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2013 2.5i Premium CVT
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85 Posts
Discussion Starter #1
Like many other reports here, my speedometer reads 3-4% faster than I'm actually traveling. Is there any way to adjust this?
 

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2011 SSM Outback 2.5i Premium
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2,103 Posts
Would you rather it reads 3-4% LOW? I'd rather think I'm going FASTER than I actually am. Considering some of the factors that go into reading the speed, 3-4% is an acceptable variance, as long as it reads high. How do you know the device used to verify your speed was entirely accurate? GPS is close but it's not 100% accurate, the roadside speed guns (the ones that tell your speed) are likely not calibrated like a radar or laser unit used by a cop.

Sure, over the long-term it can make a difference on when oil changes happen (you've actually gone 4,850 miles instead of 5,000 miles), or that you've only gone 97,000 miles instead of 100,000.
 

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2004 Outback Wagon, Mystic Blue Pearl
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4,940 Posts
3-4% larger circumference tires is about your only option.
 

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2012 Outback 3.6R
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511 Posts
Most GPS units will report the speed at 0.5 mph error or less. 0.5 mph at 60 mph gives an error of 0.8%. I would consider that to be very accurate. GPS logs have been used in court to overturn a speeding ticket.
 

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Onyx, 2008 LL Bean 3.0R and 2017 Honda Civic Hatchback Sport Touring
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4,111 Posts
I imagine that the speedo is somehow related to the odometer which is not adjustable and actually tamper-proof. Otherwise my car could have 66,000 miles on it instead of 166,000 if I were to sell it. Although, the speedo looks analog and the odometer is digital. Any thoughts on this?
 

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2013 2.5i Premium CVT
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85 Posts
Discussion Starter #6
Would you rather it reads 3-4% LOW? I'd rather think I'm going FASTER than I actually am. Considering some of the factors that go into reading the speed, 3-4% is an acceptable variance, as long as it reads high. How do you know the device used to verify your speed was entirely accurate? GPS is close but it's not 100% accurate, the roadside speed guns (the ones that tell your speed) are likely not calibrated like a radar or laser unit used by a cop.

Sure, over the long-term it can make a difference on when oil changes happen (you've actually gone 4,850 miles instead of 5,000 miles), or that you've only gone 97,000 miles instead of 100,000.
No, I'd rather it be correct using the largest diameter tire I will run (and then read a little fast as my tires wear). If I know how fast I'm actually going, an incorrect speedometer is simply racking up more miles on my car than I'm driving.

3-4% larger circumference tires is about your only option.
I don't like this option. I miss the days when I could simply change the speedometer gear in the transmission. I also know that speedometer shops used to have the ability to change the computer (e.g., in cases where you changed tire size or gear ratio). Is this still possible?
 

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01 Outback H6 VDC, 97 GT wgn w/ ej22, 98 OBW w/ej22
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1,401 Posts
i was able to determine the error in my 97 GT odometer buy resetting it and my GPS trip data and then driving 100 miles non-stop. i calculated the error as a percentage and then bought tires that much larger than my current tires. now my speedo is dead on. i was a little surprised it worked out so well, but it did.

the point is that calculating the odo error over 100 miles is more accurate than trying to read the speedo and compare it to the GPS reading.

the speedo in my 00 lego is dead on.
 

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2016 Outback Ltd 2.5 eyesight Nav push button Hole in roof, Lapis Blue
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737 Posts
Most cars error on the high side rather then lower. I seam to remember that Honda had a class action lawsuit that ended up with them having to do warranty service beyond their stated 36k. My Odyssey was always off by something like 5mph at 60 that was close to 10% error, also made you think your mpg was better then it really was too. I guess if it errored the other way we would be looking for the manufactures to pay our speeding tickets
 

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2004 Outback Wagon, Mystic Blue Pearl
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4,940 Posts
Most cars error on the high side rather then lower.
This is actually law. I believe manufacturers are allowed to have speedos that read at or above your actual speed by 10%. So most companies design speedos to read a little fast to make sure the don't end up in trouble.

Personally I've never owned a stock car that didnt read 2-3 MPH fast at 60 MPH.
 

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2005 LL Bean
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138 Posts
This is actually law. I believe manufacturers are allowed to have speedos that read at or above your actual speed by 10%. So most companies design speedos to read a little fast to make sure the don't end up in trouble.

Personally I've never owned a stock car that didnt read 2-3 MPH fast at 60 MPH.
Correct. All motor vehicles legally sold in the US have a certain amount of speedo error. They intentionally design them to read slightly fast to avoid lawsuits etc. It has not become an issue until people started using GPS and have something to compare it to.
 

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2004 Outback Wagon, Mystic Blue Pearl
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What DOT law is this? I've done a little reading and have never seen it..
Looks like I was wrong that federal regulations would allow the speedo to overestimate speed, but not underestimate. The exact wording of the federal regulation is as follows. My belief though is that these cars (and most cars) are built to meet European specs, which do not allow for the speedo to under estimate speed.

US Reg:

" The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)."


I could only find the reg for trucks and busses, but I assume its the same for normal passenger vehicles.

eCFR ? Code of Federal Regulations

European Regulation:

The speed indicated shall not be less than the true speed of the vehicle. At the test
speeds specified in paragraph 5.2.5. above, there shall be the following relationship
between the speed displayed (V1 ) and the true speed (V2).
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h


http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r039r1e.pdf
 

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I wouldn't mind the 4mph high reading I get but I do mind that my max cruise control setting is then actually 4 mph slower than intended. (03 H6 Outback) My Dodge RAM is exactly on at 45 mph and begins to read high below 45 and low above 45. But not by near as much as the Subaru. That's after going to a slightly taller tire size.</pre>
 

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'15 Outback 2.5i Premium
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My Gen-5 outback trends ever so slightly low, at least when the factory tires were new.

Resetting a trip meter and GPS odometer when crossing into Arkansas from Oklahoma (mile 0 on I-40) and comparing them with the surveyed mile markers nonstop across Arkansas shows the GPS and mile posts agreed to within a tenth of a mile or so over 272 miles. The trip meter accumulated about 1.5 miles less distance (about 0.5% low) compared to the other two. The speedometer needle was usually a gnat's eyelash below 60, and the digital speedometer would show 59 or 60 when the GPS was showing 60 MPH, which is consistent with that error. I was impressed by how close the numbers were - especially how closely the GPS distance matched the highway markings. Occasionally a milepost was off by a noticeable amount, but those didn't seem to accumulate; if one was short, or long, the next would make up the difference, and this didn't happen very often. After almost 300 miles, an error of as much as 0.1 miles doesn't amount to much.

I highly recommend checking your instruments against independent measurements over a reasonably long distance (5 or 10 miles is probably plenty) just to get an idea how accurate they are. Plus, doing stuff like helps pass the miles (especially in the very flat section of eastern Arkansas between Little Rock and Memphis) while satisfying curiosity at the same time.
 
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