Overfilling theory (input needed)

[Charon]

Originally Posted by Draigflag

The efficiency of large trucks has significantly improved since then, it's not unusual to get 12 to 17 MPG in a large semi these days

I'd like to see your source for that mileage. We have European engines in some of our trucks, too, and they do NOT get that sort of mileage. If they did, trucking companies would switch to them in a heartbeat.

[Jcp385]

4-6mpg is all you'll see in a semi. Maybe closer to 8ish unloaded

[Draigflag]

Sorry, I keep forgetting the difference, I was quoting UK MPG, and that is what the truck drivers tell me they get. At the lower end you have the big Semi trucks, whilst the 18 tonne trucks get slightly better.

[OliverGT]

Hi,

From a Hypermiling perspective, filling the tank to the brim means that you get further on each tank full, over time this means that you have to stop less times overall for the same distance. Stopping and starting have a big impact on MPG, hence the struggle to squeeze every last mile out of each fill.

For example on our Skoda, from the first click to the brim adds about 2-3 litres, which is probable good for 50km or more, we get around 600km to a tank, so just undeer 10% extra, so it effectively saves 1 fill in 10-12 fills.

Is it worth it? Probably not, one extra stop/start every 6000km.

It's dangerous to fill and leave filled, definitely for Petrol which expands etc in the heat. I normally fill up on the way home from work and have another 20km to drive after filling, so I've already used a litre or so of the extra fuel.

As for getting accurate MPG figures, makes no difference long term. If however you are driving in an MPG competition or using a borrowed car for a review, it is absolutely crucial that the tank is brimmed at the start of the test and then brimmed at the end to get an accurate figure for the fuel used.

Oliver.

[Draigflag]

I agree Oliver, keeping a constant speed is very important on long journeys. Having to stop and fill up every few hundred miles, and then having to accelerate back up to cruising speed is bound to use more fuel than staying at a steady speed for the entire journey. Having said that, carrying the extra weight of a full tank could use slightly more fuel than carrying a quarter tank, especially in a hilly region.

[Charon]

I could be wrong, but I think most folks are going to have to stop every two or three hours to empty their bladders. The basic reason for long range fuel capacity would involve travelling somewhere without convenient fuel stops, such as sparsely populated regions. However, if long range is important to you, have at it.

[Cogeneration]

Originally Posted by Jcp385

I'd say it's good practice to stop at the first click. It's why your filler cap and the gas pump will say do not top off. As I said, I miss the extra range and the (probably, maybe) more accurate fill data from stuffing the tank, but it's not worth the possibility of having to prematurely replace the canister. In mine, it's a $300 part for a car worth maybe $800.

Not sure if diesels even have a canister? Diesel doesn't evaporate like gasoline does.

Diesel's have no evap system. My 2011 Golf TDI I always filled to the brim. My 2015 XV Crosstrek Subaru gasser, first click and I'm done. Not going to take any chance on killing the charcoal canister.

[Jcp385]

Originally Posted by Jcp385

4-6mpg is all you'll see in a semi. Maybe closer to 8ish unloaded

I'd like to amend this statement...I have seen as high as 8.5 in my truck, fully loaded, but that's if I keep it rolling for 4+ hours straight on decent roads. 7.5-8mpg is more common. Urban driving kills these trucks' fuel economy, much more than any car I have driven on a % basis.

Unloaded, you'll get about 11mpg. Bobtailing I have seen 12-14.

If you're shifting, you're wasting fuel in a semi.

Numbers above have been from a '14 Freightliner Cascadia, DD13/500hp engine with tag axle and Eaton 10-speed over the last 40k miles. Lightweight model, has a slightly shorter sleeper and only one fuel tank. The company limits speed to 60mph on cruise, 63mph hammer down.

[LDB]

I put the nozzle in the filler as far as it will go. I fill on the medium setting of the 3 notches. When it shuts off automatically I manually fill until the next automatic shutoff, so one overfill. Then I shut it off and go on my way. I believe the extra 1/3-1/2 gallon will be absorbed by air gaps and other normal things and will not damage my car. I would not go beyond one extra cutoff though.

[trollbait]

I think the manual to a past car said to fill to the second click. Possibly to account for the first click being triggered by frothing.

I use the a lower fill setting when available. Some stations might only have have one.

For monitoring fuel economy, pick a fill method and stick with it for better precision in the calculations.