Quote:
Originally Posted by Nerys
then why dont i see a 15% improvement at 45 versus 55? i tried it for a tank and it resulted in less than 1mpg difference. that is not even 2%
maybe i will have to try it again for a few tanks and see what happens
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Hi Nerys,
You're on to something here.
My gas mileage is not 15% better at 45 than 55 either. And that is because at these lower speeds, the Crr contribution to the overall drag of the vehicle is probably close to let's say 30% or so.
So if you decrease your speed from 55 to 45 the decrease in aero resistance should be:
(45)^2 / (55)^2 = 0.67 of 70%(air drag) = 47%(air drag)
So now your air drag went down from a 70% contribution to 47%, and the resultant Crr went from 30% contribution to 53%. That's not to say that the Crr increased, but rather it's contribution to the overall drag is now a higher portion since the aero drag portion dropped such a large amount.
These numbers are obviously not exact, but try to make a point.
And the point is that are our overall speed decreases, and we add a boat tail or whatever, that the Crr component will start to show it's contribution and the overall reduction in fuel consumption will become less due to the boat tail.
To go to an extreme, if we could make a perfect body shape like an airplane wing, then the large contributor to overall drag at lower speeds would simply be the rolling resistance of whatever we are driving. Lighter would be better in this case.
I think this is what Aerohead has been mentioning in different threads.
EDIT:
Nerys, if I had to guess, your low decrease in fuel consumption has more to do with how your particular engine is tuned, engine size, the gearing, the rpm's your spinning and the actual load on the engine.
I would say that engine is size is such that it is basically loafing at speeds of 45 and/or 55 mph and that is why there is a such a small difference in fuel consumption.
The air drag really is dropping quite a bit at the lower speeds, but the design of the vehicle is limiting the gains that you will see by your careful driving.
This is all just a hunch on my part, but does seem very likely from what you are describing.
EDIT2:
I see that you are sometimes driving a Metro, so your mileage
should increase more than 2% under the conditions mentioned.
The larger engine sizes limit the fuel mileage gain, since a larger engine has larger cylinders, more internal surface area to heat loss, and thus lower overall gas mileage. Larger vehicles also have a higher Crr so the aero gains will be a smaller overall contribution compared to engine efficiency and Crr.
Jim.