Someone recently told me (without cite) that an all-wheel drive vehicle has less rolling resistance than an otherwise identical vehicle with fewer drive wheels. Is there any evidence for this? I can understand it’s true if the tires are slipping on loose or slippery surface, but other situations? We were discussing bicycles but if it were true, I imagine it must also be true for cars as well.
The lowest rolling resistance would be for a 2WD vehicle, likely a front wheel drive. A 4WD with lockable/unlockable hubs would be next greater, and the highest would be an AWD vehicle because there is more physical mass being turned as you climb the scale. This comparison assumes vehicles of equal size and drive train components.
Eolling resisyance is a pretty inexact term – I’ve never seen an auto maker offer a statistic on it. I know that my mountain bike has significantly more rolling resisyance than my road bike because of at least 4 factors I can point to – total weight, the amount of rubber in contact with the road at any given time, the heaver tire rims on the mountain bike that resists acceleration, and wind resistance.
I suggest there are too many factors that contribute to make blanket statement. However, it’s arguable that rolling resistance is really an overall measure of vehicle efficiency – which vehicles use the least energy to accelerate and maintain speed. AKA gas mileage. If you’re willing to accept that definition (and you may not be) then 4WD vehicles tend to be at the low end of efficiency compared to their 2WD counterparts.
Type of drive train has more to do with it and then weight of and drag of rotating parts. The little 2005 Subaru etc. VS the 1974 Chevy Blazer etc. with auto lock front axle are not in the same ball park and can’t really be compared. Throw in manual hubs that can’t be locked out and it get worse.
Does the car have a free wheel overdrive or not? What kind of tranny hook up and torque converter? Radial tires? What weight differential lube?
At 0 MPH ( start) or at 50 MPH (wind resistance coming into play )
Lots of things to put in the mix.
Are you talking resistance to rolling forward, or resistance to rolling over sideways?
Boyo Jim, and danceswithcats both pretty well nailed it.
A 2 WD car will have lesss stuff being powered, and turning than an AWD car.
My current ride is a 05 Volvo XC70 AWD. It gets about 1-2 mpg less than a standard V70 (same body, slightly different ride height) in a FWD version. The AWD has the following items that the FWD doesn’t
[ul]
[li]Angle Gear [/li][li]Driveshaft[/li][li]Haldex AWD coupling[/li][li]Rear axle (rear diff)[/li][li]rear driveshafts[/li][/ul]
It takes some power to turn all of those items, power costs gas. More power = less gas mileage.
A trip though a car dealers lot where both AWD and FWD cars are sold should verify this.
Thanks, for some reason it didn’t occur to me to look at fuel efficiency numbers…
But how about if we could isolate the rolling resistance of just the tires? Say there’s a clutch right on each wheel, and you can choose to power or freewheel that wheel without affecting mechanical resistance? And ignoring wind resistance or hills, but not ignoring effects of rough terrain? In other words, just looking at the energy loss in the tire itself and the suspension system.
Okay, if you disconnected all of the drive systems and were able to hack out anything that prevented the four wheels from being absolutely free-wheeling, and put on the same tires, same inflation, ensured the cars weighed the same and distributed the weight the same, then yes, you’d have the same rolling resistance on an AWD as a 2WD. But then by time you did all that, you’d really not have 2WD vs 4WD, but something the junk yard would give you $75 to haul away for scrap (take out your stereo, first).
For the particular AWD that I’m starting to build (2007 model), the floor pan is a little different to accomodate the drive shaft, and I think it may be a couple of inches higher, so, okay, there may be some slight differences, but it won’t be inherently related to AWD vs. FWD.
Yes different tires have different rolling resistances. Hnowever if you put a clutch on each wheel and all that, you would no longer have a AWD, FWD, or and RWD you would have a NWD that is a No Wheel Drive.
In the business we call that a trailer. 
I don’t think anyone grades tires uniformly for rolling resistance.
I think we can all agree.
Maybe no uniformity, but I know they’re used. Back when I was shopping for my '95 Civic EX, the CX (the economiser model) was touted as having a different version of the EX engine (different valve timings for grandmothers), but additionally, low rolling resistance tires.
I agree with what you are saying, some of our cars have lower rolling resistance tires. When I went to write that post I cruised over to the goverment website for tire rating and discovered that there is not a rating for rolling resistance. Tire wear sure, temp got it. Rolling resistance nada.
So I am sure that the engineers from say Michlien could tell our engineers which one of their tires has the least rolling resistance. There does not appear to be a practical way for a consumer to compare a Michlien to say a Uniroyal to determine which one will give better gas mileage.
Seems that it’s a timely question … there is a UN ECE Working Group that is apparantly due to report on the 7th April with a standardised method of measuring this parameter. Also ISO 8767:1992 seems to cover this area (“Passenger car tyres – Methods of measuring rolling resistance”). So maybe there will be widely-published data to standard conditions in the near future.
In the meantime, there IS published data on rolling resistance of cycle tyres here(here, for instance) (because they’re nuts and care obsessively about these things), and there is some limited data for particular car tyres (here, for instance) .