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Old 12-07-2010, 10:59 AM   #1 (permalink)
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Winter Driving - (does odometer register accurately if wheels slip in snow/ice?)

Hi everyone,

Since yesterday was the first big snowstorm up here in Montreal, I started thinking...

If you drive a car without traction control, and you spin your tires (involuntarily, of course), does the odometer record a longer distance than the real one?
This would be a source of error in MPG calculations (At least the ones computed by hand, I don't know about the ScanGauge)

I'll try to gather data over the next few weeks & find out, but I was wondering if someone knows?

P.S. In summer, my speedometer usually reads a distance about 3% shorter than according to the GPS.

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Old 12-07-2010, 11:16 AM   #2 (permalink)
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i think that would depend on where the odometer gets it's signal, if it's on the driven wheels it would record the extra distance, if it's on the non driven wheels it wouldn't, also turns might affect the actual distance vs the one recorded depending on what the odometer actually reads. different tire diameters will also affect the accuracy.
Some gps devices can record the distance traveled as wel as show the speed, and this will generally be more accurate as there are no mechanical variables.

I do think however you'd have to spin your wheels more than the occasional slip for it to have a signifficant effect on your milage figures, especially since the propper response to spinning wheens is to lift of the throttle and let them slow down so they can regain traction.
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Old 12-07-2010, 11:57 AM   #3 (permalink)
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I believe most speedometers/odometers get their signal from the transmission output shaft, transfer case for 4x4's. So yes, it would record extra distance.
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Old 12-07-2010, 07:09 PM   #4 (permalink)
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I have noticed the same thing via my GPS speedo, glad I am not the only one!
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Old 12-07-2010, 09:40 PM   #5 (permalink)
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I know that when I drive through deep snow, the odometer says it is a longer distance than it really is because there is some constant slip at the wheels.
It sometimes is as high as 20-30% when in deep snow.
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Old 12-08-2010, 09:20 AM   #6 (permalink)
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that's not to good, because it means your winter milage is even worse than than it would appear. especially since i imagine spinning the wheels freely on ice takes less energy than travle the actual distance.

another way to check any anomalies is to use the trip odometer and compare identical routes like you're commute with snow and no snow. also google maps can give you a fairly accurate guess on how long your actuall route is.
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Old 12-08-2010, 05:56 PM   #7 (permalink)
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I drove a 9.2mi route home from work on 4" of frozen slush (at 20mph or so), dragging an underbody panel for about 8mi of the way. Basically, perfect conditions for lots of wheelspin. The odometer registered 9.6mi.

I haven't seen any deviations from the expected odometer readings since then, but the roads have all been at least sorta-plowed.
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Old 12-08-2010, 09:02 PM   #8 (permalink)
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Old 12-10-2010, 10:52 AM   #9 (permalink)
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So, you're all confirming what I thought: winter mileage is actually worse than what it is because the true distance is actually shorter than the one displayed by the odometer?
My 23.5 mpg winter driving in a compact car suddenly looks even worse!
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Old 12-10-2010, 04:55 PM   #10 (permalink)
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...an "old" rally car story: back in the days of rear-wheel-drive rally cars, they used to attach their Halda Speedpilot™ input shafts to the FRONT wheels (not to the REAR wheels via transmission) for more accurate speed & distance values.

...why? because whenever the car got "airborne," the driven REAR wheels tended to speed-up (just like on ice) and thus incorrectly indicate TOO much distance covered and TOO fast velocity.

...by taking the speedometer input off the FRONT wheels (which tended to continue rotating at the SAME speed when airborne instead of speeding-up) the Speedpilot would register more correct distance & velocity values.

...for those who've never seen one:



Last edited by gone-ot; 12-10-2010 at 06:10 PM.. Reason: added via transmission
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mpg, odometer, precision, snow, winter

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