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Odometer Discrepancy
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mondshine
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PostPosted: Sun Jun 26, 2022 2:31 pm    Post subject: Odometer Discrepancy Reply with quote

After bitching since forever about lousy fuel mileage, today I compared my odometer reading over a measured course to the distance indicated on Google Maps.

My odometer indicated 15.1 miles, while Google Maps shows 20.3.
The odometer operates smoothly; showing no jerky motion or apparent missing teeth.

My speedometer indicates 60 MPH when I pass a roadside radar display that reads 63 MPH. That's about a 5% error.

My odometer is showing about a 34% error!

I am interested in opinions about this.

Thanks, Mondshine
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Glenn Premium Member
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PostPosted: Sun Jun 26, 2022 2:34 pm    Post subject: Re: Odometer Discrepancy Reply with quote

The odo should depend on the rotation of the cable. So i don't see how it can be that far off.
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PostPosted: Sun Jun 26, 2022 3:23 pm    Post subject: Re: Odometer Discrepancy Reply with quote

Tire size could make a difference, but I can’t imagine there would be that big of a discrepancy. Could the hole in the dust cap be worn and causing the cable to slip? That wouldn’t explain why the Speedo is kind of close but the odometer is way off. Interesting.
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mondshine
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PostPosted: Sun Jun 26, 2022 4:19 pm    Post subject: Re: Odometer Discrepancy Reply with quote

My dust cap hole is square and in good shape.
It's a huge discrepancy, at least when compared to the speedometer error.

I am wondering if the driven gear is slipping on the shaft that drives the odometer "barrels".

I have read where some Vanagon guys have reported a similar problem; like 33% odometer error.

I have a 1300 mile road trip planned next month, so I will make some tests with mile markers and a stopwatch.

The thing is, repairing this could be sort of self defeating (in a cheating sort of way), especially considering that the speedometer works well (enough).

If my calculations are correct, this odometer error would change my fuel consumption from 18 to 24+ MPG... Big difference.
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PostPosted: Sun Jun 26, 2022 4:45 pm    Post subject: Re: Odometer Discrepancy Reply with quote

There's a plastic gear on the odo shaft and it often cracks. A drop of super glue is the easiest fix.
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74 Thing
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PostPosted: Sun Jun 26, 2022 4:46 pm    Post subject: Re: Odometer Discrepancy Reply with quote

I think you can use GPS as well for some of your research.

Looking forward to the results.
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mondshine
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PostPosted: Sun Jun 26, 2022 5:17 pm    Post subject: Re: Odometer Discrepancy Reply with quote

74 Thing wrote:
I think you can use GPS as well for some of your research.

Looking forward to the results.


I loaded an iPhone app that uses GPS for speedo and odo.
I will try it out tomorrow.

Meanwhile, I don't want to take my speedometer apart until after I return from Kubel Treffen in mid-July. (Could change my mind about that.)

But I do feel better about the horrible fuel consumption I have been recording.
With the top down, 22 -24 MPG is OK... 18 MPG not OK.
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PostPosted: Sun Jun 26, 2022 9:06 pm    Post subject: Re: Odometer Discrepancy Reply with quote

Had my speedometer rebuilt a few years ago, works good, bur shows 20% less speed and miles than it should. when goin down the hisar showing 50 on speedo, I am really going 60. Maybe its the tore size, I expect, I just live with it.
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Semper_Dad
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PostPosted: Mon Jun 27, 2022 5:32 am    Post subject: Re: Odometer Discrepancy Reply with quote

My Speedo can be off by 10 miles faster than what I'm really doing. The error seems greater at higher speeds. If the speedo says 60, I'm really doing 50 as measured by GPS. Currently running Maxis Bighorn 27x8.5 R14LT. Should be no more than a 3MPH error. My original speedo was spot on but that was when I had stockish tires.

Found this speed correction formula that adjusts for tire sizes.

https://tiresize.com/speedometer-calibration/
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mondshine
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PostPosted: Mon Jun 27, 2022 7:44 pm    Post subject: Re: Odometer Discrepancy Reply with quote

I made a couple of trips today, using the “Speed Tracker” app on my phone.

On the first trip, my odometer recorded 12.2 miles, while GPS recorded 15.9 miles. About 30% error.
On the second trip, my odometer recorded 40 miles, while the GPS recorded 51 miles. About 28% error.

The main conclusion is that I am relieved that my fuel mileage is actually acceptable, considering I have been covering ~25% greater distance than I had assumed.

So anyway, I couldn’t stand waiting; I took my speedometer apart and found the 11 tooth gear that drives the barrels cracked and fitting loosely on the shaft.
I am surprised that the odometer worked at all.
So I ordered a new gear, and I’ll replace the gear in a few days.

I am not really obsessed with fuel mileage; when the gauge reads 1/4, I fill the tank.
But it will be interesting to note my fuel use on this 1300+ mile road trip coming up in July.
I expect to burn through 55-60 gallons of gas.

The trip log function of this “Speed Tracker” application functions like the trip computer on a contemporary car, with time, distance, average and max speed, etc.

Hoping for dry weather; should be a fun ride to the Kubel Treffen.
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mondshine
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PostPosted: Thu Jun 30, 2022 2:31 pm    Post subject: Re: Odometer Discrepancy Reply with quote

YIPPEE! All fixed.

My new 8.65mm 11 tooth gear fit perfectly, and now my odometer error exactly matches my speedometer error at about 3.5% under-reported (my tires are a slightly larger diameter than stock).

The gear, from odometergears.com was a little expensive ($2.82 per tooth), but I am glad it's fixed.

Safe travels, Mondshine
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