im not trying to insult but your MPGs are all wrong. you conversion is yeilding you a much higher MPG then you're actually getting.
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this calculation is based off of your last entry, the one dated july 31, 2009

101,967.9 - 101,520.5 = 456.4km

456.4km / 1.609344 = 283.59miles (1 mile = 1.609344litres)

90.280L / 3.785411 = 23.894gallons (1 gallon = 3.785411km)

283.59miles / 23.894gallons = 11.89MPG

for some reason you have 14.29 MPG you may want to check over your math
(and the 2mpg drop in numbers is going to be a real kick in the *****)
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and if im wrong can someone show me where.

 

Back to the original subject, Patman said it best. The EVAP.
This has actually been going on for years but most people think it's just their imagination, or the guage.
Check out this website. It explains it pretty well.
http://technologyinterface.nmsu.edu/5_1/5_1e/5_1e.html

Here is the basic jist:
"One major addition to the modern EVAP systems is the tank mounted vapor management valve. The purpose of this valve is to vent HC’s from the fuel tank directly into the storage canister in order to prevent the pressurized build up of hydrocarbons in the fuel tank. The addition of this valve reduces the amount of hydrocarbons that are allowed to escape into the atmosphere as the fuel cap is removed at the time of refueling."

So, what's happening is the vehicle is purposely pulling fresh air into the gas tank and moving the vapors to the cannister. When you pull in fresh air you are then going to start the evaporation cycle all over again. If the tank mounted valve turns on at say 1/4 to 3/4 then you are actually evaporating at a higher rate. The mileage your engine is getting doesn't really change. You are just evaporating more.

So, stop all your spreadsheets and calculations and just keep the tank full.

 

No harm, no foul. I knew he was joking as was I. Just throwing a bit ob bull back at him.

 

You're actually not wrong. Not right either. Lol. I had unintentionally put the calculation to use Imperial (UK) Gallons, which equals 4.55 Litres. I'm adjusting it now to be US Gallons. I'll repost the results in a few minutes.

EDIT: Here's the updated US numbers. Still fairly consistent, just lower than what I'm used to looking at.

 

It's not that much. That top number, 101976.9 is kms. In miles that's 63339.7. I bought the truck at ~87000kms (~54037 miles) back on Dec 2, 2008. I've put on almost 20000 kms (~12422 miles). In April we did a 4400 km (2733 mile) trip through British Columbia, over 3 weeks, for my vacation. Other than that, I drive about 500 kms (310 miles) every 2 weeks.

I also don't drive it like a hearse. I don't purposely keep the tach under 2000 rpms (aside from idling) and I'm pretty consistently 10-15 kph over the speed limit. I bought a truck and I was expecting much lower mileage then I'm getting. I went from a Civic getting 30 mpg to a truck and was expecting it to hit the pocketbook.

 

i thought i was crazy...turns out im not the only one


that really isnt all that much, ive had my truck for 21 months and ive already got 51,000 miles on it

 

Ive always noticed that my gauge moves faster when it gets past 1/2 I thought it was because of the way my tank was shaped.

so from now on im filling up at night when my gauges hits half a tank

 

Rich:

Thanks for the reference link.

Thanks to most everyone for the suggestions. I've been away from the computer for a few days and was surprised to see the posts. I think I will learn what's going on with my mileage and prove to my dear ol' dad that I'm not the bloomin' idiot he thinks I am.

My gas mileage claim is one of those family emabarassments that we don't talk about. That and Uncle Lou's gambling problem.

Dave