How’s my math/reasoning? (gasoline)

[ChrisGreaves]

1) From the web I garner rough data:-
1a) One British Thermal Unit (BTU) = 778 foot-pounds
1b) One gallon of gasoline delivers 114,000 BTUs
1c) Regular automobile engines run at 20% efficiency

2) I calculate that one gallon of gasoline, burnt at 20% efficiency, delivers about 17,000,000 foot-pounds of work
2a) 1 gallon = 114,000; 20% of 114,000=25,000; 22,800x778=17,000,000 ft-lbs

3) In most jurisdictions, travel by automobile involves repeated (at least slight) vertical movement.
3a)To travel from my old home (M9C 2A6) to The Montreal Deli (L4X 2V9) involves rises that total 22 feet (Google Earth) while the return journey involves rises of 22 feet.
3b) Note that this is not the overall change in altitude, for then a trip out-and-back must have by definition a net zero change in altitude; here we are summing each detectable rise over the entire out-and-back trip.
3c) It can be argued that some of the potential energy stored in lifting mass to the top of the hill is regained by conversion to kinetic energy on the downhill run; I have no figures for the efficiency of this regain of energy; I’d start with a guess of 50% efficiency.

4) Supposing 20 pounds of junk lying around in the (trunk/boot) of the car.
4a) It would cost me an extra gallon of gas to make about 20,000 trips for brunch at The Montreal Deli.
4b) The impact is, of course, greater on a longer trip, for example, lugging that same 20 lbs of junk from Toronto to Auburn (NY) and then to Olean (NY) and then back to Toronto may well cost me a gallon of gas.

5) A lighter car has less metal to lug up and down each slight rise on any trip.
5a) A Hyundai Accent (2,396 lbs) must be more efficient than a Lincoln Town Car (3,993 lbs) in terms of lifting metal up and down an accumulation of rises.

6) It follows that a car driven around Pittsburgh (PA) will produce worse mileage (mpg) than the same driver and the same car driven around Fort Walton Beach (FLA) solely because Pittsburgh is 50% hills and 50% valleys whereas Fort Walton Beach is almost as flat as The Nullabor Plain.

7) As I tootle along a restricted-access highway I visualize a rooster-tail (or plume) of heat trailing upwards and behind each vehicle that overtakes me; the plume of heated air represents 80% of the gas being burnt at any instant.

[John Gray]

That is a classic example of how the SI system of units could make life ever so much more intelligible!

[StuartR]

I once heard a delightful analysis of units for fuel consumption.

We measure fuel consumption as miles per gallon (or in Europe in liters per hundred kilometers)
Miles per gallon is a distance divided by a volume, which gives an inverse area.

Imagine an infinitely long pipe full of fuel. It has a cross sectional area equal to your fuel consumption. If you drive along this pipe it would provide exactly the amount of fuel you need to keep going (assuming that average fuel consumption = actual instantaneous fuel consumption, but I guess a very small tank could compensate for that).

[ChrisGreaves]

That is a classic example of how the SI system of units could make life ever so much more intelligible!

I agree.
But it's all to do with addressing the Lowest Common Denominator, and some Lounge residents are in the USA

[PJ_in_FL]

I agree.
But it's all to do with addressing the Lowest Common Denominator, and some Lounge residents are in the USA

Hey! I resemble that statement -- but I heartily agree with the sentiment!

Though I wonder how far the UK are doing in changing from ordering pints to liters, or will it be ha-liters for half-liters, like the venerable ha-penny from the childhood Christmas song?

[John Gray]

We already buy petrol and diesel and milk by the litre, and wine and alcoholic spirits by the centilitre, but beer is still bought by the pint. This is a Noble British Tradition which our lords and masters did not dare change! Read all (well, some) about the Metric Martyrs.

The half-penny (all that pre-decimalisation time ago) was pronounced hape-knee.

[jonwallace]

I actually started out looking for George Orwell's take on pints in 1984, and came across this interesting page about pints in Canada (it actually includes Orwell's quote, for the interested.)

The current Wikipedia page on pint glasses is here. It has some nice pictures (well, pictures anyway) of the official UK glass markings which certify that a pint glass holds a pint. I'm not going to go into lined pint tumblers, that's a bit complicated for this time of night.