If you routed the diesel fuel in the payload tank (such as Amoco) back to the engine of the semi carrying the tank, how many miles could the semi conceivable travel before running out of fuel?

This would be a differential equation problem, where you would need to know the mileage of the semi both fully loaded and empty, and of course, the volume of the payload tank.
I’m guessing the tank would be in the thousands of gallons
range.
You could also get marginally more miles if the conventional
gas tanks of the semi were used.

This is a question that has bugged me for some time.

Well, since I’m an Engineer and used to guesstimating, I won’t go stampeding towards the Laplace Transforms just yet…

It seems to me that all you need is a rough estimate. Find out what the mileage is pulling a full tank, what it is pulling an empty tank, how much fuel is typically carried onboard, how much fuel the big tank holds, and do this:

miles of range = ((Full Tank Mileage[sub]miles/gal[/sub] + Empty Tank Mileage[sub]miles/gal[/sub])/2) *(Onboard tank capacity[sub]gal[/sub] + Big Tank Capacity[sub]gal[/sub])

I mean really, there are too many variables at play here to expect a whole lot better answer.

A little investigation suggests that large tanker trucks are about 8000 gallon capacity (accident reports and so on). If that were water, that would be 32 tons, so I can’t imagine a much greater capacity, and I wouldn’t have guessed that much.

An empty semi weighs 30,000 to 40,000. The maximum legal weight for a semi is 80,000 pounds. I am not sure what a gallon of diesel weighs but I think its pretty close to water, 8 LB a gallon. With these numbers the maximum load is 6,250 gallons.

In my experience there is not a significant difference in mileage between a loaded and empty truck, most of the power is used to push the air out of your way. If you start adding hills or stop and go driving then the weight will start to make a difference.

Most trucks get between 6 and 8 miles to a gallon (Highway) so that would give you a range of 43,750 miles, plus the trucks fuel tanks (about 240 gal) add another 1,680 miles.

Of course this is only a very rough guess, to get a more accurate number you would need to know the exact engine and mileage, the terrain and type of driving, and the speed being driven.

Sounds reasonable. BTW, I arrived at my 8000 figure by poking around and turning up several news reports concerning tanker trucks. I found at least three that read like “A tanker truck containing 8000 gallons of gasoline overturned …”. One item reporting an incident said “8700 gallons”, suggesting that the reporters had been given a more precise figure in that instance. 8000 gallons would be 64000 pounds if it was water. It could be that tanker trucks, being rather specialized rigs, don’t weigh as much empty, and can carry that much while staying under the 80000 lb. limit, though allowing only 16000 for the truck itself sounds light.

Just remembered, 80,000 is the national limit, some states have higher limits. Some tankers have a third axle on the trailer which allows more weight to be hauled in some states.

Diesel and gasoline are lighter than water (they float on water) but I have no idea how much the difference is. In some states it is probably possible to get 8700 gallons of fuel on a tanker legally.

Also how aerodynamic the truck is would have a large effect on milage, a tanker has much less frontal area (then a van)and would probably be more fuel efficient. Many modern trucks (tractors) are much more aerodynamic than older ones.

Water, at 70ºF, weighs 62.31 lbs/ft³. Converting, this gives us 8.33 lbs/gal for water. Gasoline has a specific gravity of .70 and kerosene .80. I do not have a listing handy for diesel fuel, but I’m sure it’s in the same range.

So, gasoline weighs 5.83 lbs/gal and kerosene weighs 6.66 lbs/gal. 8000 gallons of gasoline therefore weighs, 46,640 pounds and 8000 gallons of kerosene weighs 53,280 pounds.

I have no idea what the fuel economy is for large trucks of this nature.

Diesel is number 2 fuel oil. Kerosene is number 1 fuel oil. Somewhat different petroleum fractions, and it would not be reccomended to run a diesel engine on kerosene, though kerosene is sometimes used as an additive in cold weather. It’s probably a good estimate for diesel fuel density, however.

I don’t know whether your math is wrong or my memory is wrong, but isn’t a gallon of water 8 lbs, the same way 1cc is 1 gram? I.e. by the definition of a gallon?

Or are you talking Imperial Gallon?
1 gal * 4 qts/gal * 2 pts/qt * 2 cups/pt * 8 fl. oz./cup = 128 fl. oz. Assuming that the definition of a fluid ounce is how much water weighs an ounce, then it’s 8 lbs exactly.