Huge swings in fuel mileage

[jbolt]

For some time now I've been getting large swings in fuel mileage tank to tank. Normal MPG is 15. 5 to 16. 5 with the day to day driving I do. One tank I'll get 16mpg and the next I'll get 12. It is never consistant. I can go for weeks getting normal mileage and then I'll get a low one. The routes I drive every day are pretty much the same so this shouldn't be a factor. Fuel filter has been changed a few times "just in case" No fuel in the oil. I fill up at the same location every time.



Now I'll throw 2 things in I've noticed.



1) EVERY time I use a fuel additive, no matter what brand, my mileage is always worse for that tank



2) Randomly, over 70mph, my speedo starts to bounce between 60 and whatever top speed I'm at.



Any ideas on what would cause large swings in fuel mileage?

 

[CDegner]

Are you getting worse mileage when your speedo is bouncing? Since the speedo/odometer are electronic, you might not actually be doing the miles your truck "thinks" it is doing. Hence the mpg difference. Don't know, just a thought.



Carl

 

[JLEONARD]

I would agree with Carl...

Otherwise

1) stop using additive

2) keep it under 70



Jay

 

[jbolt]

ECJ



Big help you are. Go float something.



The speedo thing does not happen that often but I have wondered if I'm getting false reading on my Odometer. According to the portable radar machines the local law enforement puts up around town to remind everyone how fast their going my speedo is accurate... well below 45 that is. Maybe I'll have to try going past one a 75 some time. on second thought... maybe not.

 

[Greenleaf]

I second that........

Both actually... ...



You'r speedo drive is quite possibly the culprit. Mine will read incorrectly the faster I go,and...



ECJ is no help, what-so-ever. :-laf



... "go float something"... :-laf :-laf



-S

 

[JLEONARD]

Go float something.

I'm thinking some ice cubes and 2 garlic stuffed olives... that'll be thursday when I'm in Fl



WCJ... . get a friend with cruise control to drive ahead of you with his/her cruise set at 70..... that'll give you a good idea of how accurate your speedometer is.

I've done that and found mine reads about 3 high at 70.

Hope this helps.

ECJ

 

[EZGZ]

Just borrow a GPS unit from someone and lay it up on the dash.



It will give you an accurate reading of MPH, top speed and distance to within a few feet.



Yes, it does caculate total mileage.



Careful ... I hear you can become addicted to that toy. Its as bad a PC and a good forum

 

[jbolt]

Okay I did the GPS thing. Very cool device. It even talks to you and gives directions. I gotta get one for the wife so I can sleep when she drives :-laf



The problem is NOT my speedo. Up to 90mph my speedo is accurate within 1mph.



SOoooooo... . that still leaves me with a mystery as to why I'm getting big fuel mileage differences between fillups.





WCJ

 

[dpuckett]

Are you getting fuel from a truckstop, et al? You know, somewhere that handles a large volume of fuel daily. Or are you getting it catch as catch can, where ever? That could be a difference. One place here is better than any other local fuel station, and it hapens to be next door to the Frtlnr dealer.



What about weather differences? One tank a few weeks ago (during our ONLY snow this winter) I got 14 mpg!!! But it was a lot of plowing thru slush at 25 mph; running 600+ deg instead of my normal 350-400. I normally get around 18 round here, on the rare occasions it dries out. Hate the mud, hate the mud, HATE... . THE... . MUD!!!



Daniel