BFG ATs are on and ...

Now I'm getting super shitty mileage. Wondering if there's something I'm doing wrong. Last mileage calc. before swap came out to about 20 mpg (us gal). Since then I've noticed a steep decline after going to the BFGs from the stock sport bridgestones, down to 16 mpg (again, us gal).
Is this normal? Or something to be concerned about?

And for all you "it's a truck, not a civic" people, I'm not concerned about the fuel economy, just the drastic change, trying to find out if this is solely because of the tire change, or not.

Also, would going from a 265/65/17 to a 265/70/16 throw my speedo out badly? It's off by about 7 km/h since the tire swap (and FJ TRD rims).
Thanks everyone.

 

Well I had 18 before and now like 17

 

BFGs are super heavy. I put some 265/70/17 Wrangler A/Ts on, and noticed a decrease of about 1 mpg myself, so I'm not super surprised. And yes, your spedometer actually was reading fast before you upgraded size, and is probably right on now. So your mileage was probably overstated earlier.

 

It's a truck, not a civic.






























































But seriously, the BFGs have a more aggressive tread pattern, and that's going to create additional resistance. Also, the BFGs are likely a heavier tire than your stock tires (stock tires are almost always shit).

What size tire did you have? What size tire now? What weight of wheel then? Now?

Tire pressure up to spec?

Is this mpg based on one tank, or several? Because to determine TRUE MPG trends, you need to look at an average over several tanks of gas.

 

Yeah, I feel your pain.
While I bought the truck for it's good looks and gas mileage (prior F150 supercrew), I hated the stock sissy tires.
Put on BFG AT's, 265/70/17, load range C. Heavier tire and immediately the gas mileage dropped from about 17 in town to 15. Taller tire than stock changed to speedo reading, it's actually spot on now.
I don't hit the highway much but last short road trip saw about 18.5 mpg. Better than my Ford, but nothing to get all excited about. In the end I bought the tires for the added off-road capabilities so I got what I expected.

 

Jandrews: good call. As for the mpg reading, it's been the same fuel usage for about 250 k of driving 3 trips in a row now. All this is highway too by the way.

As for tire size theories, my tire is actually near the exact same diameter, and according to the tire calc, should only effect my speedo by less than 1 mph. Remember, it is a "70" but on a 16 rim, not 17.

It is possible my speedo has been out all along. I'll try my next trip with it showing the speed I want to go. Not using gps as a speedo.

Thanks for the responses so far. Keep the ideas coming.

 

Whats your cruising speed? I can squeeze 19-20mpg on mostly highway driving. Are you a fast accelerator? Happen to be driving into the wind? Lots of things will change MPG. What PSI are you running?

Heavy Tires = More Rotating Mass = More right foot to get to cruising speed.

PS Check out my fuelly log, and add 6.6%. so 16.3mpg x 1.066 = 17.35. My best tank at 21.1 is actually 22.5. (Tire diameter difference vs odometer)

 

7km an hour at what speed? Its a ratio not an offset.

 

OP , my mileage dropped a bit when I went to heavier E rated tires , and went up to 265/70/17 ( I realize you are 265/70/16 ) .

I'm suprised that you have had such a big decrease going to a C rated tire which is virtually the same size as stock . Any chance your tire change coincides with the Alberta winter kicking in ? Colder weather , use of 4x4 will decrease your efficiency .

 

I put BF's on my Dakota, mileage went to hell. They are a very heavy tire and designed for off road use. Not fuel economy. I went with the Grabber AT2 this time, mileage did'nt suffer too much. Lost about 1 mpg from stockers. I think from moving from a 17 to a 16, is throwing off your speedo and mileage. Use a gps and figure out your speed vs odo.

 

You really wanna fuck up your mileage, slap some MTs on there.

 

Noneck: no offense, but please learn to read the whole post.

Jd: I try to accelerate without jamming on the pedal, and for the most part I'm the slow guy at the lights. As for cruising, about 66-68 mph.

OZ: winters here yes, but no 4x4 yet. And not really that cold. Maybe it's a speed thing.

 

BFG's as stated.....a heavy tire, gas mileage will drop with a heavier tire.

 

Just so we are clear for everyone who thinks they're mileage dropped when getting bigger tires - it was overstated with your old tires. If your spedometer was reading fast, your truck was accumulating more miles than you were really driving, so your mileage incorrect, about 5% on mine I believe. So even though I thought I was getting 20 mpg, I was really getting 19. So with my new tires getting 19, I'm actually pretty much the same.

You can't have the best of both worlds...higher mileage with shorter tires, and more accurate speedo with taller tires. Just making sure everyone is on the same page...I think some guys ignore the fact that the 20 mpg they thought they were getting was never happening.

 

If you use a GPS, then you're fine. I'd say most people (myself included) don't, so you have to make the adjustment.

EDIT: Yeah, if you get 12-13 mpg, then you have some issues.

 

Easiest solution is to not give a shit. When gas needle gets near E, put in more gas. Simple.

Not at all. Once you lift these things, put 33 - 35" tires on them, along with a full set of armor (an extra 400 pounds or more), 12-13 mpg around town is easily attainable and absolutely nothing is wrong with the vehicle.

See above: When gas needle gets near E...

 

I just bought some 17 in xd rims. They had some maxis bighorn m/t, im getting 15 mpg, but my truck looks badass.

'

 

FWIW , I have 265/70/17 E rated Hankook Dynapro MT's and get 19 MPG with rural driving .