Questions about my milage

[Powermoon]

So I've been averaging around 14.5-15 highway mpg since I bought my rig last July with 74,000 miles on er. I've also been using powerservice on every fill up since last November. My first question is, should I recalubrate my speedo for 285/75R16 tires and if so will my odometer start reading the correct miles after I do this? I try to hand calculate my milage after every fill up and figure since I haven't done the recalubration then I'm actually getting a little better milage then what I am showing (is this right?).

Also, last weekend I went on trip over the cascades and I got my usual average using powerservice and traveling at speeds of 65-70 mph. The truck currently has 84,000 miles on er. I again filled up at a Safeway on the westside before heading home without using an addative and actually averaged 17 mpg on the way home traveling 75-80 mph on the mountain pass. Wind was not a factor too and from the westside. As you can guess the milage was a big surprise to me and I'm trying to figure out why it went up? Is it possible that in my neck of Washington that we are still using a winter blend of diesel, or do different areas (same station though) use a better quality of fuel?

Sorry for being long winded but any answers to my questions would be much appreciated.

Moon

[Heat]

The one thing I can answer is that if you want accurate readings you need to have the speedo set to match the tire circumference difference... That will also reflect accurate mileage on your odometer and accurate speeds on the speedo...

I don't know if your mileage has changed since you bought it but they wouldn't still be using winter blend I'm sure... I get between 15.5 and 16 out of mine around town with the A/C blowing... On the highway I get roughly 20 to 21... These numbers are all based on an unloaded truck....

Ken....

[powershooter]

in my opinion no two fuel stations have the same quality of fuel. you probably got better fuel.

[sea71]

Im with powermoon on this one. I , too , run 285 tires and have never had a recalibration done. I also show roughly the same mileage he does. Not to hi jack, but how or where do you get the recalibration done? Thanks in advance.

[250KR]

I just don't know how you guys can get 20-21 out of yours. I have used Stanadyne additive in just about every tank since day one, and a I religously change the oil-(Amsoil), get it sampled, etc. In other words, I should have optimum mileage. I don't "hammer it". Some might say I don't drive it hard enough. I bought it to pull my boat. Anyway, it's an '07 with factory 20's and a 3.73 ratio. I usually go no faster than ~ 73MPH when on the highway. (With the 20s, this is about 1,900 RPM). The best I can get is about 18.2-18.5 unloaded, on very flat ground. Why? By the way, I have 19K miles.

[powershooter]

i am getting 20 mpg. little on interstate and back roads going to work everyday. if i stay on the interstate for a few hours ,4 to 5 min, i get 23 -24 mpg stock truck. i belive my milage comes from me having a manual tranny. honestly dont leave high gear unless starting or stoping, and unless rpm's drop below 900. i dont drive it hard and am conservative with driving technique. although once and a while i drive it hard to clean it out.