Hey all,

New member here, this is my first f150. Its a 5.0 2wd with 3.15s. I just upgraded from the factory or lot installed 265/60r18 into a 275/65r18 and have noticed a fair drop in MPG on the highway (was getting 21-22 at 70)(now barely getting 20 at same speeds) and the truck runs a good bit hotter as well. 10-15* on all temps.

Is this because I went to a 1.5" taller tire with more weight and have the higher end gears?

I am in between going back to the tire shop and going back down to the 265s or spending 2600$ to get 3.31 installed and a yukon duragrip locker installed. Not sure which way to go but could use the help and advice from more experience people.

 

it's in your head... your speedo is off a tad.

 

Bigger tires throw the speedo off so the truck "thinks" its going less distance per gallon.

Can be adjusted with Forscan.

 

why would this cause the truck to run hotter as well? Because of the added weight of the larger tires?

 

That I am not sure of. I put bigger tires than stock on my rig and had no difference in temps. I did have to adjust tire circumference though.

That there is an increase in engine temps just cause of slightly bigger tires might lead me to think the cooling system is weak.

 

You got some good advice from these guys.
You won't see much change from switching to a 3:31 gear, you're on the right track going bigger. To gain anything, you're going to have to go to a 3:55, and IMO, a 3:73 gear.

 

More than likely YES your pushing a brick in the wind with the 3.15 gearing and a larger tire.

Here is a tire and gear calculators to use to determine your if YOUR worries are warranted.
https://tiresize.com/gear-ratio-calculator/
https://tiresize.com/comparison/

Limited slip type rear diff is not going to give better fuel mileage. That is why most new F150's have a e-Locker type rear diff. Best of both worlds, better fuel mileage and locker when needed. The rest of the time the anti lock brake sytem will control rear wheel spin to work similarly to a limited slip.

For scan the new tire size in and drive the truck with the present tires.

 

If more throttle opening is used, more engine heat is produced from more fuel is used.
If you can get the dash display into Engineering Mode, there should be a step to adjust the Bias setting to make the MPG read closer to the actual measured fuel usage.
You still need the speed indication vs tire correction made. If not, it likely changes the shift points as well.
Good luck.

 

Doesn't sound like much of an upgrade.

 

Smiles per gallon. If you lift and add more rolling mass weight, you will lose some MPG...but the looks are worth it.