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mikemtnbike
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Joined: March 26, 2015
Posts: 2796
Location: North Carolina
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 Posted: Mon Mar 05, 2018 8:40 am Post subject: Speedo slow reading, effect on odometer |
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I think my math is right, but there's a reason I work in the social sciences...if my speedo reads slow (slightly bigger wheels/tires), that should mean odometer is slightly low on it's reading, correct?
Thanks in advance, tried searching, no obvious results, just want to know my mileage now that fixed odometer.
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1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled  https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual |
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bobbyblack 
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Joined: May 21, 2015
Posts: 4351
Location: United States, Iowa
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| Posted: Mon Mar 05, 2018 8:44 am Post subject: Re: Speedo slow reading, effect on odometer |
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The speedo needle and the Odo could be both wrong, or one of each. Also, it is possible for the speedo to show slow, and the Odo show more miles per mile... It's not always the same at any rate.
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dobryan
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Joined: March 24, 2006
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Location: Brookeville, MD
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?Waldo?
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Joined: February 22, 2006
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| Posted: Mon Mar 05, 2018 8:53 am Post subject: Re: Speedo slow reading, effect on odometer |
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| dobryan wrote: |
The operation of the speedo does not affect the accuracy of the odo.
No correlation other than they share the same input from the cable. |
While that is true, the OP is specifically asking about the effect from different sized tires. Larger/smaller tires changes the input so it will affect both speedo and odo. Larger than stock tires will cause the odo to accrue miles slower.
Last edited by ?Waldo? on Mon Mar 05, 2018 8:54 am; edited 1 time in total |
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Aryana
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Joined: November 09, 2015
Posts: 568
Location: Southern CA
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| Posted: Mon Mar 05, 2018 8:54 am Post subject: Re: Speedo slow reading, effect on odometer |
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| If you’ve put tires with a larger circumference than stock then your speedo and odometer will read less at any given speed than before you put on the bigger diameter tires. |
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mikemtnbike
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Joined: March 26, 2015
Posts: 2796
Location: North Carolina
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| Posted: Mon Mar 05, 2018 8:55 am Post subject: Re: Speedo slow reading, effect on odometer |
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So, larger tires making speedo slow would *probably* make odo read slow as well because same input?
EDIT Thanks Aryana. Good news for me, as reading without adjustment was just below 17mpg. I'm really happy with that.
_________________
1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual |
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dobryan
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Joined: March 24, 2006
Posts: 16503
Location: Brookeville, MD
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| Posted: Mon Mar 05, 2018 9:02 am Post subject: Re: Speedo slow reading, effect on odometer |
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| ?Waldo? wrote: |
| dobryan wrote: |
The operation of the speedo does not affect the accuracy of the odo.
No correlation other than they share the same input from the cable. |
While that is true, the OP is specifically asking about the effect from different sized tires. Larger/smaller tires changes the input so it will affect both speedo and odo. Larger than stock tires will cause the odo to accrue miles slower. |
Yep. Apparently reading comprehension is not my strong suit this morning. 
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Dave O
'87 Westy w/ 2002 Subaru EJ25 and Peloquin TBD
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RicoS
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Joined: December 02, 2006
Posts: 583
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| Posted: Mon Mar 05, 2018 9:08 am Post subject: Re: Speedo slow reading, effect on odometer |
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| Aryana wrote: |
| If you’ve put tires with a larger circumference than stock then your speedo and odometer will read less at any given speed than before you put on the bigger diameter tires. |
Praise dee Lawrd! The CORRECT and succinct answer.
Richie (near The Burgh) |
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Jon_slider
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Joined: April 11, 2007
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Location: Santa Cruz, Crowdifornia
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| Posted: Mon Mar 05, 2018 10:58 am Post subject: Re: Speedo slow reading, effect on odometer |
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| mikemtnbike wrote: |
| if my speedo reads slow (slightly bigger wheels/tires), that should mean odometer is slightly low on it's reading, correct? |
not necessarily
the speedo is only accurate if it is calibrated to be accurate
from stock the speedo reads high by as much as 6%, and larger tires will change how far the speedo is off
the odometer is completely independent and can be off by a completely different amount than the speedo. It makes no difference what the tire size is, the odo and speedo each dance to the beat of a different drummer
to know how far your speedo is off, with your tires, compare it to the speed from a GPS app on a smartphone.
to know how far off your odometer is, with your tires, compare to the distance calculated by a GPS navigation app on a smartphone.
it could be inaccurate to assume that the odometer is reading lower than reality, just because the speedo is reading lower, regardless what tire size you have.
do some tests, and let us know your actual GPS results for speed and distance
then you will know how far your speedo is off, and how far your odometer is off. The two numbers will most likely not be the same percentage off.
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mikemtnbike
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Joined: March 26, 2015
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Location: North Carolina
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| Posted: Mon Mar 05, 2018 12:02 pm Post subject: Re: Speedo slow reading, effect on odometer |
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My speedo is about 7mph slow at 60mph per gps. In other words, when my speedo says I'm going 60, I'm going 67. This also matches rpms/speeds before the bigger tires went on. The amount of slowness increases as my speed increases- in other words, it reads about 4mph slow at 45 mph. This makes sense to me as there are more tire rotations going on, so the error in measurement increases in relation to number of rotations.
Thanks for the answers, everyone.
_________________
1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual |
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Jon_slider
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Joined: April 11, 2007
Posts: 5091
Location: Santa Cruz, Crowdifornia
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| Posted: Mon Mar 05, 2018 2:48 pm Post subject: Re: Speedo slow reading, effect on odometer |
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| mikemtnbike wrote: |
| when my speedo says I'm going 60, I'm going 67.... it reads about 4mph slow at 45 mph. |
yes, the error is a percentage of your speed, not the same number of miles per hour at all speeds. You seem to be off by about 10%.. curious what size tires
also curious what percent your odometer is off
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mikemtnbike
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Joined: March 26, 2015
Posts: 2796
Location: North Carolina
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| Posted: Mon Mar 05, 2018 3:01 pm Post subject: Re: Speedo slow reading, effect on odometer |
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215/60/16 Nokian Entyres.
I'll check the odometer against speedo app on my phone next time I drive the van, which probably won't be until next weekend. Don't know why I didn't think of that in the first place.
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1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual |
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Jon_slider
Samba Member
Joined: April 11, 2007
Posts: 5091
Location: Santa Cruz, Crowdifornia
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| Posted: Mon Mar 05, 2018 5:25 pm Post subject: Re: Speedo slow reading, effect on odometer |
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| mikemtnbike wrote: |
| 215/60/16 |
3% taller than stock, not enough to explain why your speedo reads 10% slow
It is possible that your odometer might only be off by 3%, not 10% like your speedo..
so, if you calculated 17mpg, and add 3% that would be only 0.5 mile per gallon off
as you can see, tire size is not the only factor that can make a speedo inaccurate, and an odometer will most likely be inaccurate by a different (smaller) amount.
I say that based on the observation that my Vanagon odometers have been more accurate than their speedos.
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Aryana
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Joined: November 09, 2015
Posts: 568
Location: Southern CA
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| Posted: Mon Mar 05, 2018 6:23 pm Post subject: Re: Speedo slow reading, effect on odometer |
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Think about why that is.
The odometer counts revolutions. This is straightforward. After so many revolutions, it says 1 mile has passed.
The speed indication on the speedo is a magnet (the needle) pulled by another rotating magnet (the cable).
The indicated speed depends on a generalized coorelation between this cable spinning a magnet at a variety of speeds to pull the other magnet on the needle and point to numbers on the speedo face. |
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