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Speedometer-Odometer correlation??
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VWinVT
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PostPosted: Fri Aug 28, 2015 4:16 pm    Post subject: Speedometer-Odometer correlation?? Reply with quote

Here is my issue. The speedo reads 6-7 mph slow....does this mean that my odometer is not counting miles accurately either? Trying to nail down some MPG findings.....
Thanks
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PDXWesty
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PostPosted: Fri Aug 28, 2015 4:21 pm    Post subject: Reply with quote

That's exactly what that means.
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VWinVT
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PostPosted: Fri Aug 28, 2015 4:24 pm    Post subject: Reply with quote

PDXWesty wrote:
That's exactly what that means.


Ok...I had a feeling this was the case, so if the speedo is reading slow...the odometer reading is lower than it should be and should show more miles than it does??
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oldhuldy67
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PostPosted: Fri Aug 28, 2015 5:16 pm    Post subject: Reply with quote

My 1980 is 3-4 MPH slower than it shows, and that's after I replaced the two-part speedo cable with a single cable to remedy the bouncing needle and noise. I was wondering if anyone stayed awake in Algebra instead of sleeping behind the big kid like I did and has a mathematical formula for deriving the actual mileage...
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Currently:1980 Westy Westfalia Poptop (a total stock-look rebuild with all my favorite camping mods)
1967 stock Sunroof Beetle
Formerly:
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1973 Squareback (sold on a lark)
1973 Super Beetle (crashed..walked away without a scratch)
1972 Standard Beetle (lost in a relationship..)
1966 Beetle (the Black Beauty, my Dad's)
1964 Beetle (the Beige Bug, my beginning beater, also my Dad's) Thanks, Dad.
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VWinVT
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PostPosted: Fri Aug 28, 2015 5:26 pm    Post subject: Reply with quote

oldhuldy67 wrote:
My 1980 is 3-4 MPH slower than it shows, and that's after I replaced the two-part speedo cable with a single cable to remedy the bouncing needle and noise. I was wondering if anyone stayed awake in Algebra instead of sleeping behind the big kid like I did and has a mathematical formula for deriving the actual mileage...


Yea....I was sleeping too!! 😀
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Ahwahnee
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PostPosted: Fri Aug 28, 2015 5:26 pm    Post subject: Reply with quote

It is possible for the speedo to read low (or high) without the odo being inaccurate.

The only way to assess the accuracy of the odo is by actual calculation either using math (and tire circumference, r/p ratio, etc or by simply driving a measured distance and comparing your odo reading to actual.
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VWinVT
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PostPosted: Fri Aug 28, 2015 5:31 pm    Post subject: Reply with quote

We're planning a trip next weekend...I guess I will check,the odometer while on the highway.....
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Ahwahnee
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PostPosted: Fri Aug 28, 2015 6:00 pm    Post subject: Reply with quote

VWinVT wrote:
We're planning a trip next weekend...I guess I will check,the odometer while on the highway.....


In my experience the mile markers in some locales are not especially accurate - I suspect they may have been set by some guys in a highway maintenance truck looking at their own odometers. Still, perhaps better than nothing.

Some highways have 5 or 10 mile stretches designed for odometer checks with distances more carefully measured.
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?Waldo?
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PostPosted: Fri Aug 28, 2015 6:20 pm    Post subject: Reply with quote

PDXWesty wrote:
That's exactly what that means.


Not really. The odometer is gear driven off the cable. The motion of the speedo needle is *not* gear driven and instead the motion is induced by a spinning magnet against the resistance of a small spring. When new the odometer was designed to read accurately for stock size tires and the speedo was legally required to read high. The added age since new does not tend to make the speedo read more accurately but rather less as the resistance of that little spring weakens with repeated stretching. The odo, being gear driven, will read accurately until it is broken or someone changes the tire diameter.

Using a GPS phone to check odometer and speedometer accuracy is quite easy.
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morymob
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PostPosted: Sat Aug 29, 2015 4:14 am    Post subject: Reply with quote

A 10 mi mkr test is long enuf to get close miles on speedo, compare to a tom tom etc if u have one.
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VWCOOL
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PostPosted: Sat Aug 29, 2015 4:26 am    Post subject: Reply with quote

I once drove exactly 100km in one hour - measured by the odo and the roadside distance markers we have here in Australia - at 110km/h!

This was on a brand new (at the time, 2006) GM car

So no, the speedo and odometer in a car may not correlate to each other
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VWinVT
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PostPosted: Sat Aug 29, 2015 5:20 am    Post subject: Reply with quote

It sounds like I should spend some time with my GPS and try to get some MPG data that way. It sounds like accurate measurement is an issue across the board. Perhaps I should devote more time to this skill in my elementary science science classes!
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Big wheel turns by the grace of God.
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You're bound to cover just a little more ground.
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Ahwahnee
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PostPosted: Sat Aug 29, 2015 5:29 am    Post subject: Reply with quote

In the US there was a federal law for 1984 that imposed a penalty if a manufacture's speedo read low - VW made sure they were okay by having the units in the 84s read way high.

So high that there is a conversion formula in the Bentley for calculating your actual speed based on what the gauge said. Yeah, that's handy.

In 85 the Fed realized they needed to say a bit more and also specified a limit on how high a reading was okay.

I finally just mounted a bicycle digital speedometer (14 bucks):

Image may have been reduced in size. Click image to view fullscreen.


