Odometer, MPH wrong (new tires)

[madler]

I just put new tires on my 2005 Prius (the originals lasted 30,000 miles). Now the mph and miles are reading higher compared to reality, which I measure with my portable GPS. My Prius does not have its own GPS. I have determined that the Prius is now counting miles, and therefore mpg as well as mph, high by 2.15% +/- 0.05%. On the original set of tires, the Prius was reading only around 1% high.

Yes, the tires are the right size -- exactly the size specified in the manual and what was on the car originally. Yes, they are at the proper pressure. Apparently there's some variation in circumference among tires of the same specifications.

So the question is this: is there some way to tell the Prius to count fewer revolutions per mile to get its accounting to be more accurate? My bicycle computer provides that capability.

[nicoss]

<div class='quotetop'>QUOTE(Rick Auricchio @ Jan 21 2007, 07:50 PM) [snapback]378848[/snapback]</div>

2. You should check the odometer and speedo against measured miles on freeways. Don't believe the GPS without testing against a measured mile.
[/b]

Not so true:
http://www.traditionalmountaineering.org/F...PS-Accuracy.htm

[auricchio]

1. There's no adjustment that I know of.

2. You should check the odometer and speedo against measured miles on freeways. Don't believe the GPS without testing against a measured mile.

3. As the tires wear, they'll get smaller. Unfortunately, this means more revs per mile and a larger speedo error.

[lenjack]

Definitely no adjustment. What you are experiencing may well be due to the fact that different tires will get different mileage, regardless of their circumference.

[Bob Allen]

<div class='quotetop'>QUOTE(Rick Auricchio @ Jan 22 2007, 11:50 AM) [snapback]378848[/snapback]</div>

1. There's no adjustment that I know of.

2. You should check the odometer and speedo against measured miles on freeways. Don't believe the GPS without testing against a measured mile.

3. As the tires wear, they'll get smaller. Unfortunately, this means more revs per mile and a larger speedo error.
[/b]

I doubt that the mileage recording capability of the Prius is sensitive enough to reflect the reduction in tire size due to tread wear. The GPS is probably more accurate than the mileage posts on the side of the highway. My Garmin shows speed to within 0.2 mph, or less, of what the odometer itself shows, so, to me, the margin of error is below any meaningful threshold. Statement number one, "there is no adjustment, etc." I think is true because the engineers have to allow for differences in tire between manufacturers, and variations in air pressure, tread wear, etc. This isn't rocket science.
Bob

[San_Carlos_Jeff]

<div class='quotetop'>QUOTE(Bob Allen @ Jan 22 2007, 12:26 PM) [snapback]379052[/snapback]</div>

.......Statement number one, "there is no adjustment, etc." I think is true because the engineers have to allow for differences in tire between manufacturers, and variations in air pressure, tread wear, etc. This isn't rocket science.
Bob
[/b]

It's also because it's illegal to tamper with odometer readings (to prevent fraud) and having control over this in the computer would allow one to do so.

[auricchio]

I'd still run the measured-mile test just to verify. If the results are different, try another measured mile elsewhere (preferably on a state vs. US road, or in a different state).

In any event, there's no way to adjust for the revs per mile of the tires. You might try higher inflation pressure (within the tire specs, of course). This will make the tires larger and reduce revs per mile---a little bit, at least.

[daveleeprius]

I've noticed that when I drive by those electronic "Your Speed Is" signs, that the display on the signs and the speedometer readout are always the same. You might want to check that when you drive by those signs.

I wouldn't worry about a little error. Look at it this way, when you're speedo says you're going 70mph, you're really going 68mph, so you won't get a speeding ticket in a 65mph zone.

Dave

[Tom_06]

<div class='quotetop'>QUOTE(lenjack @ Jan 21 2007, 10:55 PM) [snapback]378851[/snapback]</div>

Definitely no adjustment. What you are experiencing may well be due to the fact that different tires will get different mileage, regardless of their circumference.
[/b]

Actually, tires of the "same size" are often slightly different in circumference. The OEM Integrities are specified as having 855 rotations per mile. If you peruse the Tire Rack listings of tires of the same size as OEM, you will readily find variations in the spec for rotations per mile.

Over the life of the tire, the diameter changes as much as 1/2" due to wear. On a tire of 24.5" like ours are, that is in the ball park of a 2% change in odometer, mph, etc. (low when new, higher when worn). That is one reason worn tires get "better" gas mileage. For around 50 mpg, that's 2 mpg better at the end of tire life that is not real, just the change in your odometer.

- Tom

[donee]

Hi All,

Ken1784 put on a link to a U.N. document. It stated that by international agreement, all speedometers were supposed to be nomally 2 % fast (I think the range was from 0 to 4 % fast). SO , it sounds like your Prius is right on the design value.