Why am i not getting good MPG?

th3godf4th3r · #1 Mon, March 30th, 2009, 01:43 PM

Hey everyone, i posted this over at f150online a couple days too but thought i would post here also

Well i just got home from a 300 mile trip.

anyway about 175 miles of the trip was flat and i had it at cruise control on 65

the other 175 were kinda hilly and going over one mountain pass and had it at cruise on about 75

by the time i pulled into my driveway, my truck computer said 15.2 mpg and my gryphon said 15.1

what im confused about is people like FishChris (on f150online) who said towing his boat he gets 16 mpg.

heres the modifications i have done to it

2008 Ford F-150 Lariat Supercrew 4x4
True Dual magnaflow exhaust with an X pipe
Gryphon programmer with custom tunes
33' BFG A/T's 18" XD monsters
K&N CAI
3.73 gears

City driving which is 95% of the time i only average about 12-12.5 mpg and never really get above that

i drive pretty conservatively too, no rabbit starts. I also do live in Seattle which has alot of hills but i would think i could get better than 12.5

now i didnt buy my truck to get good gas mileage but i just wish i could get some better mpg than i am getting

somebody said with my exhaust i probably lost some low end compression, this would hurt me going up the hills, is that true?

do you think i should try my custom towing tune? any other comments?

soutthpaw · #2 Mon, March 30th, 2009, 01:50 PM

I am sure Jack and others will jump in here too, but MPG reports are like fishing stories!!! (especially when the user's name is FishChris)

So take it with a grain of salt. Tire rolling resistance will make a big difference. a highway tread Low rolling resistance tire could easily net +1 or more MPG's. Wind resistance is a big factor too. I'll see if I can find a graph to illustrate that and post it. Asphalt vs concrete highways will affect MPG too.

th3godf4th3r · #3 Mon, March 30th, 2009, 02:00 PM

well i did just change my tire size to 2541mm which it should be with 285/65/18 tires.

the programmer said it was at like 2742 or something

hmm i didnt know asphalt and concrete made a difference!

soutthpaw · #4 Mon, March 30th, 2009, 02:20 PM

Here is an in-depth article on MPG and tires. there are a couple of interesting diagrams on page 40 that show where energy losses occur in automobiles.. There is also reference to road surface effects on MPG
Tires and MPG

***88Racing*** · #5 Mon, March 30th, 2009, 02:49 PM

Try using your custom tow tune.

But like Southpaw addresses there is a lot of different factors out there and they all add up for you or against you.

Take for instance:
Tire type.
Tire wieght.
Wheel weight.
Gearing.
Cai.
Exhaust.
Cats.
4x4.
Headers.
Trailing weight.
The list goes on.......

Lars

soutthpaw · #6 Mon, March 30th, 2009, 03:55 PM

Fuelly F150 MPG's

Is a good link to get more realistic MPG's

JWBFX4 · #7 Mon, March 30th, 2009, 04:29 PM

Well just to put my 2 cents in here... The 33'" tires aren't helping the mpg (heavier tire and more rolling resistance) My first thought was you might haven't correct your speedo with the turner...but I see that you did. Wind also plays a factor. But two of the biggest things is 5600 lb truck vs hills equal bad gas mileage, and anytime you get over about 65 in these trucks throw the gas mileage out the window.

Here is a little data I gather about my truck on a 280 mile round trip. Most of this is interstate. I found that if I put cruise on 67 mph (65 according to the gps) I can get around 18.9 Droping down a little more will get me 19,4+
I also found if I put the cruise on at 77(gps=75 actual speed) I drop down to around 15.5-15.9 Sometimes a little lower but thats about the average. Now before I get slammed for anything I said the conditions was pretty much the same on winter blend fuel running canned level 2 3.73 rear end. Also mpg was being calculated by miles driven divided by gallon's of gas, not programmer.

But just to throw this out there if I run around 55 pulling the ole bass boat I can get around 17.5 then the mileage drops after that. But with the tire upgrade coming those figures will be off now

***Jackpine*** · #8 Mon, March 30th, 2009, 04:35 PM

Southpaw and Lars pretty much covered it. I know Fishchris' truck, because he did a TON of postings in the "Appearance Products" forum in f150Online when he was getting his truck.

The boat he pulls is very lightweight, something less than 1000# with the trailer. His truck is an SCab, not an SCrew, so it's somewhat lighter than ours. It's 2WD, not 4WD like ours. It has 17" wheels. I don't know what his gearing is, but I suspect it's lower than 3.73.

So, everything I know about his truck tells me he SHOULD be getting better gas mileage than we do.

I averaged about 15.5 mpg in just under 1400 miles on our last trip at 70-75 mph (hand calculated at each fillup). I get around 12.5-12.7 when we pull our 3600# trailer. My tires are 275/65-18 Goodyear Wrangler Silent Armors and they have a fairly conservative tread - I'm sure mine have less rolling resistance than yours.

Both you and I have more bells and whistles in our trucks than Fish has. For instance, we have climate control, and it's ALWAYS on in my truck. That consumes energy.

I don't trust either the Gryphon's or the truck's MPG displays. The truck's is always lower than the Gryphon's average numbers and my "hand calculated" results fall between the two.

One point - unless your loaded tire circumference is correct, your speed and odometer values will both be wrong. For my tires, I've subtracted 3% from the unloaded tire circumference. Using Bill's calculator, that corresponds to a 7% rubber squish (since the wheel doesn't squish). I don't know what your unloaded circumference is, but you might try entering a value that is 3% lower for the tire size. (A larger number shows up as higher speed and more distance driven per tire revolution - and, when the Gryphon's speed readout is correct, the dashboard speed will be 2 mph faster, but the odometer will be accurate).

Finally, each truck is an "individual". Some will simply perform better than others and there's not a whole lot we can do about it.

Two more posts appeared as I was writing this "book".

The "fuelly", site was quite interesting, Southpaw. Looks like the trucks getting about 15 mpg are in the majority. And JWBFX4's remark on the tires is dead on!

- Jack

th3godf4th3r · #9 Mon, March 30th, 2009, 11:17 PM

ill trade ya fuel economy cory!

F150 Man 4Eva · #10 Sat, February 13th, 2010, 10:13 AM

Its me again. I searched for miles per gallon and find this thread. I just had a few question about my gas mileage. Before I start, let me say that I know that my 35" Nitto Trail Grapplers that weigh like 80lbs. a piece are a BIG factor in my mpg loss. Here are the facts..which within lies my question. With my mods (K&N FIPK and Flowmaster 50 series) and my 35" tires, but before I ever got the Gryphon...my MPG would vary between 12-14. Now when I got the Gryphon but only had canned tunes...I ran the canned 87 Tow tune. Mileage around town was about the same, maybe a little better. On the interstate however (going like 75) I was getting around 15. This made me happy. Now maybe I was thinking wrong, please tell me if I was, but I assumed when I got my custom tunes and ran the custom 87perf. that I would get better MPGs. Well going around 50, I can get like 16-17, going on the interstate (70-75) I am getting right at 11. Whoa now. What happened? Is the custom perf. supposed to give the best gas mileage? Let me also throw in that I am NOT out to get 17-18,hell even 16 Mpg, and I know I have a big 4x4 truck not a car. And I also know it is pretty much give and take with power and MPG. I just need to know if this is normal. Thanks yall...Travis