Air density: it's a real drag
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As air becomes cooler and denser the aerodynamic drag on a truck increases and so the demand for more engine power rises.
The density of air is approximately related to its temperature and pressure by the following calculation and we're indebted to Leyland Daf for their work on the calculator):
According to LD, as drag force is directly proportional to air density, o 4% increase in air density is equivalent to a 4% increase in drag (or a 40/0 increase in drag coefficient, expressed as Cd).
A further rule is that x% change in Cd relates to x%/2 in mpg in service. In other words, at O'C the mpg of your truck compared with that at 15'C could be down by 2.5%. In other words, if your wagon is returning an average of 8mpg at 38 tonnes at 15 'C, when the temperature drops to freezing it falls back to 7.8mpg. So what's 0.2mpg amongst operators? Probably not much until you turn it into litres.
If the difference between 35.3114/100km (8mpg) and 36.2214/100km (7.8 mpg) is 0.91 lit/100km, then for every 100km your wagon travels it's burning nearly an litre extra of diesel.
At an average pump price of 61,6p/14, if your tractor's covering an average of 400km a day and you get five days where the outside temperature doesn't go above freezing (as happened earlier this year), then compared with a week of
temperatures around 15'C your fuel bill will rise by nearly £12 per truck—and that's before you take account of headwinds.