I know this might sound a little odd, but I'm curious if any of you have noticed any relationship between elevation and fuel mileage with your PSDs.

The reason I ask is because two months ago I bought my 2000 F250 PSD, 4x4 CC in Bend, OR. (elevation approx 3,400 feet I think). It had 41,000 miles on it. Within three weeks of getting it home (elev. 900 feet), mileage had dropped from an average of about 17.5mpg to 14.5mpg -- highway driving -- and on my road trip this weekend (flat freeway driving, no towing), I got 13.5mpg. I got these figures using a calculator, not the overhead computer, and always fill the tank as high as it'll go.

I think I've eliminated the obvious mechanical variables. After posting an inquiry on FTE a month or so ago about why it would be dropping, I've replaced the fuel filter, air filter, water pump (slight leak), put in Stanadyne lubricity additive at every fillup for the last 5 fillups and don't run it past about 2000-2100 rpm on the freeway.

The altitude difference just donned on me tonight, and it might be completely baseless. But this driving me nuts, because other than this the truck is running great.

Anyone else had this type of experience? Thanks for the input.

 

Mileage is a topic that has been beat to death in here. There seems to be such a huge delta between what folks are getting in terms of mileage. I know that I purchased my '99 in Bozeman, Montana and the owner promised that he got 17 mpg. I got 14.5 on the highway when I drove it home (Boise, Idaho). I am not sure of the elevation change but I know I am lower in elevation here in Boise than in Bozeman. I get 12.5 mpg around town and it hurts. In my opinion, we should see an increase in mileage as we drop in elevation not decrease (if any change at all). I am basing this theory off of having "denser" air at lower elevations than at higher elevations. I would also think that if there was a significant enough change in altitude to effect mileage (if such a thing can happen) then we would see some performance issues as well (good or bad, depending on if you gained or lossed altitude). I would love to hear from the physics majors or anyone who paid attention in high school science as to whether or not I am correct.

Craig

 

Some things to think about. The 2000 f-150 5.4 I used to own, my wife's 2000 Focus and my '96 F-250 460 gassers *all* get noticeably better mileage at higher altitudes, like in our mountains at 8,000' and above.

I think that's because the mass airflow sensors in the gassers sense there's less air going into the engine, so the mixture is leaned out proportionately. Of course, you also get less power.
So high altitude=less air=less fuel=less power.

How does that apply to a PSD? I *think* the 7.3s have a barometric pressure sensor that senses the altitude and adjusts the fuel accordingly. And I do know the new 6.0L diesels have a MAF sensor like the gassers do. If my guess is right, then the same should aspply to a PSD, lower altitude driving might tend to make mileage worse.

This is all the opposite of the carburator gasser and mechanical diesel days when it was common to see vehicles belching clouds of black smoke up in the mountains from overly-rich sea-level fueling.

FWIW, my '02 PSD auto with 3.73 axles gets 15.1 to 22 (not towing, with a couple hundred pounds of tools on board). The lower figure is 40% city, 60% highway mostly on two-lane 65 MPH blacktops, the upper figure is a constant 70 MPH, cruise locked on, summer fuel on long road trips. I don't have a mileage computer, I use a pencil and paper to calculate.

I have also noticed that extended idling seems to use and waste a lot of fuel.
Fuel sold in different areas will give you different mileage. Straight #2 like they sell on the west coast and south where it never gets really cold is the best, I lose up to 2 MPG using the winter blend fuel they sell in Wyoming and Montana. And there seems to be differences in performance between brands, too.