ThetaChi715

I have an 06 regular cab STX 4.6 4wd with 3.55s and 125,000 miles. I have recently fitted a set of 275/70/17 BFG All Terrain KO2s (up from 255/70). Other than Rancho QuickLift struts, the rest of the truck is completely stock, motor included and no tuner.

My problem is, after fitting the new tires, I've decided to check my mileage after a months of basically ignoring it. When I bought the truck 2 years ago I regularly checked the mileage and up to about 6 months ago (before I stopped keeping tabs on mileage) I was getting 15-16 MPG, sometimes 17-18. Now, after a few weeks of checking, I'm holding steady right around 11 MPG. I hadn't checked the mileage for several months until installing the tires so I am not sure when the mileage dropped. I'm assuming that going from a 31.1" diameter tire to a 32.2" tire wouldn't drop mileage by 4-5 MPG. Even after correcting my math for the lower tire revs/mile, I am still under 12 MPG.

A lot of miles are city miles but I also checked interstate miles twice since noticing and still around 11mpg.

My question is, has anyone had experience with this first hand? I know there could be several things; plugs, COP, filters, O2 sensors... But has anyone actually found a singular culprit? I'm not convinced it is winter gas because I use to check mileage regularly during the winter at 15-16 mpg and I'm not even sure they've made that switch yet, the weather is still warm here.

Air filter is in good shape, oil change is still a couple thousand miles out, never had a check engine light even flicker since owning the truck. I replaced the fuel filter after buying the truck. Does still have the stock plugs in and I'm sure they need changed but there's no miss or rough idle. Doesn't burn any oil. I try to steer clear of discount gas stations and use BP, Shell, Marathon gas almost always (but cheap gas does sometimes happen).

What should I start with? MAF Sensor cleaning? Plugs? How can I check that my hubs are completely unlocked - I did have the clicking noise last year but has gone away. Is there any way to be certain my front hubs are not losing vacuum like a lot of these trucks have problems with? Some kind of link to diagnose the hubs would be awesome but there's so much talk on these forums it's hard to find exactly what you're needing.

Sorry for the long post, fellers. Just trying to be as specific as I can be. Let me know if you have any other questions/concerns. I'd sure appreciate any direction you can give.

thebreba

I believe tread has an effect on MPG as well. You went with AT's. Did you get different/bigger rims as well? All that will have an effect. I went from Stock rims and Tires on my 09 5.4. I was getting 20-21mpg highway with stock rims and tires. Now I put 33's with MasterCraft All Terrain tires. On the 2014 Raptor SVT Rims which have a 34mm offset. I'm getting 18-19 highway now @70mph I can get 20 if I do 65.... However I do have aftermarket Headers and Exhaust which may be helping with an extra MPG or 2.

gone postal

The weight of the tires is probably having as much if not more of an effect than the size.

Takes more HP to get a higher rotating mass moving.

nrivera04

Same with my 4.6 when I had it... Same setup as yours also. Ran 10 plys and fuel mileage was 12. Always 12. You're going to have to regear and tune if you want your mileage to go back up to what you were running.

Mod (Ret.)

What are you using to check your mileage with? If it's a tuner, or from the dash information center, it's going to be off, as you are using a slightly larger tire. When you go to bigger tires the circumference of the tires is larger, meaning the distance around the tire is longer. Thus, with each revolution you are traveling further than before.


Your specs: New - 275/70/17 BFG All Terrain KO2s Old - 255/70/17

Your speedometer is going to be off as well. At 65 MPH with the old size tires, the new tire speed is actually 67.31 MPH

ThetaChi715

Thanks for the replies, fellas.

Bucko, I do not have a tuner and my old STX doesn't have any dash read-out for mileage. I am doing it old school with miles driven over gallons to fill up. I have also been figuring in the difference in revolutions per mile between the 31.1" overall diameter of the old tire and the 32.2" of the new tire by the percentage (.04) difference. Still under 12 MPG.

If the change in tire size is the culprit then so be it. I do quite a bit of 4wheeling in this truck, which is also my daily commuter, so the tires are a necessity not vanity. I was just curious if anyone else has noticed this much of a change or if something was affecting my mileage elsewhere.

What really got me curious is my dad has an 04 STX 4.6 and 3.73s with a Superchip tuner with 265-70-17s and gets a legitimate, and consistant, 18-20 MPG with a heavier foot than I have. Oh, and 210,000 miles on the same plugs, COP, PCV... Maybe I'm just jealous...

wrscpc07

I use a digitial tuner on my 05 stx with 103k. Althought it may not be 100 percent accurate, i can at least see if the mpgs on the tuner screen change or not. Anyway, how i got the tuner when i was on 35s and stock wheels. Then i moved to 37s and aftermarket wheels..it went down maybe 2 mpgs..and thats with the stock gearing. So i highly doubt your small change in the tire is dropping it that much. On 37s and stock gears i was getting 12-13 mpg

ThetaChi715

This is exactly what I am looking for. I did not think going to a 275 from a 255 should cause this drastic of a change.

So... Does anyone have any suggestions on where to start or what to look for?

wrscpc07

You could always try some fuel injector cleaner in your tank. I add some every few hundred miles

Adam013085

So you had decent MPG, put on bigger tires and the MPG went down? Seems pretty easy to pinpoint the issue here or am I missing something?