Home Forums Gear Hunting Trucks and Four-Wheel Drives F250 Diesel Engine MPG question

What mcmurphrjk said. When Ford went to the 6.4L they also had to include additional emissions equipment and mileage is down. Ford now has 3.55 gears available in the diesels to help with mileage, and going forward (2010 model year) they no longer offer 3.73 gears in a single rear wheel (SRW) truck. You now have 3.31 and 3.55 gears available in hopes of increasing mileage still. Obviously as rear wheel torque is a function of gearing you lose a little torque due to the higher gears (numerically lower) but will maybe squeak out a mpg or two better than previous years. If you go to the diesel forum on ford-trucks.com you'll get a good idea of mileage in the new Ford diesel trucks. By the way, the F250 and F350 are the same engine and drivetrain, so don't let that sway you.

There were some surprised and pissed off people when the 6.4L came out and the new emissions requirements had to be met. The mileage was much less than some people thought and it appears Ford was listening and that is why they are trying to use higher gearing to get a little better mileage.

Good news. My '97 Dodge diesel with a 5sp and 3.55s never hurt for power. I'd gladly run 3.31s in the '06 Ford I've got now, if Ford would build it that way so everything coordinated.

Run a taller tire to get an effective gear ratio that is lower numerically.

They've got me believing it's not that simple anymore, and I don't understand why it wouldn't be. From what I gather, if I put 2" taller tires on my F-250, mileage will go down.

What mileage do you get with it now? My 2003 f250 gets 16 mpg on the highway. It has 315-75-16 tires on it which are supposed to be 35", although they are more like 33".

As far as the USLD, it doesn't seem to be influencing mileage in the summer, but the Winter diesel costs me 15% in fuel economy in the Semi. I hate the stuff.

FWIW, I was delivering to a customer the other day with my '97 F250 (which still gets 18-19 MPG with 280,000 miles on the clock), and the customer was complaining about his 08 only getting 10 mpg. He wanted his '98 back.

As an aside, how can it be good for the environment to burn 50-80% more fuel? A country bumpkin wants to know.... JMO, Dutch.

My 97 Cummins with the 3.55s does well as I can get 20 mpg pretty easy if I keep my foot out of the pump. The 2010 emissions will be a bugger. They are talking urea tanks on our dump trucks. I have not heard of too many good reports as far as fuel economy goes with the DPF trucks. My dad's 6.0 can average close to 20 if you keep it 60-65. He has the 6 sp. manual. My brother is in the process of putting a 1995 Cummins in his 1995 F350 4x4. He is going to use a Superduty 6 sp. manual and transfer case. Economy should be similar to my Dodge as he has the 3.55 gears.

I have 91 DODGE 3/4 TON 4X4 WITH 12VALVE 5.9 CUMMINS .It has 200K+ and gets 17-25mpg the only thing I do is add 1 QT of SEA30 oil in with the fuel on every fill up and plug her in when it gets cold.

Moving up to 35" tires is virtually guaranteed to cut your fuel mileage,unless you have a 4.10 axle ratio.

What are your thoughts on this then: I am mounting up some 37x12.50x16.5 Goodyear Hummer radials on my 2003 F250 diesel (6.0), due to the fact that a gent nearby sells the military surplus version of these tires (95% tread) for $100 each. This beats the heck out of the BFG All Terrain 315-75-16s that cost me about $210 each that I have been running. (As an aside, I laugh everytime I see people with the large offroad tires that are 20" or larger rims, as these tires usually are $400+ EACH!!! :o). I have 3.73s now, and with the 315 tires have NEVER ever gotten better than 16 mpg empty. I am also installing 4.30 gears, this should bring back the RPMs into the "sweet spot" hopefully. Now I am curious what I will get for mileage.

Seems like you'll have to go through an aweful lot of those discount tires to pay for the gear swap.

That would be true had I not gotten the rear end gears for free from a friend who swapped out his gears, the same friend who will be installing them for me (again for free). Just have to buy the fron gears ($250 or so) and have the same friend install. One set of these milsurp Goodyear tires will more than pay for what I will spend swapping gears!

That changes the equation Sounds like a good deal.

plus all the wicked clearance you'll gain.

did you have to get a set of 16.5" wheels?

Yes, I found a used set of Weld Racing "Mountain Crushers" 9.75" wide for $240 with 37" tires that had only about 25% tread left. Sold the tires only for $150, so I have "net" $90 in 4 rims that sold for $350 each new!! God, I love craigslist!

sweet!

My 7.3 still gets 20+ mpg with my special fuel mix. Every fill-up with that cheap watered down junk is mixed with 12 ounces of ATF. Boosts the fuel mileage right back up to where it used to be before the junky fuel came along.

Any of you guys tried mixing Marvel Mystery Oil to the new diesel? My mechanic said to try it. But I have reservations about damage in my '99 7.3

I doubt Marvel Mystery Oil will do any harm to your 7.3.

One of the mechanic here has been running his truck on used ATF for the past several years. Not something I would do but he gets the ATF for free.

OK. But will it do any good, then?.....