I just replaced the factory bf goodrich rugged trails lt 245-75-17, with michelen ltx at2 36,000 miles. They softened the ride. Don't chatter when taking off. are little noisier. But i lost 3 mpg's. Can't get past 15 mpg. I won't be complaining when the snow flies though.

 

45.1 lbs ea for the BFG vs. 44.8 for LTX. Why the 3 mpg loss? I cant see the rolling resistance making that much difference between those 2 tires.

 

Do they have the same load rating? If the new tires have a higher rating, may want to check your tire pressure. Stock pressure will be too low.

 

old and new are both E load rating. 55 front 60 rear psi from sticker on door jam. Maybe because of more aggressive tread.

 

I would think winter fuel may be the culprit, or something else that changed at the same time. 3mph for the same size tires doesn't add up. Tires don't reduce your mileage by 17% when they are that similar in weight.

 

I don't know if they switched to winter fuel yet. will have to look up. I got these 3 weeks ago, or about 500 miles ago. Maybe they need to break in???

 

I don't think the break-in is the problem. If your tires are the same size, same weight, and properly inflated, you shouldn't see a reduction like that. I didn't get that much drop going from stock to almost a 35" and much heavier tire.

 

Agree. Might be winter blend, might be that you've somehow been driving differently. But either way, you shouldn't ever have seen a drop like that from tires alone. I'd be shocked even if you lost 3 mpg with Mud Grapplers in a bigger size.

 

I think they switched to winter gas in mid sept. But I lost mpg's right away after tire change. Same tank of gas. At one point I got 19.4 mpg hwy, old tires. I'm usually 16-18 mpg, city hwy mix. I don't know.

 

Just a few questions. We all have assumed you're using the same exact size tire as stock right? Which engine do you have? And last, are you getting your gas mileage from the dash gauge? The digital display? If so, that is a fluid number based on driving habits as well. I babied the throttle on a tank to see how much impact that had on fuel mileage and saw great benefit. But within that same tank, when I aggressively got on the throttle, I watched it immediately start dropping. Was there any abnormal idling or driving habits following the tires being put on? How many tanks of gas have you observed this drop in gas mileage on?

 

Yes same size tire LT 245/75/17. Specs about the same From bf goodrich rugged trail to michelen at2. 4.6l3v, 4x4, 3.73 ls. Getting numbers from dash, same driving habits and roads. I noticed right away drop in mpg. Would a difference in tire height from new to worn affect mpg. I think the spedo would be off a little. In tire rotaition.

 

I'll try a manual calculation at next fillup.

 

The speedo would be off such a negligible amount, legitimately minute and almost non-existent. Manual calc won't show you anything unless you've done it for the old tires as well. This is all very puzzling if all other variables have stayed the same. Every logical possibility for why it's off by 3 mpg would yield a very small deviation that won't explain this at all. If it's truly off by 3 mpg somehow, it would be worthwhile switching to different tires. I doubt that's the case though

 

Yeah this is bizarre for sure. Unless the new tires are all but melting to the asphalt and creating that kind of resistance, I can't see how the tires have caused your drop. Like I said, when I was stock tires on my 2010 FX4 SuperCrew with the 5.4, I averaged maybe 15.5 to 16mpg (fairly aggressive driving habits). When I upgraded to new wheels and tires (in signature), I increased diameter by 2.5", width by around 1.5" I believe, and significant weight, and I'm currently (with a tuner to recalibrate speedo for tire size) averaging around 14mpg.

 

And also, agree with KMAC, manual calculations aren't going to help you at this point if you have nothing to compare them to with the old tires. The dash is all the info you can go by. I had missed the part where you had put 500 miles on the new tires and was just questioning if you had finished a tank yet.