I am writing to inquire about fuel economy decreases after switching to General Grabber LT tires. I drive a 2011 Ford F150 with the 3.5L Ecoboost engine, supercrew cab and 5.5' bed, stock tires were Goodyear Wranglers size 265/60 18. I occasionally tow a 32' travel trailer that weighs approximately 7000 lbs and I prefer a stiffer tire for stability and loading so I switched to the General Grabber LT275/65 18 tires, as recommended by my tire dealer.

Before I changed tires, my fuel economy consistently averaged 20.3 MPG (not towing) and 10.5 MPG while towing. The previous Goodyear tires had 95% tread remaining, so nearly new.

After changing tires, the fuel economy decreased immediately to 18.1 MPG (not towing) and 7.5 while towing. I calculated the fuel economy after the first 500 miles, and again after putting 2500 miles on the General Grabber tires and the readings were precisely identical.

All other conditions were constant including driving habits and the roads I run on. I am surprised to see a 10.8% - 28.6% decrease in the fuel economy. On a different vehicle (Ford Expedition), I changed the Goodyear tires to Michelin LT tires and saw increases of 1-2 MPG.

With either set of tires, the inflation is 35 psi.

Why would the fuel economy decrease so much with the General Grabber tires? If there something I could do different to increase the fuel economy with this setup?

 

35 PSI is pretty low for an LT rated tire ....

LT's are heavier and stiffer than P rated, and have more rolling resistance. All Terrains have more rolling resistance than All season tires.

so you have a heavier stiffer tire with more rolling resistance, a more aggressive tread with more rolling resistance, and are on the low side for inflation leading to more rolling resistance.

looks like you have control over 1 of the 3 ... try airing them up and see if that helps.

as for the other two .... as they say ... pay to play.

post up the max load and max PSI on the tire and your GRAWR from your door tag and i'll give you a good indication of what you should be running for pressure.

or, better yet, read all about it here:
https://www.ford-trucks.com/forums/1...tle-stiff.html

BTW, welcome to the forum!
lots of good knowledgeable people here ... myself excepted.

 

I'm pretty sure that the circumferences (revs/mile) of the two tires are different. If the new tires are say 4% larger, then you are actually going further than your odometer is registering and your mileage will be low by approx. 4% either on the in-dash computer or hand calculated. Sadly Ford trucks with the Ecoboost V-6 can only have the speedometer/odometer programmed (by a dealership) to the sizes of tires the factory installs. If yours are bigger, you'll just have to correct mileage for the tire size difference. Even correcting for the larger tire size, you will not get back to the mileage you were getting with stock tires as the stock tires are much lighter in weight and have less rolling resistance than the LT tires you've replaced them with.

 

Thanks for your reply,

The max load and PSI is 3415 lbs, 80PSI.
What is GRAWR? Do you mean GAWR? Front: 3450, rear 3850
GCWR: 15,300 (I have the 3.55 axle)

Also, the new tire is the General Grabber HTS, which is listed as a low rolling resistance tire. Is that just a fancy marketing term, or is there a quantifiable number to compare the resistance (drag) force at various speeds to the P-rated Goodyear?

 

+1

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Good call. I grabbed my portable aviation GPS and put in on auto mode, set the cruise on an indicated 60 mph and the actual speed was around 62-62.5 mph. This certainly explains that part of the decrease is simply due to indicator error.

Here's proof:
Indicated Speed
Actual Speed

 

Gross Rear Axle Weight Rating, sorry, should have been more clear.

with those tires and your Rear Axle weight rating you should run about 45 PSI to be safe. you could actually reduce that number further, but that's not the goal in your case ... unless you find the ride at 46 psi unpleasant.

3850/((3415x2)/80) = 45 psi rear
3450/((3415x2)/80) = 41 psi front (i rounded up)

i'd start there, but at those numbers you can run at your maximum GAWR both front and rear without concern of overloading your tires.

to reverse that equation at 35psi your tires can only support 1494 lbs each before they are overloaded. which is 2988 per axle or 5976 for the whole vehicle. That is, if you go by the safest formula (above). you can split hairs and figure out more exactly what the lower limit for air pressure is, but that will be counter productive for the goal at hand.

try 41 front and 45 rear and see what you think.

or go nuts and put 80 in each corner ...

as for the low rolling resistance rating: they will have a lower rolling resistance than any other tire in that category. very seldom will any LT have a lower rolling resistance than any P-rated tire of the same size.

did you upsize tires at the same time? that will have a HUGE affect on rotating mass and MPG calculations.

 

Thanks again! I'm also a numbers guy so that is very useful. The original tires were 265/60/18, these are 275/65/18. There did not seem to be too many LT tires in the original size.

 

air those tires up. running around at 35 psi on LT tires (looks like E rated as well) is going to overheat the sidewalls and potentially cause a blowout.

I recommend you start at 45 psi rear, 50 psi front and dial them in based on ride and wear patterns.

 

what's the weight difference of each tire ?

 

Update: I now run them at 50psi minimum and have since purchased a tuner and some other bolt ons. The tuner allowed me to set the actual tire size based on the diameter and the computed mileage issues went away.

 

Adding mass to the rotating system is way worse then putting that same extra mass in the cab. The added tire mass increases rotating inertia and has a very strong negative effect to fuel economy.

 

Yeah we get that. It's been discussed earlier, I sent the update to advise that the issues were not as bad as they appeared. I just had to do some things differently, i.e. air pressure, and tell the computer that it has bigger tires. I am quite pleased with the fuel economy now.

 

Ford picked the Goodyear Wrangler SRA's for stock use because they are a p metric tire that rides smooth and does have one of the lowest rolling resistances.
My Goodyear Wrangler LT's say to run 35 psi on the door jam. I run that unless I am towing then I up the rears to 50 and fronts to 45

 

I switched from the stock Goodyear 275/18 (cord broke at 17,000 miles) to E rated 285 BFG ATR's. I run them at 65 psi all around and have lost 1 mpg, but it did make a noticable improvement pulling a camper.