V6 MPG after lift -- Does this sound right?

[Cornelius]

[long post warning... sorry]

So a few months back I installed a 3/2 Toytec lift with BFG 275/70R17 meats. Super happy with it so far, but my fuel economy has been far worse than I had anticipated.

Before the lift, I consistently got around 20.5mpg @ 75mph and just under 17mpg in town -- and yes, that's after calculating at fill-up. None of that shiny happy sunshine crap that the onboard computer tries to tell me I'm getting.

Anyway, I did a little experiment with my fuel economy and wanted to see what those of you with similar lifts thought of the numbers. First I thought maybe something was wrong with the truck, but now I'm thinking that the increased wind resistance is the culprit and I'm right where I should be. I'm starting to think that those who are reporting negligible mpg loss after lifts must be trusting their onboard computers too much or something...

70mph = 19mpg

75mph = 17.5mpg

80mph = 16 mpg

I did this little experiment on the same 50-mile stretch of highway at the same time of day (no wind or traffic) in the same direction. Speed driven is actual speed, adjusted for speedometer error with the bigger tires and verified with GPS.

It seems I'm losing 1.5mpg for every 5mph over 70. That's bananas! Anyone seeing similar numbers?

I hate driving the speed limit, but it looks like I'll be revising certain driving habits from now on.

Now, if I could only explain the 13mpg I'm getting in the city...

 

[Buckles]

Sounds about normal for someone with a heavy foot.

 

[cpark_12]

That sounds about right for the jump in tire size.

 

[CXS]

Yep, sounds right to me too.

 

[xpeppyx]

I started out averaging 17mpg with stock size Nitto Terra Grapplers. Then I added 265/70-17 BFG KO's, ~2.5in lift, steel bumper and I now average between 13.5 and 14mpg. I get about 250 miles out of 18 gallons. I honestly didn't see a drop between the lift and bumper. I think the biggest hit was the tires.

I also drive like a grandma.

 

[BlackWorksInc]

Keep in mind when going to larger tires that your speedo and consequently your odometer is going to be skewed. i.e. when going from P265/65R17 to my LT285/70R17's there's a 6.6% reduction in accuracy of my speedo. This will skew some of your numbers a bit.

 

[wyanrard]

Sounds right.

 

[Guitarplaya39]

Yup. IIRC, i read on a jeep forum that the difference between 55 and 65mph was a 5% decrease in mpg. I get my best mpg below 65.

 

[Cornelius]

Alright thanks for the replies, everyone. I guess it was a bit of wishful thinking for me to believe I would only lose 1 or 2 mpg after upsizing to those big ol' heavy BFGs.

I have a feeling that, around town, it's a matter of getting those heavy tires moving after a stoplight. And on the highway, it's a matter of pushing all that wind.

I may go broke from gas, but I'll look damned good doing it! :bounce:

 

[rubbertoe]

How is the actual mpg #value calculated ,if its by tire rotations ,would it be off just as your mph after you go up in tire size ? just asking the question not even sure how the trucks computer comes up with mpg ?

 

[scw1991]

I'd anticipate the tread compound on your new set of tires is also different (softer) which could contribute to more rolling resistance.

 

[gm350reratliff]

Sounds about right. Taller, more wind resistance along with heavier, more aggressive tread tires equals worse mpg. I wish we could have our cake and eat it too. How much air do you have in the tires? You can try running a little more air on the street to gain a little mpg back but the ride may suffer.

 

[Cornelius]

How is the actual mpg #value calculated ,if its by tire rotations ,would it be off just as your mph after you go up in tire size ? just asking the question not even sure how the trucks computer comes up with mpg ?

Well from what I've been told, the onboard computer simply uses an intake vacuum reading to determine about how much fuel it thinks you're ultimately delivering, and it's generally pretty inaccurate. Before the lift and tires, mine was usually about 2 mpg too generous -- when I first bought my 'Runner, I was blown away that I was getting 23 mpg on the highway. Then I filled up my tank and was a bit disappointed. Since installing larger tires, it's now reliably about 1 mpg too generous.


For an accurate mpg calculation, I used TireRack.com to get the stats (circumference & revs per mile) on the OEM tires, then compared that to the BFGs I ended up with. All in all, my new larger tires are taking me 5.4% farther (and faster) than my odometer and speedometer think I'm going. So every time I take an odometer reading to get mpg, I take that into account (multiply miles driven x 1.054). It's the only way to really know what kind of fuel economy you're getting.

 

[Cornelius]

How much air do you have in the tires? You can try running a little more air on the street to gain a little mpg back but the ride may suffer.

That brings up another nagging question that I couldn't really nail down, even after all my searching here...

I'm running about 42 psi (cold) in the BFGs, as I was told the E-rated tires should have more air pressure. IIRC, that's what the guys at the tire shop set them to upon install.

Question is: If I'm not hauling some ginormous load, I shouldn't need to air up, right? I feel like 42psi is a bit on the high side, and I still feel like I'd rather have them at about 36psi unless I'm hauling a really heavy load. But either way, I'm assuming the higher pressure isn't hurting my mileage at all...

 

[xpeppyx]

I run my C load BFG's at about 42psi. I don't find the ride any worse than when I tried running them at 36psi.

 

[Martinjmpr]

I may go broke from gas, but I'll look damned good doing it! :bounce:

Or you could just slow the hell down.

Optimal MPG is typically between 55 and 65. Once you get past 65 your MPG will drop like a rock.