Odometer, MPH wrong (new tires)

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

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

 

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.

 

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.

 

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

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

 

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

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.

 

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.

 

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

 

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

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

 

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.