Just picked up my 2011 XTR 5.5 with buckets.
Tires too small... anyone know what the largest tires I can fit without doing any mods. I like the ride hight but the wheel wells need to be filled with rubber..

Thanks Coach

 

I believe the largest you can go without lifting is 33s. Just to let you know, upping the tire size will zap the heck outta mileage! I went 1" taller that factory on my 2011 EB, and it knocked 5+ MPG off!

 

Ed - when you went with the larger tires did you get your PCM reprogrammed?

 

Ed, for the sake of argument and just plain good learning, I would love for you to remount your larger tires and leave your other mods off and then check your mileage.

The reason why I'm so adamant about this is because my '04 Expy has the 4.6L and 4 speed transmission and 3.73 LS axle. It's running factory sized tires that are an inch larger than my 2011 tires and it's getting 20 mpg's on the highway.

I dunno, thoughts?

 

IIRC, Ed didn't you go to a taller, slightly wider, 10ply tire? I'm thinking that weight could be more a contributor than overall diameter at this point in the venture.

Went about 1" larger on my '06 and saw little to no drop, but I stayed the same width, and didn't go to an E-rated tire.

 

We did that. The time before last, when it went into the dealership, we stripped off all the accessories, with the exception of the 275/70R/18 Big-O ATs that I had put on it. Hooked it up to a recorder and drove it....MPG was 14.8mpg over a 38 mile round trip on the highway at 70mph. Then they changed the tires to factory 275/65R/18s (Goodyear wrangler AT/S) and drove the same route and speed.....we got 20.4mpg on that run.

We actually weighed/measured the tires....the Big-O ATs are a total of 14lbs heavier for all four, or 3.5lbs each, than the Goodyear wrangler AT/S tires. There was no width difference, the Big-O AT tires are 1/2" taller, and 1 1/2" larger around the outside. The only guess I can come up with is the "rolling resistance" difference between them.

I tried to have the PCM reprogrammed when I went to the larger tires, but Ford only has calibrations for it's factory size tires, so it could not be done.

I had them leave the factory Wrangler AT/S on the truck, and will be buying them. I have the Big-O tires in the shed, and will save them to trade in when it comes time for new tires. I'm even checking closely on tire air pressure......I'm running them at 40psi right now, but think they would be more comfortable at 35psi. The max. inflation is 80psi on these tires.

The only thing I've put back on the truck is the topper, and the mud flaps (which I took off again after my last test drive) I'm gona take the drive again here in a few and see what I come up with.....I'll report the findings both here and on the PCM update thread.

 

What type of recorder?

If you didn't update the PCM with the correct REVS/Mile, diameter, or circumference, then your distance travelled wasn't accurate. In fact it was low (since you went to a larger tire).

Not saying this makes up all the difference, but it does raise a question as to whether the MPG numbers you're trying to compare are really accurate.

 

I only lost 2 mpgs with my 34" tires.

 

Thanks guy for the info.
I am not too worried about gas milage. I just want the largest tire possible without changing the ride hight.

 

I'm not sure exactly what they call it, the guys at the dealership called it a "black box" that connected to the OBDII port. I was along for the ride to witness it. It had a green LED screen on it, with a number of buttons that allowed the recording of just about everything going on with the truck. We did a run with the Big-O AT tires (275/70R/18) and then came back, changed out the tires to the factory size (275/65R/18) and took the same drive. Everything else was the same except the tires.....the 70 series tires yielded 16.5mpg @ 65mph, and over the same course, the 65 series tires yielded 20.4mpg @ 65mph.

The only problem with that is that around here, most two lane roads have a 70mph speed limit, and the highways are 75mph.....creeping along at 65 will get you ran over!

The PCM could not be updated for the larger tires.....the dealerships only have calibrations for tire sizes that come from the factory...and they did/do not have a calibration for 275/70s. Hopefully if somebody comes out with a tuner for the EB, that won't be an issue in the future....although then, the question of tuners and warranty comes into play.

 

But (and yes, this horse was dead a long time ago) - if the tire size is wrong in the PCM, then the instantaneous and average MPG displayed by the PCM will be wrong and so will the dealer's recorder (it's just reading the PCM calculated value).

And again, I have no doubt the tires are negatively affecting your MPG, but it might not be as bad as you think.

 

What size rims? If they are 18" you could go 1 size bigger to a 275/70. I went with a 2" leveling kit up front and run 295/70-18 Nitto terra Grapplers ( 34.4x11.4 wide) I think it looks just right. For now... Welcome to FTE