h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How much does having a dirty car effect your fuel economy?

I've seen a report that glibly threw about a figure of 10%, which looks suspiciously round, and I suspect is at best, misleading. I've also seen claims about the 'golf ball effect', which seem to not apply to randomly dirty cars.

Intuitively, I would have thought the answer would have to be a big 'it all depends', what with all the variables at play. Clearly there are different levels of dirtiness, so there must be different levels of effect. And we know drag becomes more significant at higher speeds.

So would anyone care to offer an answer, for an averagely dirty, average car, at average speeds?

It certainly wouldn't be 10%.

I used to own a small under-powered Renault 5, to which we had fitted a "pop-up" sunroof. This sheet of glass hinged at the front and the back could be raised a few inches, to allow air to circulate in the car. This had quite an effect on the performance of the car - when attempting to pass somebody out, I would occasionally ask Mrs G to shut the sunroof, for the extra boost in power it would give.

But I'd estimate the total effect as only about 10%, so I very much doubt that some dirty paintwork could achieve as much an effect as this.

It must have some effect but 10% seems awfully high.
Intuitively, air-conditioning would have a rather greater effect (?) -or an open window?

Whatever, keeping your car clean /all\ the time sounds like a rather more wasteful exercise...

Interesting.

Me & my mucky car are waiting with interest.

Incidentally, I always think having a bit of a mucky car stops people trying to bully you out of the way to a certain extent. If you look like you take lots of pride in your car you're more likely to give way to pillocks, or something.

Deb

Was this 'research funded by the national car wash association'?

>>Was this 'research funded by the national car wash association'?

I see that coming up to Easter, we have a news story that chocolate 'might' make you thin.

My question was prompted by an piece* on Radio4 yesterday, where the car washer claimed a clean car helped fuel economy, but didn't even offer a figure. My research revealed nothing trustworthy.

*The piece compared washing a car with a bucket to washing it with a hosepipe (fitted with a trigger). The results were basically the same, which called into question the usefulness of the hosepipe ban.

If you mea pressure pumps when you write hosepipes...don´t. They hammer the grit into the paint and when it´s washed out you have a car with cellulitis.

I suspect that there is more to it than simple cleanliness.

It may be to do with the general attitude towards the maintainence of the thing. I suspect for instance, that clean cars are more likely to have correct tyre pressures. And be regularly serviced. Etc. (OK, I can't actually THINK if an etc).

PP

It depends what you mean by "dirty".

I have a friend who drives a Land Rover. It would not be uncommon for his to be driving round with probably twenty or thirty KILOGRAMMES of mud up the wheelarches and around and about.

Similarly, when my car is "dirty" on the inside, it's because I'm carting around ridiculous quantities of rubbish that I should have left at home or binned.

Additional WEIGHT on the car will affect fuel efficiency... a bit. Bear in mind your average car weighs well in excess of 1000kg. Cars driven exclusively on the road would do well, I think, to build up even 0.1% of that in grime before they became impossible to see out of, or possibly, to find.

But if you think a layer of grime less than a millimetre thick is going to affect the aerodynamics of your car, I can assure you it's not.

You're not going to get clever boundary layer effects making it better or worse from just the rubbish you pick up off the road.

And yet F1 cars are polished to a mirror finish, suggesting a slippery surface has an effect.

The grime on my car has naturally been fashioned into venturis by the wind, creating downforce and allowing me to take corners quicker

I'm not sure aerodynamics is the reason. Have you seen how much sponsorship and advertising there is in F1 racing? A grubby car isn't quite the image most sponsors pay the big bucks for.

>>F1 cars are polished to a mirror finish, suggesting a slippery surface has an effect.

I expect it has more of an effect at 200mph, and any tiny advantage might make a difference in a race.

There's an interplay of forces in determinging fuel efficiency. If a bit of dirt on the outside of your car is the only adverse factor, then you probably shouldn't worry about it too much. if you weigh 200 kilograms or 400 pounds, you are not ever likely to get good fuel efficiency from your car, no matter how clean it is, nor how regularly the tire pressure is achieved. Or you might be svelte and lightweight, but you load your car with too much stuff. This has a similar effect.

Or, maybe you are virtuous in every respect, but your car is usually filled with people. Maybe you car-pool, or you have a spouse and two strapping teenaged sons who travel with you most of the time.

In this last case, you should not be calculating miles per gallon [or kilometers per kilogram of fuel]. Instead, you should be calculating the amount of fuel per miles per passenger.

...I heard another unsubstantiated claim that a clean car is more fuel-efficient, again on Radio4, who really ought to know better...

Well, if all other factors are equal, a clean car probably *is* more fuel-efficient by a tiny degree. But we don't use cars in conditions where all other factors are equal.

"we don't use cars in conditions where all other factors are equal." [KB]

Very well put!