Are cars more economical on wet or dry roads?

[Ari]

Something I was pondering on a wet motorway recently, are cars more economical in the wet?

On the one hand you've got lower resistance (presumably), on the other you've got water to displace I guess.

So which is it?

[mnkiboy]

I always presumed that displacing the water would reduce economy slightly.
Where does the lower resistance come from?

[Nedzilla]

Water causes drag....just try walking through the stuff,so I would imagine a car would be economical on a dry road.

[deltashad]

Less tyre wear, and usually driven with a lighter foot. Also, does the car not run better in wet weather keeping the engine cooler?

[WorAl]

mnkiboy said:

I always presumed that displacing the water would reduce economy slightly.
Where does the lower resistance come from?

I presume he means there's less friction against the road surface, hence being able to wrap your car easier., however, I think the water causing drag would be greater than the reduction in friction.

[911p]

Less economical in the wet.

[kambites]

I doubt it's measurable either way.

[davepoth]

deltashad said:

Less tyre wear, and usually driven with a lighter foot. Also, does the car not run better in wet weather keeping the engine cooler?

Cold, damp air is denser so more fuel can be burnt - If you drive for peak BMEP then you might make a small improvement on economy but whether the denser air will cause enough drag to negate that I don't know.

[Chris71]

deltashad said:

Also, does the car not run better in wet weather keeping the engine cooler?

The moisture in the air will also provide a degree of charge cooling, which might improve thermal efficiency.

Getting back to the tyres, I've often wondered this about my bike. There's definitely a more slick feeling to very narrow, very lightly treaded bicycle tyres in the wet, but I suspect overall the water resistance and spray drag still make it worse.

[veevee]

Less grip in the wet = less efficiency, surely?