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:angel:
It always makes me laugh when people complain about their fuel economy when towing something.
"Its just so much more expensive to tow at 100kph"
I have no doubt thats true,,
but what is the actual difference in fuel economy between towing at 90 or 100?
If the engine and transmission etc efficiency is the same at the two speed, all you have to do is square the two speeds and dive. This gives the difference in air resistance as a percentage. For example, for the two speeds you quote, the difference (increase) from 90 to 100 is about 23%.Quote:
Originally Posted by Pedro_The_Swift
Now resistance to motion is not entirely air resistance - there is rolling resistance, and the engine and drive efficiencies may vary over this speed difference, but at those speeds, most of the resistance is air.
Other effects will come in, such as not having to change to a lower gear on hills with the higher speed, but it is probably reasonable to conclude that you can expect a reduction in fuel economy of around 20%.
John
so, 80-90?
100- 110?
no I'm not making trying to be difficult John:D, I'm interested in the differences between speeds, is it 20% at any 10kph speed difference?
No, it is not that simple. The difference in resistance 80-90 is 26% increase, and 100-110 is 21%. The figures for fuel consumption would not be that dramtically different for 80-90, because by 80 the air resistance is beginning to be a smaller proportion of the total resistance. (10km represents a 10% increase at 100, but 12.5% increase at 80. Also be careful to remember that 10% decrease at 110 is 9%, and 90-80 is 11% - the denominator matters when you talk about increases or decreases, and the difference gets more when talking about squared numbers>)Quote:
Originally Posted by Pedro_The_Swift
Although it will vary with the vehicle design, a usual rule of thumb is that over 60kph the air resistance represents over half of the total drag. Of course, this speed will be higher for well streamlined vehicles and lower for unstreamlined ones. But not as much as you would expect, because vehicles where the manufacturer has put effort into reducing air resistance usually also have effort put into other losses. And ones with high air resistance tend to be also four wheel drives with high transmission losses and high tyre losses.
Of course, if you put a large unstreamlined caravan behind any vehicle, you might just as well pull a drag chute!
John