about 15 lbs heavier than stock. I calculate fuel mileage by hand but multiply the number on my trip meter by 1.035 to get the actual mileage. For example if I fill up a 365 miles I multiply 365X1.035=377.75 actual miles driven. The difference isn't huge, about 1/2 mpg difference. The difference in fuel mileage was minimal compared to the factory tires. Around 1-1.5 mpg after correcting for the miles driven.

As far as the speedometer is concerned I'm actually doing about 72-73 mph when my speedometer says 70. That is close enough, especially at speeds under 70. At 40-50 mph I'm only off by about 1 mph. There is no place where I drive where speed limit is over 70 and no one is going to stop me as long as I keep it under 75. Actually closer to 80 here in GA.