Economy of speed

February 14, 2007

I'm occasionally challenged by environmentally minded people on how I can justify having fast gas guzzling vehicles instead of dull slow sensible economical vehicles. Well, I've just finished doing the test on the bike so I've got accurate figures*

Vehicle: Vauxhall Corsa 1.4 A/C
Power-to-weight: 81bhp/tonne
Pace: Pathetic
Excitement: Risk of coma
Efficiency: 36.6mpg (test average)
(source: Autocar)

Vehicle: BMW 1 series
Power-to-weight: 87bhp/tonne
Pace: Bearable, barely
Excitement: Risk of staying awake
Efficiency: 38.6mpg (test average)
(source: Autocar)

Vehicle: Lotus Elise S1 1.8
Power-to-weight: 160bhp/tonne
Pace: Briskly satisfying
Excitement: Risk of big grins
Efficiency: ~40mpg (did the test ages ago and have lost the exact figures now)
(source: personally measured)

Vehicle: Yamaha FZS600
Power-to-weight: 502bhp/tonne
Pace: Beserk
Excitement: Risk of heart attack
Efficiency: 44mpg
(source: personally measured, weight/power from On Yer Bike)

* To accurately measure a vehicle's miles-per-gallon usage, do the following:
  • Brim the fuel tank to the absolute top
  • Zero the trip counter
  • Drive around NORMALLY until the tank is nearly empty. If you drive more economically than you normally do you'll skew the figures
  • Fill the tank to the top again. Write the trip meter's mileage and the amount of fuel it took to refill onto the reciept. Do it at the pump so you get it right.
  • When you get home, pump the figures into google calculator as shown in the links above and out pops the magic number.


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  1. stephenw 2 dup dup * dup * sqr sqr swap / says:
    February 14, 2007 @ 17:10 — Reply

    "pump the figures into google calculator"? 'Home made RPN calculator' meant you surely?

  2. Ciz says:
    February 14, 2007 @ 21:52 — Reply

    If the tree huggers want something to complain about then they should look closer to home. Their precious Mother Earth is much less efficient than any modern car. The kinetic energy of our planet's rotation is 2.137 * 10^29 joules, giving a thoroughly unimpressive power-to-weight ration of just 0.23 bhp per tonne. So next time you come across a giant spinning gas cloud, don't wait for it to condense into a planet. Use it to power something relatively economical, like a Bugatti Veyron.

  3. CMU says:
    February 15, 2007 @ 05:00 — Reply

    There's some shonky physics in your calculations there. You've magic'd joules into watts for the purposes of extra humour. The real figure for the planet is probably 0bhp/tonne, as it's not actually making any power but just sitting there in a fairly static orbit.

    There is fun to be had here though. Think of the Speed Kills campaign. Orbital path linear speed of the Earth is 66629mph, and nobody seems to be dying from that.

  4. Anonymous says:
    February 15, 2007 @ 19:40 — Reply

    How very dare you. 1 joule = 1 watt second (= 1 Nm). I would never make up some ludicrous lie for the purpose of comedy.

    And another thing. You know when you see a © symbol? It doesn't mean "copyright", like most people think, it's actually a speed limit sign.

  5. CMU says:
    February 16, 2007 @ 15:17 — Reply

    I very dare me because I'm right. You've confused seconds (time) in your second equation with seconds (angle) in your first. Note how Google's unbelievably clever calculator has examined the "seconds" you wrote in your search and decided you must mean arcseconds, since that's the only way you can get to power with what you wrote.

    To put it another way, do you really think the Earth has a 1.4x10^21 horsepower motor pushing it along somehow? Are there large rocket motors atop Everest that we've all missed? Perhaps a huge outboard motor dangling off the North Pole?

    The Earth has no major resistive forces slowing it's path through space, hence needs no power to stay in orbit. In an analogous way lots of objects that follow circular paths require no power to do so, other than that required to overcome friction/air resistance etc.

  6. Matt says:
    February 15, 2007 @ 11:40 — Reply

    I did a test with my Elise on a trip to france last year. The trip from home to Folkstone at mostly motorway speeds came out at about 45mpg (Miss Daisy Mode!) but once in France (Miss Daisy Mode disengaged) this went down to abuot 30mpg. But over the 900ish miles it was about 37mpg, which I thought was pretty good!

  7. CMU says:
    February 15, 2007 @ 13:16 — Reply

    Once I was in my Elise following a friend towing our shared rally car on a trailer. We travelled several hundred motorway miles at a fairly steady 55-60mph (proper Miss Daisy stylee). On that trip the Elise returned a measured 55mpg.

    That figure isn't comparable above hence I didn't mention it in the article, but it's pretty amazing for a car that can do 0-60 in 5.5 seconds and was hardly built with fuel efficiency in mind.

  8. Gavin says:
    February 19, 2007 @ 05:02 — Reply

    Whimper. I once managed to get to the end of my driveway without running out of fuel.

    It's not a long driveway.

    (Some of this post may have been exaggerated for comic effect)

  9. Anonymous says:
    February 17, 2007 @ 20:20 — Reply

    mmm, Bugatti Veyron.

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