I think there are several areas where rain cold be having an effect:
1 - Rolling resistance
If there is water laying on the road then the tyres will meet resistance as they try to push the water away. However, I would guess that if the road is just damp then resistance may in fact be reduced.
2 - Wind resistance
I know someone who flies gliders and he once told me that flying in the rain affected the performance of the gliders due to the extra turbulance caused by the droplets of water. He said that they would sometimes go into a slight dive to increase speed to remove the water if they had just flown through rain. As cars are nowhere near as aerodynamic as a glider I would image the effect would be tiny enough not to be noticed.
3 - Driving style
I think this could go either way. Slower traffic could mean driving at a more economical speed. Or it could mean more stop start traffic and therefore lower mpg.
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I know someone who flies gliders and he once told me that flying in the rain affected the performance of the gliders due to the extra turbulance caused by the droplets of water. He said that they would sometimes go into a slight dive to increase speed to remove the water if they had just flown through rain. As cars are nowhere near as aerodynamic as a glider I would image the effect would be tiny enough not to be noticed.
But on the other hand, water on the cars body means there is surely less friction between the car body and the air - the rain acts as a membrane between the two.
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If a road is damp then rolling resistence is reduced and you should get more miles per gallon. However, once a road becomes wet enough, whereby the tyres have to perform work to pump water away, then additional work is required which will result in an increase in fuel usage. The wider the tyre the more work that needs to be done and the more noticable the negative impact on fuel consumption.
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If a road is damp then rolling resistence is reduced and you should get more miles per gallon.
I thought rolling resistance consisted mostly of hysteresis loss in compression of tyre rubber, plus some friction in the bearing and brake surfaces - not much affected by rain I would have thought. Correct me if I am wrong?
And is there a measurable effect of hitting raindrops as well as air? EDIT - I don't expect to get an answer to this ....
Edited by Andrew-T on 14/03/2019 at 23:24
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Good point - I guess it depends on if the water beads or just forms an even film across the surface.
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Two recent fills on my car - both BP Ultimate diesel from the same station, reset trip computer, 3 mile to mway, dawdle 22 miles to work, and 22 miles back along the motorway home before turning off. Both sets of journeys were comparable, light traffic, cruise control set to dawdle in 6th, no queueing at the work end of things.
On the first fill, it was a lovely sunny dry day, and the comp read just over 60mpg when pulling off the motorway. The second fill was last Friday, rain both directions, torrential on the return journey (which is the more economical one), the trip this time read a much less frugal 51mpg.
I've noticed this sort of pattern before (wind/rain heavily effecting economy), but this is the only time I have had such a close comparison.
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Wind is another factor that affects fuel economy, unless it's a following wind.
As air friction in front and over/under a car increases then more fuel used to overcome it.
A combination of strong wind (head or cross) and heavy rain, coupled with smaller factors such as using wipers, lights etc., would account for the differences you experienced.
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Fuel consumption can vary due to so many factors (including the method of calculation!) that I wouldn't attach any significance to a 3 mpg difference.
--
L\'escargot.
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Wind is another factor that affects fuel economy unless it's a following wind.
Wouldn't a following wind improve fuel economy?
I once helped investigate a case of a camper van whose elevating roof allegedly lifted by itself, causing the wind to blow the camper into another vehicle. "The Case of the Sailing Camper" was eventually put down to just an old bat whose foot slipped off the pedal. Tests conducted during a gale showed that the brakes were easily capable of stopping the vehicle, even with full sails set.
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>Wouldn't a following wind improve fuel economy?
Wind ought to be pretty much a zero-sum game - unless you're on a round-the-world trip! I know sometimes it blows on the way out but not on the way home, or whatever, but over a tankful I'd expect to spend as much time driving into the wind as with it.
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Following wind should improve fuel economy, however if it exactly matches the speed of the car and its direction, then there would be very little air flow past the rad, so at higher speeds the electric cooling fan cuts in and so uses more juice - does this balance out the lower air resistance?. But I guess these type of conditions are very rare (what with turbulence around structures etc and variations in wind speed direction). Do any car manufacturers have open days at their design and testing facilities? Maybe should book a trip and fire these questions at their computer modelling guys?
