Ok I had 13.6 mpg to 14.9 on my 99 PSD 7.3L 4x4 With 3:73s LS, Stock Sized Tire, Ext Cab, Short Bed, Oh and a bug deflector!

I went crazy trying to find my loss, I had only owned it for about 4 months and thought that was really bad, and that it never changed was also wierd! I took it to Ford and they guess checked and missed real bad, I was told it may be turbo or what ever, NOPE!

I was up there late one night and talking to a senior tech and the guy asked if I ever changed the tires, I had not, but the guy before must of, but they were still stock size, but as per his reccomendation, he said use a WDS ( I think) to check what the computer says the size should be.
To my suprise the tire size was ____________! thats right ford forgot to set it!!!! it had nothing there, with about 4-5 clicks on the WDS he set the tire size and engine rpm setting for those tires. It fixxed tons, my MPG went up to 17.6 - 19.1 and more power less lag! This was at 75mph avg hwy, with a couple stops around town. I still havent seen city driving, I will see that soon!

But check that setting I got it done for free, just ask late at night and maby give the guy 10$ or something and they will do it, It works better that my 100$ deductable for the new turbo!

Jt

 

Glad things worked out for you!

 

I haven't heard that one before. Cool. I think that maybe I should check that out. I have a fairly good relationship with my dealer and maybe he will check mine out for me. I only get about 13 to the gal right now. I thought that it was because it was new and was hoping that it would improve.
The more I visit this room the more I learn. Thanks Guys.



God Bless our Troops:

 

I'm curious how that would affect real mileage and power... as far as I know, there's nothing in the programming that uses that data for fuel mapping on the PSD. I could see how it would affect *calculated* mileage, since your odometer would be reading incorrectly.

 

The only way I can see that the tire size setting would affect your mileage is that the miles recorded on the odometer would not be accurate, thus the wrong mileage would be used in calculating MPG's. If you have a trip computer, the wrong input data would be used to produce the computation.

Physically, larger tires would effectively lower (numerically) your final drive ratio, and vice versa, affecting mileage. This change would be slight, IMHO.

 

I'd tend to agree with LW555, other than transmission shift points, I don't think anything that would affect actual mileage is affected by the tire size reading in the PCM.

I guess that could account for some gain, but I don't think it would be all that significant. Maybe someone knows definatively?

Waxy

 

According to ford and the WDS it sets a RPM at # of tire revolutions, if that is not acheived, aka towing, it gives more power, that is how ford's PSD gets more power when loaded.

As for mpg, it keeps the engine from working with "load" all the time.

This is the explanation I have received.


But! - after that trip, I have driven around town and my city MPG appears to be at 12 (less than before!) I havent figured it yet, but I am at just above half tank and only 120miles on it :-(

Will post update.

Jt

 

I have the PSD as well, and whenever I press the MPG button, it ALWAYS reads the same MPG. I get 13.1 MPG all the time. Is that good or what? Cause I think thats terrible lol

 

I never trust the digital gague, it is on a mathmatical function, something like tires roatation / rpm = mpg (just BS, some one knows, tell me) or what ever, but it is real un-accurate real way is to fuel at the same way, on the same pump (Really not just the same place, but SAME PUMP) and record your miles per full tank, then devide those by how many gallons you take to fill up, then round to the nearest decimal.

As for 13.1 I am getting that in city now, people are not clear on this board (sorry guys I am not crackin on you) they say they get 18mpg, I do too you probably too, that is no load, highway with cruise control, hell I got 19!, but city I fell to 13.4 in winter, and summer I suspect 15 or so.

Also for driving it keep it below 2grand (RPM) when highway, I was at 2g doing 74 to get 18mpg, then at 2300 at 80 I got 17.4mpg.

One last thing your truck is a big difference too, what gears and bed, and cab, and is it 4x4 all this affects mpg.

Hope all this helps!

Jt