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VWinVT Samba Member
Joined: April 21, 2013 Posts: 1541 Location: North East Kingdom
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Posted: Fri Aug 28, 2015 4:16 pm Post subject: Speedometer-Odometer correlation?? |
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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 _________________ 1981 Westfalia with a 2001 Cabrio ABA
Small wheel turns by the firing rod,
Big wheel turns by the grace of God.
Every time that wheel turns 'round,
You're bound to cover just a little more ground. |
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PDXWesty Samba Member
Joined: April 11, 2006 Posts: 6235 Location: Portland OR
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Posted: Fri Aug 28, 2015 4:21 pm Post subject: |
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That's exactly what that means. _________________ 89 Westy 2.1 Auto |
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VWinVT Samba Member
Joined: April 21, 2013 Posts: 1541 Location: North East Kingdom
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Posted: Fri Aug 28, 2015 4:24 pm Post subject: |
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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?? _________________ 1981 Westfalia with a 2001 Cabrio ABA
Small wheel turns by the firing rod,
Big wheel turns by the grace of God.
Every time that wheel turns 'round,
You're bound to cover just a little more ground. |
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oldhuldy67 Samba Member
Joined: November 25, 2008 Posts: 506 Location: somewhere with a southern exposure
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Posted: Fri Aug 28, 2015 5:16 pm Post subject: |
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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... _________________ 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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VWinVT Samba Member
Joined: April 21, 2013 Posts: 1541 Location: North East Kingdom
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Posted: Fri Aug 28, 2015 5:26 pm Post subject: |
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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!! 😀 _________________ 1981 Westfalia with a 2001 Cabrio ABA
Small wheel turns by the firing rod,
Big wheel turns by the grace of God.
Every time that wheel turns 'round,
You're bound to cover just a little more ground. |
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Ahwahnee Samba Member
Joined: June 05, 2010 Posts: 9776 Location: Mt Lemmon, AZ
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Posted: Fri Aug 28, 2015 5:26 pm Post subject: |
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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 Samba Member
Joined: April 21, 2013 Posts: 1541 Location: North East Kingdom
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Posted: Fri Aug 28, 2015 5:31 pm Post subject: |
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We're planning a trip next weekend...I guess I will check,the odometer while on the highway..... _________________ 1981 Westfalia with a 2001 Cabrio ABA
Small wheel turns by the firing rod,
Big wheel turns by the grace of God.
Every time that wheel turns 'round,
You're bound to cover just a little more ground. |
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Ahwahnee Samba Member
Joined: June 05, 2010 Posts: 9776 Location: Mt Lemmon, AZ
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Posted: Fri Aug 28, 2015 6:00 pm Post subject: |
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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? Samba Member
Joined: February 22, 2006 Posts: 9752 Location: Where?
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Posted: Fri Aug 28, 2015 6:20 pm Post subject: |
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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 Samba Member
Joined: November 09, 2007 Posts: 4683 Location: east-tn
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Posted: Sat Aug 29, 2015 4:14 am Post subject: |
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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 Samba Member
Joined: June 02, 2006 Posts: 1821 Location: Down under
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Posted: Sat Aug 29, 2015 4:26 am Post subject: |
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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 Samba Member
Joined: April 21, 2013 Posts: 1541 Location: North East Kingdom
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Posted: Sat Aug 29, 2015 5:20 am Post subject: |
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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! _________________ 1981 Westfalia with a 2001 Cabrio ABA
Small wheel turns by the firing rod,
Big wheel turns by the grace of God.
Every time that wheel turns 'round,
You're bound to cover just a little more ground. |
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Ahwahnee Samba Member
Joined: June 05, 2010 Posts: 9776 Location: Mt Lemmon, AZ
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Posted: Sat Aug 29, 2015 5:29 am Post subject: |
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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):
Can even tell me how many calories I've burned. |
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Ahwahnee Samba Member
Joined: June 05, 2010 Posts: 9776 Location: Mt Lemmon, AZ
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Posted: Sat Aug 29, 2015 5:32 am Post subject: |
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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 Samba Member
Joined: September 05, 2005 Posts: 4143 Location: San Francisco-Bay Area
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Posted: Sat Aug 29, 2015 6:33 am Post subject: |
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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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jimf909 Samba Member
Joined: April 03, 2014 Posts: 7404 Location: WA/ID
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Posted: Sat Aug 29, 2015 7:10 am Post subject: |
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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. _________________ - Jim
Abscate wrote: |
Do not get killed, do not kill others.
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Current: 1990 Westy Camper - Bostig RG4, 2wd, manual trans w/Peloquin, NAHT high-top, 280 ah LFP battery, 160 watts solar, Flash Silver, seam rust, bondo, etc., etc.
Past: 1985 Westy Camper - 1.9 wbx, 2wd, manual trans, Merian Brown, (sold after 17 years to Northwesty who converted it to a Syncro). |
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oldhuldy67 Samba Member
Joined: November 25, 2008 Posts: 506 Location: somewhere with a southern exposure
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Posted: Sat Aug 29, 2015 8:07 am Post subject: The joys of electro-mechanical minds |
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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 Samba Member
Joined: July 02, 2008 Posts: 3262 Location: Vancouver, B.C.
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Posted: Sat Aug 29, 2015 8:46 am Post subject: |
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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! _________________ '86 Syncro Westy SVX |
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jobenaus Samba Member
Joined: March 27, 2014 Posts: 100 Location: Yukon
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Posted: Sat Aug 29, 2015 10:22 am Post subject: |
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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. |
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Wildthings Samba Member
Joined: March 13, 2005 Posts: 50262
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Posted: Sat Aug 29, 2015 10:39 am Post subject: |
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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. |
The Yukon must have changed a bit since I ventured up that way. |
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