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>...if it [wind] exactly matches the speed of the car and its direction, then there would be very little air flow past the rad...
Attention all shipping! The Met Office issued the following gale warning to shipping at 1254 BST today...
A gale (Beaufort force 8) starts at 39 mph. Motorway speed is equivalent to force 11 or 12 - 'violent storm' or 'hurricane'. If the wind is blowing as fast as you can drive, you probably shouldn't be driving at all!
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Neither the wind velocity vector or the velocity vector of the car remain constant for any period of time. The wind vector is also varies in a complicated way with respect to position, because of turbulence, blocking effects of buildings and boundary layer effects from the ground itself. The case you ask about is so unlikely that it isn't woth bothering with.
However, the power required to drive the fans is truly negligible when compared with the power required to overcome aerodynamic drag at any reasonable speed. Most experimental and concept cars do not include a large air intake, because the drag and losses from this far outweigh the benefits.
While the question which kicks the thread off is technically quite interesting, I can't see that changing the climate to give us 3mpg extra is ever going to be high on anyone's agenda. Also, while there are many reasons for wishing to leave these shores for better cliamtes, getting an extra 3mpg isn't one of them!
Number_Cruncher
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BMW 535D, I live in Lincolnshire where it is very flat, normally average about 40/41mpg on a good run in decent weather when I tickle he gently or 36mpg across a tank of mixed driving. Just had new tyres and increased size on rear from 245 to 275 (more resistance) and then done a 300 mile journey up to Glasgow. Obvioulsly this had far more elevation change than usual local driving and the weather was horrendous for 50% of the journey with heavy rain, wind and standing water. A pretty clear run traffic wise at night! So maintained cruise at 70mph for a good proportion of the trip. Averaging about 65mph. Saw consumption drop to 36.2 so combination of wider tyres, win, rain and elevation cost about 10%mpg.
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An old thread, but no harm in resurrecting it. Welcome to the forum.
I find that wind has more of an effect on mpg than rain (although with rain, wipers, lights etc will make a small difference).
I go up the A303 fairly often from Dorset to Berkshire. Last Sunday, with a strong westerly wind blowing, I got 42 mpg on the way there (2.0 petrol Audi Q2) and 36 mpg on the way back. Normally the wind is less strong and if it's from the west the difference between going wasy and going west is about 2 mpg.
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.... the difference between going wasy and going west is about 2 mpg.
I like that word, Avant .... :-)
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.... the difference between going wasy and going west is about 2 mpg.
I like that word, Avant .... :-)
Blimmin' auto correct on a phone/tablet! I changes the word when you don't want it to, and then doesn't when you do!
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Since you like it I won't edit it! 'Going west' has more than one meaning.....
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Drag co-efficient used to be all the rage as quoted by car manufacturers before the brick culture of the MPV/SUV and the rest began appearing on our roads. I would not buy one of the latter because of the obvious wind resistance of the vehicles( but concede could be useful in a tail wind!) For the boffins on here how much extra weight is being carried by the vehicle in heavy rain, water on the car or washing underneath.
I have noticed or is it just psychological that my car seems to cut through the air better after it has had a good polish
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Driving through even a half-inch of surface water during heavy rain there is considerable drag as the tyres shift water aside. Slush and snow have the same effect, though in those conditions speed is usually lower anyway.
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Drag co-efficient used to be all the rage as quoted by car manufacturers before the brick culture of the MPV/SUV and the rest began appearing on our roads. I would not buy one of the latter because of the obvious wind resistance of the vehicles( but concede could be useful in a tail wind!) For the boffins on here how much extra weight is being carried by the vehicle in heavy rain, water on the car or washing underneath.
I have noticed or is it just psychological that my car seems to cut through the air better after it has had a good polish
Mines definitely more economical after a weekends 8 stage polish job LOL
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So...Return journey, Generally Downhill, very little rain, less wind...39.7mpg
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