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Sounds like you're digging into it carefully. Getting your speedo is about as easy as it gets. The local HP folks will have you, either on the road or on a dyno, go to different speeds: 50, 55, 60, etc., based on what you are seeing on your speedometer. If on the road, they'll either pace you or have you go by a calibrated radar gun, which will provide what they will see (better option for road driving). If done on a dyno, same thing, but they get the readout off their meters (more accurate for technical uses).
If there's a speed shop in your local area that does full rebuilds, they often have their own dyno where you can check and calibrate and, perhaps even more important, run an output check and tell you how much horsepower you're actually putting out, both at the engine and to the ground.
As to gear ratios, as you've already learned, mileage does vary, even with the same tires, by gear ratio. You are probably already aware, but just in case, the higher the numerical ratio (3.91 vs 3.73), the better the torque for pulling power and acceleration. Unfortunately, it also means lower mileage. A lower numerical ratio (3.31 vs 3.73) will give you better mileage overall (with a few exceptions), but it also takes longer to get up to speed and you can't tow quite as well, particularly uphill.
There are some other ideas I've sent forward to folks on the forum that I found worked well for me, resulting in Highway mileage in the mid-20s, and just under 20 while towing, and my city mileage is still averaging in the high 18 range. I also replaced my tires, but didn't go with as large as you did.
Good luck and I hope some of this is of use. If there's anything else we can do to suggest improvements, be sure to sound off.
I was figuring out a mathematical way to figure this out since you have diameter, you have circumference so then it's just a math problem......then I just got lazy and googled to find this converter online. So my speedo difference between the old and new tire is exactly 5.5% based on the diameters on this converter. Ironically exactly the same percentage difference between the two tires in height.......fluke? The circumference is also 5.5% different so it could have been as easy as that.
So my average MPG on my gauge should also be off by 5.5% meaning my 10.8 average it's giving me since the tire change is actually 11.4 MPG which helps a little. It's also improved a little over the last week or so with more highway driving.
Question if anyone knows.... is the average MPG on the gauge a total average of all miles driven since the last reset .... or a certain number of last miles driven, like say the last 500 or 1000?
Last edited by southcst1; Oct 21, 2015 at 05:50 PM.
Question if anyone knows.... is the average MPG on the gauge a total average of all miles driven since the last reset .... or a certain number of last miles driven, like say the last 500 or 1000?
Its since the last reset.
Dont forget we are driving a brick so just because your on the highway dont mean always great mpg.
6th gear kicks in between 45 & 50 mph in a stock truck. That is when you will get the best mpg. The faster you go from there the more drag you will make and your mpg will start to drop.
Question if anyone knows.... is the average MPG on the gauge a total average of all miles driven since the last reset .... or a certain number of last miles driven, like say the last 500 or 1000?
Its since the last reset.
Dont forget we are driving a brick so just because your on the highway dont mean always great mpg.
6th gear kicks in between 45 & 50 mph in a stock truck. That is when you will get the best mpg. The faster you go from there the more drag you will make and your mpg will start to drop.
I figured probably last reset but wasn't sure. I know they have no aerodynamics and worse since I lifted mine. It's been kicked around on several threads and probably this one that the best mileage is under 2000 RPM. That seems to be the consensus from what I've seen. I haven't really paid attention on mine since the lift and tires what speed that is in 6th gear but I'll check it out.
good advice, and he didn't mention what speed he was doing on his highway trip. even so, he still beats most of the 2.7s on fuelly.
I was going between 65 and 70 MPH on the highway,I could understand if I was going 80 or more mph and getting less mpg.It was also real windy on the trip,the wind was bad enough it was blowing the truck around,so Im guessing on the same trip with no wind and the truck gets broke in it will get better.
Here are some cherry picked miles for ya'. Full disclosure, the whole trip was 26.5mpg and the truck was warmed up when I started. Still...damn near 30 for awhile there.
5.0, 3.55, 4x4, 275/65/20 Baja ATZ, 1" level, 5 Star e87 PT tune, Resonator Delete w/ Magnaflow Muffler. This was the best mileage I have seen in the year I have owned it, I had 1 passenger and an ATV on the back as well.
Wow...I have a 2013 FX4 5.0 SCREW, 3.73 gears. I had the stock Pirelli 275/55R20 all terrains on it. Just put a 3" Icon coilover leveling lift on it with Method 17" wheels and 35" Toyo M/T's. Looks great but the mileage went all to hell. I was expecting a fair drop but for a few months all around city and highway I was averaging a very consistent 14.2 mpg. It's only been a couple weeks but my average has dropped to around 10.8. I'll give it some more time but don't see that number changing much. Driving style hasn't changed. That's about a 3.5 mpg drop! Keep checking under the truck to see if I hooked onto a boat anchor somewhere. Maybe I'll trade this thing in on a 6.2 Raptor so I can get into something economical.
Here's what happened to you, I believe: 1- the lift kit disrupted the airflow around the bottom of the truck. Oddly enough, that is considered when the engineers design the vehicles, so optimum flow includes things like the lower valance panels, air dams, etc. 2- I'd be willing to bet that with the change in tires, their overall diameter is bigger than the stock Scorpions (I had the same tires on my '11 FX4), as well as being substantially heavier than stock rubber. 3- the more aggressive tread of a M/T tire vice a M/S or A/S tire also decreases mileage a bit. Combining those three factors, it's not surprising that your mileage numbers dropped significantly. It's a moot point now, but I bet if you went back to your stock configuration, mileage would go back up as well.
Unfortunately, there's a tradeoff to be had when you go for a more aggressive look and better off-road grip. Wish there was better news to be had.
Here's what happened to you, I believe: 1- the lift kit disrupted the airflow around the bottom of the truck. Oddly enough, that is considered when the engineers design the vehicles, so optimum flow includes things like the lower valance panels, air dams, etc. 2- I'd be willing to bet that with the change in tires, their overall diameter is bigger than the stock Scorpions (I had the same tires on my '11 FX4), as well as being substantially heavier than stock rubber. 3- the more aggressive tread of a M/T tire vice a M/S or A/S tire also decreases mileage a bit. Combining those three factors, it's not surprising that your mileage numbers dropped significantly. It's a moot point now, but I bet if you went back to your stock configuration, mileage would go back up as well.
Unfortunately, there's a tradeoff to be had when you go for a more aggressive look and better off-road grip. Wish there was better news to be had.
I knew all those things would affect my mileage, I just didn't think by so much. Now that I've put a lot more all around miles, and more highway miles on it I'm up to about an 11.6 to 11.7 average. Factor in the speedo being off by 5.5% from the tire height increase and I'm actually getting about 12.3 to 12.4 MPG which is more like what I was expecting.
