ecbrewer2224

hey i havent changed my fuel or air filter since ive owned it and i got the truck in july of 2006 and have put 7500 miles on it since i bought it my dash isnt calibrated to my 315s but my a2 monitor had a thing on it where i could put my old and new tire size and the monitor tells the right speed and the dash doesnt when the dash says 60 the monitor says 67 and my truck had 265s on it from factory not sure of the rearend gears like i said on the highway it seems like i could use a 7th gear to keep the rpms under 2000 also the way i calculated my mileage wasnt by hand i just waited till my gas light came on and put gas in then as soon as it came on agian i looked on my dash at the trip counter always right at 300miles so 300 divided by 30-32 gallons 9-10 mpg

xtoyz17

Ecbrewer,

Read my posts again a few times. Replace filters, replace fluids, do all your regular maintenance, check to make sure brakes aren't dragging, and a dozen other things listed. To get your accurate mileage you're going to take the miles driven, divide by gallons put in, and then multiply 1.06 or so to get the correct number. Try driving your truck with the chip turned on, etc...

Until you've tried all that I don't know what else to tell you. Your mileage isn't going to get any better by just talking about it.

stickler

That's it right there. Here our max LEGAL speed limit is 62, and at that is where I'll average 22-24mpgUS. Slow it down to 55 and she picks up 2-3mpg. Down to 50 another 2mpg at least. My truck's only been over 62 once in its (relatively short) life, and I highly doubt if it ever will be again. The difference between 62 and 75 dropped mileage to 15, just like so many here are complaining about. I'll never be in that big a hurry that I can afford to throw money away on speeding tickets and wasted fuel.

To each his own though, whatever floats your boat. But....I don't care WHAT you're driving, you just are not gonna get your best mileage in a 7000+lb vehicle running at 75mph and higher. You could have 3 or 4 MORE overdive gears and you still to the point where wind resistance overcomes gearing. Even big rigs will normally get their best mileage somewhere down around 60-64mph.

cwgtex

Not exactly.

The speedometer calculates how fast you are going off the odometer, which is calculated by how many times your tires turn.

Example:
(RANDOM NON-ACCURATE NUMBERS)
Lets say stock your engine computer knows that after your tires rotate 100 times you have traveled a mile. Therefore the computer knows that if your tires turn 100 times in a minute then you are going 60 mph (1 mile per min, 60 min in 1 hr).

Then you get bigger tires and now it only takes 80 rotations to make a mile. If you don't adjust the computer, it still thinks that 100 rotations is a mile. Therfore for every mile the computer thinks you traveled, you really traveled 1.25 miles. This results in the speedometer error, in our example you would be actually going 75 when the computer thinks 60.

So with an unadjusted engine computer, even hand calculations are inaccurate because you are basing it on the incorrect odometer. When I first upgraded from stock tires to BFG 315s, my overhead was reading between 12-16, and my speedo was 5mph off at 70mph (according to my GPS). After I bought a smarty and adjusted the computer, besides my speed being correct within 1mph, my fuel economy shot up to 17-20. But this is still less than the 22+ I was getting before. Several factors attribute to this.

1. wind resistance
2. more tire = more weight
3. bigger tire = more friction with road

Larger tires definately do affect fuel economy. And it even looks worse if you don't adjust the computer. Just my $0.02

Night Rider

Hey ecbrewer,

I have a 01 3500 and I get 21 to 24 highway and around 17 to 18 city. One thing that I learned is that speed is the biggest killer for your mpg. I drive down to Florida alot and did an experiment one time from Maryland to South Carolina I never got above 65, averaged alittle over 23 mpg. The rest of the way I did the 75mph that you stated you do and got right around 19. Figured out real quick that my lead foot cost me more than just the tickets I used to get.

Try and keep your rpm's under 2000 while cruzing on the highway and don't wring it out while your shifting and you should see a big differance in your mpg. Just my opinion on this. Oh and I have the NV4500 5 speed in mine.

Dangerous Dave

According to my GPS my odo is right at 2 mph optimistic at 60-65.

Right at 1850 RPM is 65.2ish.

And that is the speed I rarely exceed.

I got 705.x miles for 35.xxx gallons for 19.2xxx mpg on my last fill up.

rick nick

Question 2001 5.9 auto ran out of fuel refilled and now milage is 13 14 mpg , before running out of fuel was getting 18 to 22 what happened?

patdaly

Do you have a fuel pressure gauge or light? Could be your pump isn't running right. Verify LP pressure and volume.

rick nick

I have a fass 95gph and a light seems to be lots of fuel good air filter and fuel filters.

patdaly

Have you done the key on trick to see if any codes are present?

I can't think of anything positive that could drop fuel mileage that drastically other than the obvious things you have checked.

rick nick

there,s been no codes , probably need a new pump, thanks for the input Rick

patdaly

I wouldn't bet the farm on a new pump, I would say you should see a code if the pump is harmed.

Perhaps a 24V guy could help you more, I am still stuck in the farm tractor era......

rick nick

thanks for the info patdaly

patdaly

Why don't you start a thread up in the 24 Valve forum?

https://www.dieseltruckresource.com/...train-f89.html

I would bet you get a pretty good response, we have some of the best guys around.......

Gumby79

14 months worth of 22avg 27high last