Hello Scotty, I got a 2012 Ford F-150 XLT 4X4 with the 3.5 eco boost engine at 168,000 miles. Yesterday I noticed that my display on the dash had 430 miles when I went out driving 3 miles I noticed my ‘miles to empty’ had dropped to 415. Same deal today when I did an 11 mile trip, it dropped 20 instead of the expected 11. I did some research and a forum suggested it might have been a bad canister purge valve. I swapped out the part with a new one and same issue. What do you think?
P.S.
i have a dual catch can system mod I’m not sure if that might have something to do with it?
Those gauges just guess based on past mileage and current driving.
Were you driving a bit more city, or less straight highway perhaps? For that 3 mile trip, it was a start, idled, then a very short trip amounting to bad mpg I would guess.
My prius used to jump all over, I didn't pay too much attention to it. Drive the entire tank out and see how much you actually get before worrying much.
No mileage guessing gauge is going to be perfect, unless you drive the exact same route, with the same traffic, at the same speed, every day.
@nlord Usually I take my truck to work so it at least would get on the highway for 70% of the trip and its that 11 mile trip I was talking about. Yesterday that 3 mile trip I went to a restaurant which was mainly city driving which I guess I don’t really do. That’s when my miles started dropping like crazy like a mile every 30 seconds it just has been worried it’s a gas leak or something?
Thank you for your two cents
It is good that you are paying attention to your truck. I would suggest just setting your trip odometer when you fill up, and after driving at least half a tank, check it for mpg. Only worry if it consistently drops like crazy, even when doing steady highway mileage. I drive my Matrix every day for work, and it will vary between 24 to as high as 37. The thing is, I may be in bad mountains with city and idling, or I could be flat level highway and getting crazy good mpg. Normally I get about 30 though.
As @dad2lm2 said, don't change any more parts unless you are sure there is a problem.
If you like working on your vehicle, and like practicing, get an Audi. Those will give you all kinds of codes, pretty dash lights, and issues you can replace parts for, and you will for sure know when something is broken on those! 
Welcome new member.
Lots of things can do that. I doubt the catch cans are the issue, but don't quote me on that. If you have a scan tool, make sure there are no EVAP codes.
At that age, it could be the fuel pump assembly or the sending unit. But those kind of small variations do not necessarily indicate a major issue brewing. As long as it is running well, I would keep an eye on it but wouldn't change any other parts until something more happens.
Good luck
@dad21lm2 Thanks a bunch for your insight. As far as I can tell it’s running like normal I’ll definitely keep my eye on it and see if something else pops up, I don’t even have a check engine light or anything like that either
Please check the FAQ - improving fuel economy.
As others have said .... not a good idea to replace things that aren't broken.
Built-in fuel computers are not accurate. They're only meant to be a guess.
Take a proper measurement:
- fill your tank (stop when nozzle clicks off)
- record your odometer
- drive until mostly empty
- fill your tank (stop when nozzle clicks off)
- print a receipt and write your odometer on it
Calculate fuel economy:
- subtract odometer reading #1 from reading #2. This is your distance driven
- divide this distance by the fuel quantity (gallons, from the receipt you kept)
- this is your true MPG
- Do this a few times to get an average.
