In the UK, we have always expressed fuel economy (or 'gas mileage' as Colin Chapman called it in the 1950s) in terms of miles per gallon, while Europeans express it in terms of litres/100 kilometres. Some car adverts you see in the UK give both.
(One is clearly the inverse of the other, ignoring the different units. A good mpg is a big number, a good l/100km is a small number. The quick way to convert from one to the other is to divide 282 by it, so 40 mpg = 7 l/100 km and vice versa. For completeness, 16.8 mpg = 16.8 l/100 km).
Both are inherently flawed. All you really want to know is how much it costs in petrol to drive a certain distance, and they both require one more step than necessary:
UK example - a 40 mpg car doing a 50 mile trip, petrol is £1.20/litre:
- 50 miles divided by 40 mpg = 1.25,
- 1.25 times 4.55 (to get from gallons used to litres used) = 5.7 litres (and I defy you to do that in your head - shortcut is times by 5 and knock off 10%)
- 5.7 litres times £1.20 = £6.83
European example - a 7 l/100 km car doing an 80 km trip, petrol is €1.40/litre:
- 80 km by 100 = 0.8
- multiply 0.8 by 7 = 5.6 litres
- then multiply by €1.40 = €7.84
Far better would be to express it in terms of miles/litre (or km/litre in Europe). 40 mpg = 8.8 miles/litre; 7 l/100km = 14.3 km/litre. That only needs two steps:
UK
- 50 miles divided by 8.8 = 5.7
- 5.7 times £1.20 = £6.83
Europe
- 80 km divided by 14.3 = 5.7
- 5.7 times €1.40 = €7.84.
Just saying.
Monday, 26 November 2018
Miles per gallon vs litres/100 km. Both rubbish units.
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16 comments:
OK. Here's my problem. The quick cars are doing 1'15" laps. We're doing 1'23" secs laps. It's a one hour race. We do 0ne litre every two minutes. How many litres do we need to complete the race with a small safety margin/ FYI race fuel is about £2.40 / litre.
It is useful to know the actual amount of fuel consumed as well as the cost, for the following reasons:
(a) to make comparisons between different car models,
(b) to monitor your own usage in terms of journey type and driving style
(c) to get a feel as to whether anything is going wrong with the car which is shown up by worsening fuel consumotion
As fuel is actually sold in litres, it might make sense to use miles per litre, but mpg is the established figure, and creating a hybrid unit combining Imperial and metric units would annoy a lot of people on both sides of the debate.
If you mainstain a spreadsheet of fuel consumption, as everyone who is bothered about it surely does, multiplying by 4.546 isn't exactly difficult.
L, one hour race, litre every two minutes = 30 litres.
PC, fair points but
a) by knowing miles/litre, you *can* make comparisons.
b) and c) are separate topics. I was just talking about the headline figure for fuel economy of a car (i.e. mpg), not how it changes. It doesn't matter what units you use, if you want to monitor how it changes, that is a whole load more faff.
If you keep a spreadsheet, then all calculations are easy.
Unless you keep a record over a few fills, you never know your true mpg. Maybe a spread sheet is overkill. I had a scrap of paper on the shelf below the speedo. Twenty year old R reg Saab 9-3, about 31 mpg - maybe a bit more after the service. I'm in love with my car. I've got a feel for my automobile.
The best is to start with it expressed in litres used per mile.
Then there is no division required.
0.1138 litres per mile = 50 * 0.1138 * £1.20 = £ 6.83
JB, old cars are the best.
Din, agreed, but it's easier to remember and calculate with a big number like 9 m/l than a small number like 0.11 l/m.
MW :-) I thought you might spot the trick question. More necessary for us is to work out the laps we'll do.
L, winner will do 60/1.25 laps = 48. You'll do 60/1.4 = 43.
MW. You've just elected yourself pit manager...
MW next question. When will we be lapped?
L, after 7 completed Laos, give ior take and 14, 22, 28, 35 and 42.
MW. You've got the job!
L, thanks. That's what we love about F1, a team employs a dozen (or more? no idea) people to run real time what-if scenarios on what the other teams are doing and are likely to do and working out tyre and fuel strategies.
MW. In my amateur world it's so much easier.
When doing maths you don't have to use long division and decimals, instead you can put the expression in the form of fractions and simplify in steps.
for example starting with sum
50 miles divided by 8.8 = 5.7
5.7 times £1.20 = £6.83
that can be expressed as (50)*(10/88)*(12/10)
=(50*10*12)/(88*10)
=(25*3)/(11)
=(75/11)
= six and nine elevenths.
Din, distance travelled and price are variables. But if price were stable, you can just work out pence per mile and multiply up where nevessary job done.
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