constant 3/4 fuel tank filling
 

Hi - I have an 1976 VW Campervan / Campmobile and I don't like letting the fuel in the tank to drop lower than about 1/4 this is due to not knowing the condition of the bottom of the fuel tank and the possibility of 37yrs of gunk entering the fuel system.

I always fill from 1/4 to the top and the fill is always about 45ltrs or so, will not filling the tank from empty(ish) change the accuracy of the MPG readings and calculations, I will always have about 10ltrs in the tank when I fill up?

Thanks
James

So you're letting it get down to 25% fuel remaining before you fill? That's as close to an empty tank as anyone should ever get anyway. It's fine, don't worry about it. Granted, I often go all the way to E, but I oughtn't.

Now, on the other hand, if you were only using 25%, so your gauge still read 75% full when you fill it, that would certainly result in each fill's calculation being very sensitive to minor inaccuracies. Long-term averages would still be accurate.

My 1980 Buick had not been driven much when I made it my daily driver in 2010, and the first time I drove it I ran it out of fuel. I put in 4 liters roadside and drove to the filling station, filled up, and it's been fine. I haven't even kept up with replacing the fuel filter per schedule.

I always let mine run very low, I have a theory that if you get rid of nearly all the fuel, all that mysterious "gunk" doesnt get a chance to build up.

A possible issue with going down to E is that the fuel pump uses the fuel in the tank for cooling. Less fuel there, the hotter it can get. Which could mean nothing, or shorten the pumps life.

True, but lets not forget that manufactures always over compensate, there is always much more fuel in there than what people think. Did you see the Top Gear episode where they drove some 800 miles on one tank of fuel? The warning light was on for miles and miles and even when they reached thier destination and drained the tanks, there was still fuel left. Think Hammond got over 80 MPG for the whole trip.

I've heard this from many people, even the instruction book for my cressida (but that's an old car and things may work differently). With most pumps; they are cooled by pumping fuel through themselves,not by sitting in it. They have a regulator which just dumps extra fuel back into the tank. If the fear is the smaller amount of fuel getting hotter, I can't see there being more that 10 PR 15 degrees F when you are on your last half gallon. I guess that's not a good thing for the pump, but meh. I guess if it did heat up a lot the added vapor pressure could do bad things to the evap system. But I've never heard that warning of running on E.

I saw that Top Gear episode also. Yes, Jeremy Clarkson did arrive at his destination without running out of fuel but also they did not "drain the tanks" to see how much fuel was in them. Where did you see that? Also, Top Gear is an entertainment show, I really doubt any of them got 80 mpg otherwise we would all be driving cars that get similar mileage.

It probably got cut from the US show, but right at the end back in the Studio, they said they drained the tanks, and there was still some left. 80 MPG is the UK figure, you need to take a bit off for the US MPG, it probably about 67 US MPG. Dont underestimate the efficientcy of the diesels here though, a Gold TDI recently got 97 MPG over a 995 miles trup across Europe, and a Ford Fiesta diesel also got 108 MPG in the famous MPG Marathon in the UK last year!

That really is amazing mileage. I don't see how the Top Gear boys did it though, non of them seemed to have any Hyper-miling skills and Jeremy was actually trying to waste fuel by turning on the heated seats and other accessories.

Driving style tends not to affect diesels as bad as petrols. In my previous gas car, if I drove quick, it would only do 26 MPG, but if I drove quick in my old diesel, the lowest I could get it to drop to would be 56 MPG.