Speedometer and mpg reading is off

<div class='quotetop'>QUOTE(saechaka @ Apr 16 2006, 05:09 PM) [snapback]240578[/snapback]</div>

25 feet in one mile is less than 1/2%. As your tires wear, it will probably come into spec and then go wrong the other way. Given the diameter variations between tires of the same size but different brands and types, this is way within what you would expect to see. If you dig into tire specifications, you can find a rotations-per-mile spec and see how it changes from brand to brand. VW used to consider a speedometer in spec for 5 mph wrong. If you are going to be out of spec, slightly high is better than slightly low around speed trap towns. I don't think you have anything to worry about.

BTW, my 2006 with 1500 miles on it read out less than 1/2 mph difference against a separate WAAS enabled GPS on a straight flat road.

- Tom

 

toyota cars read about 3 mph fast.

 

I just did a test of this the other day, in fact. Plotted a route
of exactly 40 miles on the Garmin "mapsource" package on the laptop,
and then went and drove it. [I didn't try to set up the route *on*
the GPS, I just took note of its length as calculated in the roads
database.] After the run, the GPS unit itself said 40.0 miles, and
by going a little farther and noting when the tenths kicked I
extrapolated to 40.04 miles. By the same extrapolation procedure,
the car had tallied up 39.62 miles. About a 1% difference, with
the car logging slightly less mileage.
.
What's interesting is that the speedo reads about 1/2 mph *above*
the GPS calculated speed, which is an interesting disparity.
.
This is with stock integrities at 44/42 or thereabouts. Now, if you
want your car *more* accurate, pump those suckers up! Soft tires
definitely spin a little faster -- I've already proven that with the
differential-speed analysis prototyping I've been doing.
.
The reason for correlating the GPS run against the mapping database
is that I could easily see changes in the GPS accuracy radius as
being interpreted as slight wanders off-course, which would make the
path longer. But this doesn't seem to have happened, at least not
with any appreciable impact. Sometimes as the thing is acquiring
birds, it does a sudden jump of several feet from its last known
position, leading to a logged "maximum speed" of like 175 MPH. Hey,
a little more hardware and I could have a transporter!
.
Next opportunity to do a long highway run with mile-markers, I will
do that correlation too. From what I hear the highway markers are
supposed to be *dead*-accurate. Anyone know how those are actually
measured out?
.
_H*

 

<div class='quotetop'>QUOTE(hobbit @ Apr 16 2006, 08:44 PM) [snapback]240636[/snapback]</div>

It did not used to be true that they were dead accurate. Most states had stretches of measured miles to check your speedometer against - 5 to 10 miles marked at 1 mile increments was common. At least until the mid 1970's there used to be one of these measured areas between Detroit and Lansing on I-96. These markers had their own posts. Generally I have found the interstate mile posts to be pretty good when compared against a speedometer checked against a measured mile stretch, but I have seen some that were noticably off. I think the posts with reflectors are placed at regular intervals and the mile signs are added later. Lately with GPS I haven't bothered to check for quite a few years. Being an engineer and a nerd, I used to pay attention to those sorts of things on multi thousand mile drives - not much else to do in the midwest cornfields.

- Tom

 

A local neighborhood has a radar system that reports your speed on the main road. My speedo seems to be exactly right, or at least it agrees with the radar unit.

 

<div class='quotetop'>QUOTE(saechaka @ Apr 16 2006, 05:09 PM) [snapback]240578[/snapback]</div>

AFAIK, speedometers on virtually ALL cars (not just toyota) are calibrated as much as 3% HIGHER than actual speed. This is by design.

The simple reason is legality, such as speed traps and lawsuits. Having this margin reduces the possibility of someone 'accidentally' speeding.

Also, nobody will sue Toyota after an accident because they were travelling SLOWER than they thought...

Kevin

 

There is a calibrate feature within the Nav. setup I believe that is supposed to calibrate the tires to the reading.

 

I have the exact same problem! Dash is 1-2 mph faster than the NAV and faster than the radar signs. Odometer is okay (I think) I compared the odometer against the NAV's distance to destination feature and they match.

What gives?

Nate

 

<div class='quotetop'>QUOTE(Doutt It @ Apr 17 2006, 02:07 PM) [snapback]241075[/snapback]</div>

I believe this is just so the nav can measure speed and distance. It doesn't affect the speedometer and odometer---otherwise every car would have it, and I think non-nav cars don't.

 

Quite possible. I was told to use it when changing tires sizes so the car would read the correct MPH. Worked for me.

 

<div class='quotetop'>QUOTE(saechaka @ Apr 16 2006, 02:09 PM) [snapback]240578[/snapback]</div>

At that rate, after actually traveling 100,000 miles, you can sell your Prius as a newer car with only 99,525 miles. That should be worth a buck or two of extra pocket change . . . and the higher MPG you can quote should be worth another buck or two.

The mileage/MPG differential will have earned you enough extra on the sale to treat yourself to a mocha at your favorite coffee shop. Think of it as a departing gift from your Prius. :lol:

 

Car and Driver had a story on speedometer accuracy long ago. See:
http://www.caranddriver.com/features/1906/...er-scandal.html
http://www.caranddriver.com/features/1906/...ndal-page2.html
http://www.caranddriver.com/article.asp?se...article_id=1907

 

<div class='quotetop'>QUOTE(saechaka @ Apr 16 2006, 02:09 PM) [snapback]240578[/snapback]</div>

Oh, please....


I've owned 54 cars in 45 years. NONE of them has had a dead-on speedo reading. The most dramatic was a '96 Acura RL, which was off a full 7%. Anything under 3% is great.

To suggest some kind of conspiracy is laughable. Try to find any mass-market car with a dead-on speedometer. Our '06 appears to be reading about 1 mph fast at 60, which is as close as any car we've owned in the past 20 years. Our '02 MB C240 was about 2 mph off at the same speed. And as noted above, even tires with nominally the same size will affect this - we've switched ours from the awful Goodyear [Badyear] Integrities to Costco Michelins - same size, but the Michelins are slightly larger in diameter [more tread, for starters], and so they bring the speedometer as close to spot-on as it is ever going to get. As they wear, it will gradually get faster again.

These are approximations; same with fuel gauge readings AND GPS locations....the errors are small, but unavoidable with the current technology.

 

<div class='quotetop'>QUOTE(saechaka @ Apr 27 2006, 02:53 AM) [snapback]245996[/snapback]</div>

If you were using the regular freeway mile posts as opposed to a measured speedometer check area, you are not comparing against anything accurate. Over a few hundred miles they average out to "not too bad", but over 5 miles it is a crap shoot. I've seen some spots along the PA Turnpike that were really off.

- Tom

 

neat calculator

http://www.csgnetwork.com/speedocalibcalc.html

 

today for no apparent reason my speedometer was showing a really high speed although I was keeping pace with traffic in a 40mph zone, my speedometer read almost 70....really wierd...it seemed to happen shortly after I went through a car wash...hmmm...any ideas?? I'll definitely have the dealer check it out since I have less than 4K on it...