ontwowheels

2018 F150 XLT, recently swapped out the stock 18" wheels for a set of Ford 20", think they are the ones that come on the King Ranch, 10171s.

Anyway I am noticing my gas mileage dropping, most likely the speedometer and other sensors need to be changed for the tire size. I searched and see this should be able to be done with Forscan but shouldn't the dealer be able to do it for me? Since they are Ford wheels? They likely would have if I upgraded the wheels at the dealership correct?

I need to take it to the dealer shortly for a fee oil change so it could be done then. I'll give the dealer a call on Monday, but thanks for any input!!

ontwowheels

Interesting....used a GPS speed app and it's spot on with the truck lol. Maybe a non issue???

idrive

iTrader: (3)

Going from 18's to 20's has most likely increased the un sprung weight.... it doesn't take too much additional un sprung weight to bring your mileage down.

ontwowheels

Interesting, had to google un sprung weight ;-)

Guess not really concerned about the mileage as much as the speed/miles being off but according to the GPS app I looked spot on at varied speeds. I know the tires are larger, but not two inches, they are a little lower profile than the 18s.

Ger1942

compare old to new

techrep

iTrader: (1)

going from 18 to 20” wheels will not effect the speedometer... changing tire size will.

marshallr

Going from 18" wheels to 20" wheels doesn't necessarily change the overall size of the tires on the truck. Just the size of the hole in the middle. You can get the same tire height and width with 16", 17", 18" and 20" wheels. It is impossible to say without knowing the tire sizes before and after.

It most likely isn't enough different to matter.

The effects of unsprung weight is negligible on our trucks. Someone posted a link a while back showing how important it was for drag racing. Saving 200 lbs of unsprung weight helped a drag car get across the finish line about 3' sooner. That 3' in a 1/4 mile race might be important to a drag racer, but not on your truck. I've had tires as light as 38 lbs and as heavy as 53 lbs on the same truck. It made absolutely zero difference in fuel mileage or performance. At any rate, while a 20" wheel has more weight, a 20" tire will have less so those should come close to cancelling each other out.

There are 2 major ways where tires effect fuel mileage. Inflation is the biggest one. The difference between running my tires at 35 PSI and 40 PSI is at least 2 mpg. The ride is a little stiffer, but it is worth it to me. The 2nd thing is tread pattern. More aggressive tires have much greater rolling resistance and will cut fuel mileage by at least 2 mpg. Most tires with an aggressive tread weigh more and people mistakenly believe it is the weight that is hurting their fuel mileage. As said above, I've had tires as much as 15 lbs heavier each, but since I stayed with an AT tread pattern that had very little rolling resistance it had no effect on fuel mileage.

I suspect your problem is related to either air pressure or the tread type being different. I've never had 20" wheels/tires and have no advice about what type of air pressure you need to be running.

idrive

iTrader: (3)

^^^ All I can say is un sprung weight absolutely makes a difference regarding GAS Mileage on our trucks.

It has been proven over and over the additional weight (tires-rims) lowers gas mileage.