Maybe I'm wrong but I've been told be several people that tire size doesn't hardly matter with these diesels. I have the 6.7L auto 4 door 4x4 long bed with 373 gears and 35" tires. The truck drives straight so I don't think it needs an allignment. I have the tire pressure up. Even when I'm easy on the throttle I can't get over 12mpg! Most of the time I'm getting 10.8mpg... the mileage didn't really bother me till I started thinking of what I could buy with the money spent on fuel! I could buy a brand new car that gets 30+ MPG and pay for the gas for it for the cost of my fuel alone. That would save the miles on my truck and keep the value up but I really like driving it. What do you all think. Maybe there is something I'm missing? I know it ain't even close to being broke in at just over 5000 miles but I was getting better than this with my '05 just off the lot. I did notice that I seem to get better mileage if I leave it in 5th gear. Would there be a better gear set than 373's?

 

that seems pretty low but it usually takes 20k or more miles before fuel mileage starts to increase a little, i have had 6 or 7 CR's and it has taken at least 20k on everyone before i noticed any increase, you may try an intake and drive the he** out of it, S&B and Air Raid have intakes for the 6.7l, my dad usually has the best running trucks that get the best fuel mileage, but he will take a trip when he first gets one and drive in like 3rd gear at 3k RPM's for 100mi or so, im sure as we learn more about the 6.7 dodge will come out with reflashes that will help them out. good luck!!

 

I don't think the 6.7's are going to get great mileage to begin with. With all the emissions stuff it's just not as efficient as it could be. Don't think removing the exhaut components will help either. I think it has alot to do with the EGR and all the exhaust gasses it's recycling. These new motors will probably just stink in the fuel mileage department no matter what.

 

So you own the first 6.7 and you and your friends built it , yall probably EDIT it up when making it... Just kidding, cool that you have that anyhow.

Might want to do some 4.10 gears seriously that sounds like your problem there.

12mpg is unacceptable.

 

WOW! that is lousy mileage, God help yu when prices go up for the summer!

That can't all be due to displacement.

I thought 373 got better mileage that 410???? Arent they stock?

 

Couple things:

I thought all six speed auto's came with 4.10's?

I think if you are having to keep it in 5th gear you need to upgrade gears for sure and it will be worth it... I'd go deeper than 4.10's if you are having problems here. maybe 4.30? Calculate it out well and see what stock people on stock tires are doing with 4.10 gears too.

Also if you are using the overhead and they finally made it accurate if you haven't adjusted your speedo it's going to read low since you are traveling less miles per gallon burned according to the computer.

I'd also do intake and catback and check everything over to make sure nothing is clogged.

 

I have a 2007 6.7 with 6 speed auto and 3.73 gears. I am also running 37 inch toyo mt's. On my maiden voyage from Idaho to Las Vegas, I got 17 mpg, all freeway. I am getting around 15 - 16 mpg now, 60% freeway driving and 40% stop -n -go. All of these numbers are hand-calculated and I had to convert my tires to get the right speedo and odometer readings too. I am pretty easy on the skinny pedal too. I haven't towed with it yet but plan to in 2 weeks. I'll let you know my results. I hope this helps. Cheers!

 

Did you recal your speedo to your mew tires? Is that MPG hand calculated, or the overhead? The overhead is often off.

 

mine got crappy mileage like that when I first bought it but it gets better the more miles I put on it and I have 37in tires.

 

Nightowl, If your original tires were 265/70R17 your tire higth was about 31.6" by going to 35" tires your 3.73:1 diff gears are now in effect 3.30:1 gears.

That is to say that when your spedo shows 70 mph you are actualy going about 79 mph.

Also for your truck to have the same amount of power, towabilty/rating it had with stock tires you would need to change the gear ratio in your diff's to about 4.20:1

Could this affect fuel millage? sure it could thats why you do better in 5th than you do in 6th, not to mention you could get a speeding ticket, be under powered when hauling or towing a heavy load.(well you could just use a lower gear than you would normaly have to).

If you check out this site you may want to check out the calculators on the left side of the page, quite usefull. http://www.4lo.com/calc/geartire.htm

 

From personal experience, Id stay away from Airraid.
I put a dropin on my 05 and after 8000 miles, the prefilter had been sucked into the pleats of the filter.
There is a pic in my gallery.
I changed it out for a AMsoil and am very happy with it.
As for the mileage, it should get better as the engine breaks in.

 

I test drove an '07 with 3.73 gears and it was only turning 1900 rpm at 75 mph with the 6 spd. auto . Putting 35" tires on it would have dropped the rpm to about 1600 at that speed and I would think even as powerfull these new engines are, you are lugging it quite a bit considering they aren't the slipperiest critters on the road.

 

I have a question that goes along with this thread. I have 315s on my 06 mega and have been trying to figure out my fuel mileage. If the tires effect the speedo, then it would effect the odometer as well.. Since I am not able to adjust my speedo/odo, then how can I get accurate mileage?? I used my gps and figured that my speedo is off about 10% on the low side. By hand calculations I am getting about 15 using the odometer.. the overhead is saying 19.

 

Big Tires = lots of rolling drag = crappy mileage. Always has and always will.
You want big tires, then you pay at ther pump. Simple eh?

Oh yeah, I only get 5 mpg at 108 mph. Something must be wrong with my truck too.... Why can't i get 24 mpg like all the other guys?

 

Joe T Thanks for that info. Man I feel like a dumba**!!! Really, I do! I have told numerous people to not go by the over head console for anything else but an idea as far as milage goes. Guess what I was doing! Man, somebody slap me!!!