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Old 12-20-2012, 03:49 PM   #1
jadawgis732
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Default Speedometer/Odometer Question

My speedometer is off, according to my Garmin GPS and an app that uses GPS to track distance traveled over time. When my car's speedometer says 84, I'm actually only traveling 80. 53mph on the dash is only 50mph according to GPS, so naturally I became curious. My questions are:

1. Does the odometer feed off speedometer's reading? If so, were I to travel 80mph (according to GPS) for one hour, would I actually have put 83 miles on my car's odometer?

2. How does the odometer work? If I put bigger tires on the car, would I actually travel farther than my odometer would have me believe?

The vehicle in question is a 2006 Subaru Impreza 2.5i. Thanks for any and all help. And please pardon my NASIOC naivete if this is in the wrong section.
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Old 12-20-2012, 03:54 PM   #2
JordanM9
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If you are running different sized tires from stock than you can expect your speedometer to be off a little bit. I am sure this is the same for your odometer.
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Old 12-20-2012, 03:57 PM   #3
rdowner22
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Just thought I'd throw in a quick note that different tires/tire pressure and all the little calculations can make it be off. I'm not sure if your familiar with mightycarmods but there was a YouTube video explaining it as well as a speedometer correcting device. It would be a pain to wire/not even worth it but it explains it all.
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Old 12-20-2012, 04:15 PM   #4
jadawgis732
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So the odometer is dependent on the speedometer's reading, is that correct?
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Old 12-20-2012, 07:07 PM   #5
epistax
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Quote:
Originally Posted by jadawgis732 View Post
So the odometer is dependent on the speedometer's reading, is that correct?
My understanding is they don't depend on each other, but they do depend on the same thing. Breaking your speedometer won't stop your odometer from increasing.
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Old 12-20-2012, 07:09 PM   #6
BrDb
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Quote:
Originally Posted by jadawgis732 View Post
So the odometer is dependent on the speedometer's reading, is that correct?
I'm pretty sure the answer is no. I am basing that off the fact that in most cars, if the speedometer were to not work for some reason, the odometer should still continue to function. If the odometer were dependent on the speedometer, then when the speedometer is not functioning, the odometer should not function as well.
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Old 12-21-2012, 10:30 AM   #7
rkramer
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They both read the VSS wire from the trans... So if your speedo quits, the odometer may or may not also quit incrementing, depending on what caused the problem in the first place. (failed stepper motor in the speedo, odo keeps working. failed VSS signal for any reason, they both go dead.)

And yes, having an incorrect speedo because of gearing/tire changes also means the odo will read incorrect.
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Old 12-21-2012, 02:47 PM   #8
ArkansasDave
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I'm also pretty sure all cars come from the factory to read a little high. It gives them a cushion zone, so when you are going the speed limit you are actually under and won't get a ticket. My 06 Sti and 05 mini both do the same thing and by the same amount as yours. As far as if your odometer is reading actual mileage I do not know. I would assume it counts tire rotations times circumference of a stock sized tire. While your speedo can have the computer tell it to read 4% higher than what it's calculating.
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Old 12-29-2012, 09:56 PM   #9
jadawgis732
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Thanks for all the answers. The last two were especially informative. Thanks again!
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Old 12-29-2012, 11:24 PM   #10
MD04WRX
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Subarus are about 3%-5% off on average. They read faster than your actual speed. There was a class action suit about this issue where Subaru agreed to extend warrenty claims MY 02-07.

" Subaru of America, Inc. will extend by 5% the mileage-based coverage periods of certain warranties, extend annual mileage allowed by vehicles leased from Subaru Auto Leasing, Ltd. by 5%, and reimburse certain repairs and excess mileage charges."

http://classactionlawsuitsinthenews....ction-lawsuit/
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Old 12-30-2012, 09:59 AM   #11
katsimbris
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Quote:
Originally Posted by MD04WRX View Post
Subarus are about 3%-5% off on average. They read faster than your actual speed. There was a class action suit about this issue where Subaru agreed to extend warrenty claims MY 02-07.

" Subaru of America, Inc. will extend by 5% the mileage-based coverage periods of certain warranties, extend annual mileage allowed by vehicles leased from Subaru Auto Leasing, Ltd. by 5%, and reimburse certain repairs and excess mileage charges."

http://classactionlawsuitsinthenews....ction-lawsuit/
+1 Beat me to it.

Mike K
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Old 12-30-2012, 10:14 AM   #12
thescreensavers
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On my 2013 gauge and GPS is dead accurate at 75mph
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Old 12-30-2012, 10:26 AM   #13
MD04WRX
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Quote:
Originally Posted by thescreensavers View Post
On my 2013 gauge and GPS is dead accurate at 75mph
My 96 and 04 both read fast. My 04 is dead accurate with 235-40-18 tires but with stock size it reads 3 mph fast at 60 mph. I have to add that every car I have owned read fast according to my GPS and the speed warning sign on the side of the road. Average on both my Fords is 2 mph fast.
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Old 12-30-2012, 11:16 AM   #14
J-hop
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Every car I own reads fast as programmed by the manufacturer. No big deal I'd rather it read fast and save me a ticket than the other way around. The speed limit is a maximum speed not a target speed so if someone yells at you over driving 4 mph under the speed limit that is their issue not yours....
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Old 12-31-2012, 06:00 PM   #15
bugeye829
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yeah mine reads about 3 mph fast at freeway speeds. and i've always had stock size tires since i bought the car.
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