I just have recenlty put 317/65/17 bfg allterrains on my 2005 f150 5.4 triton. Since then I have only been able to go to 80mph topping out. I would like to know how to fix this any tips would be nice thanks.

 

You will have to get the computer reprogrammed for the new tire size.

 

May just have to live with it.. When i had my 03 KR i put a set of nitto terra grapplers on it with one size up from stock and it felt like i was driving a 4 cylinder.. Acceleration sucked fuel mileage when from 18mpg down to 12mpg.. So i went back to discount tire and went back to the stock size and put a set of yokohama's on it and everything was normal again

 

Why is 80mph necessary? Be careful out there, its not a sports car.

x2 on speedo recalibration.

 

Here in texas speed limit's range from 70-85mph (yes 85mph) It would suck not being able to top 80mph especially trying to pass

 

I would recommend you get a programmer with custom tunes - you need to correct the speedo/odo/PCM for the larger tires, and have the speed limiter raised to a higher speed.

The speed limiter is causing the topout at 80 mph due to incorrect speed calculations.

 

I agree. I think the stock trucks top out at 98 mph. But the larger tires have freaked out the computer.

 

The tires *should* (key word there) not have messed with your speedo computer that way, it should have been the opposite. Bigger tires fool the truck into thinking it's going slower than it actually is. You should still be able to hit your regular speed cap of 98, but actually be doing 105+.

I have plus sized tires that yield exactly 10% error from my speedo. When doing 40, I'm doing 44. 70 is 77. Both GPS and speed signs have confirmed this.