Quote:
Originally Posted by JulianEdgar
I am happy to discuss the relationship between aerodynamic drag changes and fuel economy changes - and they don't appear to match either Aerohead's rule of thumb or the ARC one (for which we don't know what 'highway' speed is).
Before you can even start to describe such a relationship accurately, you need to know:
1. The starting Cd (a car with a Cd of 0.35 will have a higher aerodynamic contribution to overall resistance than a car with a Cd of 0.25).
2. If at a constant speed, what that speed is (aero drag goes up at the square of the speed; rolling resistance does not), so if the term 'highway' is used, we need to know what highway speed is.
3. If it's in a drive cycle test, what is that drive cycle?
The nearest I can find to a rule of thumb for modern cars is this, from the Bosch Automotive Handbook (8th ed):
On an average production vehicle, 10 per cent reductions in weight, drag and rolling resistance result in fuel consumption reductions of roughly 6 per cent, 3 per cent and 2 per cent, respectively.
Note the 'roughly'.
Motor Vehicle Fuel Economy (Stone) describes a drag coefficient reduction from 0.45 to 0.35 (22 per cent) giving the following constant speed changes:
50 km/h - 6.7 per cent
80 km/h - 15.2 per cent
120 km/h - 20 per cent
160 km/h - 22.5 per cent
Aerodynamic of Road Vehicles (5th edition) does not even try to use any rule of thumb, suggesting it is too closely tied to speed and vehicle type (ie it depends on what percentage contribution to overall resistance is made up of aero drag - my points (1) and (2) above.
Road Vehicle Aerodynamic Drag (Barnard) gives no rule of thumb.
People love simplistic rules of thumb, but the real world of car engineering is nothing like that - in any area: suspension, engine management... or aerodynamics.
Without extensive qualifications and caveats, the title of the thread (10% = 5%) is misleading rubbish.
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I might add, I've written pretty well all of that here before. Aerohead ignores it, waits a few months, then again posts the same misleading stuff.
People pick it up just as before, and so away it all goes again.
I honestly don't understand why people don't say: "The 10%=5% rule was debunked last time, so why are you posting it again?"
(Or, at its most generous: "Everyone should note that the source Aerohead is quoting is 23 years old, and car drag coefficients have changed a lot since then. Also, it applied only at a fixed 55 mph. So the heading of this thread is really deceptive and shouldn't be applied to drag changes you make to your own car.")
Instead - ME Andy says:
The info he posted this time seems 100% reliable
It's very odd.