Can even tell me how many calories I've burned.
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Ahwahnee
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PostPosted: Sat Aug 29, 2015 5:32 am    Post subject: Reply with quote

morymob wrote:
A 10 mi mkr test is long enuf to get close miles on speedo, compare to a tom tom etc if u have one.


I don't use a GPS much so I am unsure -- but wouldn't it measure distance travelled in a straight line (vs over-the-road)?

If so then one would need a long straight stretch of road.
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RCB
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PostPosted: Sat Aug 29, 2015 6:33 am    Post subject: Reply with quote

oldhuldy67 wrote:
My 1980 is 3-4 MPH slower than it shows, and that's after I replaced the two-part speedo cable with a single cable to remedy the bouncing needle and noise. I was wondering if anyone stayed awake in Algebra instead of sleeping behind the big kid like I did and has a mathematical formula for deriving the actual mileage...



I also have an air cooled and replaced the speedo cable with a single one. Got it from CiP1 as it had pretty good marks ( stars ) actually. The mechanic who replaced it claims the aftermarket ones are not as good a fit as the OEM. Its bouncing a bit after a few hundred miles but only at speeds of 45-60.
How is the one you have behaving.

Go-Westy has a remedy for the bouncing needle and that's to slap the side of the instrument cowling but Id rather not do that.
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PostPosted: Sat Aug 29, 2015 7:10 am    Post subject: Reply with quote

GPS measures speed by calculating distance (based on continuous tracking of individual locations) and time. Time comes from atomic clocks so it's accurate. Locations are tracked using triangulation using 3 or more satellites, i.e. The gps will track elevation changes, lane changes, etc. quite accurately.

Fleet vehicle manufacturers manage their significant liability by tracking vehicle speeds with GPS. The big orange Schneider trucks, metro busses, etc. travel the speed limit because their big brother is watching and logging their GPS speed constantly.

I rely on GPS indicated speed (and thus distance) to set my speed to keep me from getting a ticket -it's more accurate.

Over long distances the stock odometer errors will be cumulative while individual GPS location errors average out.

Methods to check speedo, odometer accuracy:
1: corectly setup bicycle speedo - I've used one for years on my motorcycle (or simply do the math yourself using transmission/final drive ratios and the tach)
2: odometer mileage check points occasionally found in interstates
3: GPS after comparing the three, you'll probably come to trust the gps.
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oldhuldy67
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PostPosted: Sat Aug 29, 2015 8:07 am    Post subject: The joys of electro-mechanical minds Reply with quote

Ahh, Dr. Libby, I presume...thank you, for as I have so long suspected and been the ever grateful benefactor of, there are those of infinitely greater electro-mechanical insight who stand ever ready to come to the aid of the curious ( and woefully uninformed) and address the ever-present enigmata that so often accompanies art and science, and plagues the minds of those of us who consider ourselves fortunate indeed to simply be known as amateur automotive adventurers.....thank you so very much
So, I can guess I can relax now...let my OCD loose on something else..I think..I think I will just try get my radio working so I won't think about mileage...or speed...and continue to enjoy the drive.
Thank you! Whew...I feel a lot bet now about driving 40 miles an hour in a 45 MPH zone...thanks!

Andrew A. Libby wrote:
PDXWesty wrote:
That's exactly what that means.


Not really. The odometer is gear driven off the cable. The motion of the speedo needle is *not* gear driven and instead the motion is induced by a spinning magnet against the resistance of a small spring. When new the odometer was designed to read accurately for stock size tires and the speedo was legally required to read high. The added age since new does not tend to make the speedo read more accurately but rather less as the resistance of that little spring weakens with repeated stretching. The odo, being gear driven, will read accurately until it is broken or someone changes the tire diameter.

Using a GPS phone to check odometer and speedometer accuracy is quite easy.

_________________
Currently:1980 Westy Westfalia Poptop (a total stock-look rebuild with all my favorite camping mods)
1967 stock Sunroof Beetle
Formerly:
1978 Westy Campmobile, the"EM50 Urban Assault Vehicle" code named "The Pickle"
1973 Squareback (sold on a lark)
1973 Super Beetle (crashed..walked away without a scratch)
1972 Standard Beetle (lost in a relationship..)
1966 Beetle (the Black Beauty, my Dad's)
1964 Beetle (the Beige Bug, my beginning beater, also my Dad's) Thanks, Dad.
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Howesight
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PostPosted: Sat Aug 29, 2015 8:46 am    Post subject: Reply with quote

My rig has a 0.70 fourth gear and 16" 215/75 BFG tires. Interestingly, my odometer reads about 99.5% accurate, according to my GPS. Of course, a 4th gear gearing change does not affect speed drive speed, but the tire change definitely does.

My speedo is a different story. It reads 6 mph slower (in the 40 to 80 mph range) than the actual vehicle speed as indicated by my GPS.

I suppose it's true that YMMV!
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jobenaus
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PostPosted: Sat Aug 29, 2015 10:22 am    Post subject: Reply with quote

You could check your speed at one of those flashing signs they put up at school zones occasionally....just don't go through at 60 mph or it could get expensive. Smile
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Wildthings
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PostPosted: Sat Aug 29, 2015 10:39 am    Post subject: Reply with quote

jobenaus wrote:
You could check your speed at one of those flashing signs they put up at school zones occasionally....just don't go through at 60 mph or it could get expensive. Smile


The Yukon must have changed a bit since I ventured up that way. Wink
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