I just returned from the Adirondaks (nice family vacation) and am proud to report I got 18MPG for the average on this 1200 mile journey. This was true calculation using the trip Odometer and filling the tank till diesel hit the overflow at the top of the filler neck each time to be sure I was getting the same amount of fuel in.

I inflated my tires to 55lbs
I kept my speed down to 65mph which kept the turbo boost down to about 3-4lbs at the most

The truck ran like a champ!

(One funny aside was that we were guest of some friends at the Ausable Club, a very fancy-smancy private club on 8000 acres of the Adirondaks. The vehicles of choice of the NYC'ers "summering" there were Volvos, Mercedes, BMW Lexus, etc. I loved the looks I got rolling up in my PSD!!! You would have thought I was an alien landing )

One question- If the stock tire for my truck is 235/75/R16 and I have 285/75R16, isn't my odometer and speedo off just a bit so in fact I am getting better than 18mpg? Off by how much?

 

Hey! Great to hear about the mileage! Those are figures I'm hoping to get... I've found too that 65 seems to be an optimal speed for good mpg and is my speed of choice when I'm towing as long as I don't impede the "big rigs" that are trying to make a living. Did you have an additive or were you running straight "pump" fuel? I've been looking for Diesel Kleen but haven't found it in my area yet, if anyone knows where to get the stuff in southeastern New Hampshire please post, I'd like to try it.

 

Straight Pump diesel!

 

To answer your question about tire size......
Go here: http://www.4crawler.com/4x4/4LoCalc.shtml
A nice place to play with the numbers! By the way, the final gear ratio for the automatics is .71.......have fun.

 

you can get diesel kleen at most wal-marts... price is about the same as anywhere else... grey container for summer... white for winter i think, i remember the last time i bought the wrong blend... but no matter.... it still helps lube the injectors...

 

Thanks - Thats exactly what I was looking for!
-Ed

 

Hmm. Check again. The stock tires on my truck were 235/85-16. This has approximately the same rolling diameter as 265/7516. So, not a big diameter difference between 235/85 and 285/75.

 

Good catch - you are correct, my mistake. I just checked Tirerack.com to be sure!

I was just out testing the theory today on the highway, cruise control set at 60mph. I was passing mile markers between 56.5 and 58 seconds. The variance due to the fact I was using a stopwatch program on my palmpilot and just eyeballing start/finish. But I took a dozen readings and they all fell in this range. (my Stats Prof is weeping in his grave)
<O
So I have to agree with you, arninetyes. I am no math genius but what these results say to me is that if I figure a 3 second average faster time that is a difference of 5%
<O
So when I am traveling at 65 on the speedo, my real speed is closer to 68.25 mph and 18MPG is really 18.9. This qualifies for bragging rights to 19MPG in my book! Not the Holy Grail of 20MPG but I'll take it.

<OBoy I am glad I am not real particular about details! It makes research so much easier to result in the conclusion one was looking for in the first place

 

Old truck (7.3 CC 4x4) with 285 75 16 got 17.8 @ 70 mph @ 2000rpm New truck 7.3 CC 4x4 dually Got 15.3 with 235 85 16 @ 70 mph @ 2250 rpm Hope I can get better once I get new tires

Larry

 

Boy, I wish I got close to 20 MPG. but I'm happy with 16 - 17 MPG

 

Just recently purchased used 1999 Ford 250 PSD, internet purchase in Dallas, Tx. 3:73-1 gears, 6 speed manual, 2wd, bare bones no frills. Drove it from Dallas to Minot, ND. 75 mph Interstate speeds, tach was some where around 2150 I think....flat 20 mpg all the way...wow was I surprised. Bought it to pull a 24ft 5th wheel I had attempted to pull with my short bed gas outfit...yuk...not safe at all, trailer was driving me and knuckles were constantly white.
PSD had 67000 miles when I got it, now has 70000 so still has to prove itself as far as longevity and dependability but what a difference towing, completely comfortable, never drop out of sixth gear on the freeways, my worst overall fuel mileage has been 14.6, that was a mixture of towing and in town driving. I haven't driven out a tank of complete towing yet so can't give actual figures but the tank I got 14.6 on was 120 miles of towing and 250 in town, stop and go. I may be talking 14 - 15 towing??? Could I be so lucky?
BTW...so bored driving from Dallas I checked trip meter for accuracy several times against mile markers, it's right on, if any thing reads a little short but so minute it would be hard to calculate...

 

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14 to 15 towing? Now I'm upset. That is what my PSD gets when town driving EMPTY. Maybe I need to tow something to get better mileage...

 

I have read a little on how keeping the rpm's below 2,000 and/or keeping the speed below 60 mph to get better mpg. So installing taller tires will help with keeping the r's down right. Any body know how tall a tire can be installed before getting a negative effect?


Right now I have on some 305/70/16's for the summer and run 315/75/16's (35") in the winter.

 

Depends on your rear end gears. I have read repeatedly that 3.73 gears get best mileage with stock sized tires - either 235/85 or 265/75. I went from 235/85 to 285/75 and lost almost 2mpg on the highway, although I gained at least 0.5mpg in town.

If you have 4.10 gears, you can use a bigger tire without hurting your mileage. Supposedly, you can improve your mileage with bigger tires - to a point.