Headwind

[Draigflag]

What kind of an effect does a moderate headwind have on economy? I just got back from a 400 mile round trip. I took the motorway assuming it would be slightly more economical, despite being a longer journey overall. I was expecting good economy as I kept to a constant 70 Mph with cruise on.

On the way down, I fought a headwind most of the journey, with heavy rain too. My trip economy dropped from 64 mpg down to just 56 by the time I got there, possibly my worst economy ever for a long trip. Similar long motorway stints have resulted in mid 70's mpg wise. On the way back, I reset the computer and on the motorway stretch I was getting a consistent 70 ish mpg. Then on the home straight, I hit another headwind and my economy dropped to 57 by the time I was home. Pretty shocked to think a headwind could cause such a massive increase in fuel consumption, I kept everything as consistent as I could, taking the exact same journey both times.

Anything more to this? Is it better to increase or decrease speed in headwinds? Any other tips?

[Charon]

The main source of drag on your car is aerodynamic drag - wind. A headwind is exactly the same aerodynamically as a higher airspeed, your airspeed being the sum of your ground speed plus the headwind. Because aero drag increases as the square of airspeed, a headwind will hurt your mileage more than the corresponding tailwind will help it on the journey the other way. For better fuel economy, decrease your speed.

[Draigflag]

Thanks for the advice/input.

[Draigflag]

I still kind of feel sceptical about the figures though, I mean 15-20 MPG is a huge discrepancy in my opinion. I would have guessed a headwind would maybe increase the consumption rate by 4 to 6 mpg max, but I guess it is what it is. I'm justifying it by not taking the toll road so I saved £6.50 anyway

[luv2spd]

I learned in engineering school that around 89km/h or 55mph, the air flow around most cars start to go from laminar to turbulent. When the flow becomes turbulent, it sucks more energy out of your engine. If you look at the below graph which shows a Jaguar, the graph is somewhat linear up to 55mph, above this speed the force on your car increases exponentially with speed. So, if you are driving 70mph with a head wind of 10mph; to your car’s engine it would feel like going at 80mph.

RR in the graph represents rolling resistance by your tire. It is somewhat constant and has a smaller force on your car than air drag. The skinnier your tires, the smaller the rolling resistance force is, the tire material can also have an effect on rolling resistance.

A car that is more aerodynamic will have a higher speed where the laminar flow turns into turbulent. The ‘16 Prius has a drag coefficient of 0.24 and has skinny 195mm wide (low rolling resistance) tires. My Mercedes has a higher drag coefficient of 0.27 and has very wide 225mm and 245mm wide (sticky) tires, it also weighs 500lbs more. All these things have an influence on fuel economy. Hope this helps understanding air drag. Charlie’s explanation above is very good and I thought I attach a graph for better visualization.

[Jay2TheRescue]

What you experienced is about a 20% drop due to strong headwinds. It is considered normal in just about any vehicle. The variance increases as your total economy increases. If you were driving a vehicle that only gets 20 MPG highway, you probably would have only dipped 4-5 MPG. It is the same %.

[Draigflag]

Thanks for the input guys. Logically I should have slowed to save fuel I guess, however Sunday evening, slightly hungover, just wanted to get back, I chose time over economy ha! I guess that's the downside of higher mpg cars, even though the rate of consumption percentage would be similar for large cars, the higher the mpg, the bigger the loss in numbers.

[Jay2TheRescue]

Yes, exactly. Just as it is easy to take a high MPG vehicle above its rated economy, it is also easy to take it below. For example, on a car rated for 50 MPG, a 10% drop would be -5 MPG. For a car rated for 20 MPG, a 10% drop is -2 MPG, which in the grand scheme of things, is barely noticeable.

[sparkn]

From your original statement you also mentioned rain. That will definitely contribute to sucking the life out of your mileage, too, wind or no wind. Nearly every factor that affects fuel mileage is affected by rain in some way. I'll leave the math for someone else, but, from experience (not scientifically documented) I can say that rain can affect economy up to around 15%. Mind you, I'm not talking about a simple damp road, but, rather a rain storm with standing water on the roadways.

[Draigflag]

Yes I had guessed that too, given the "drag" effect you can feel when going through standing water, it can really slow you down. Also maybe the wipers affect the aero drag slightly too, splitting atoms here, but I'm sure it has a small effect given how sleek the windscreen is initially.