MY ’03 f150 4.6L came with BFG’s 235/xx/16’s.
I “upgraded” to Cooper Discoverer AT LT245/75/16’s at 33K.
(nice thick semi-aggressive tread, I like ‘em)
They are slightly larger, heavier tires than the stock BFG’s.
I was averaging approx. 270 miles on ¾ tank of gas with BFG's.
Now I am getting about 180 miles on ¾ tank of gas
The truck is running like $h!t.
I can’t fathom the new tires killing this much mileage.

I had 30K service done this summer.
Fuel filter was replaced last year.
I added injector cleaner last week.
I changed my air filter this week.
I didn’t get a front end alignment when I had the tires mounted.
Could the alignment be a possibility?
The truck isn’t pulling, is it worth getting an alignment?
Cold weather only accounts for so much in power and mileage loss.

ANY THOUGHTS? HELP!

 

What pressure are you running the Coopers at? The BFGs were probably P series tires. Running the Coopers, which you show as being LT tires, at the same pressure would certainly make mileage go down and be dangerous to boot. If those are load range C tires, they should be run at 38-40. If D or E tires even higher.

 

Tire pressures....
These LT's are "C" ply.
I set them at 40 psi a month ago.
Was thinking of going to 45psi.
I'll check the pressure later today.

(I know running at a low pressure isn't good.)
Can't find either of my pressure gauges.
(misplaced them...somewhere)
Think I'll go buy 3 more.
These things disappear so fast.
I have a better chance finding one if I have 5 to look for.
HA!
Thanks.

 

40 should be sufficient. More and you will just wear the tire prematurely.

Other things that will affect it. Colder weather coming on. Also most states are now running a 10% ethanol blend that will reduce mileage some.

 

Also, as noted by Wandell in other posts, the odometer also isn't going to read as accurately as before due to the new size. It probably isn't as bad as it shows on the odom. That could take up a little bit of the reason for the difference. Your new tires travel farther in one revolution than the old ones (not by much, but over 3/4s of a tank of gas, it does make a difference.)

 

You probabyl had 235/75R16's so the size difference is not much. i'm sure the new LT tires weigh more than the p-rated OEM tires which will have an affect as well. Unfortunately, using the guage is not consistent. I always divide miles by gallons pumped.

 

http://www.miata.net/garage/tirecalc.html

 

Shouldn't larger tires give you better mileage on the highway? Or would the extra friction take that away?

 

these trucks don't necessarily lose any fuel mileage when bigger tires are added...with all that power, 3.73's, and something like a 35 or less, you're probably gonna actually get the same or better mileage than with the stock tires....up to a certain point...

you've gotta look at what engine you're running and the gear ratio especially...

There's a web site with a chart that shows the best gear ratio for certain size tires...here it is...

http://www.4lo.com/calc/geartable.htm

...hope this helps..._motes

 

NHFord

Somethings surely wrong with your truck. I've had 2 F150s with 4.6's, 4x4, 3.55, LT265/75R16s, supercabs, that average 14 - 16 mpg.

Ask yourself the following, when did this change happen? Is it just from a tank or 2 of gas, or after the NH temps started falling, are you using 4wd more, etc. You mentioned the fact the truck is running like crap, I'd be chasing something other then tires.

FYI, in NH we've been running 10% ethanol since at least summer and by now, we're using the winter mix, which will not help your mileage. When the weather got consistently cold, my 4.6 mileage dropped to 12 - 14. Still not the huge drop you described.

Curious, where in NH are you from? I'm up near sunapee.

 

Hey svt2205, this is NHFORD.
I am down in Nashua, NH.
I was trying to connect my last oil change (from a Ford dealership) to my loss in power also.
I didn't feel that same zip in the rpm's after my oil change.
That oil change was a week or two before I bought tires.
Started wondering if they used the original oil filter on the engine.
I plan to mark my filter for any future oil change I have done.
-NHFORD

 

I went from 235/70/16 highway tread to a 265/75/16 agressive allterrain tread, then added a power programmer to correct for the bigger size. I'm still getting the same mileage(14-16mpg around town).