Speedometer and mpg reading is off

[saechaka]

Since I got my nav system, i've noticed that my speedometer seemed to be reading 2mph faster than what my nav was stating. so, i used the 5 mile speedometer checkpoints to see if my miles were calculated correctly and noticed that the first mile i was off by about 25ft(as in, i reached 1 mile on my car and wasn't at the 1 mile checkpoint yet) and the second mile was about double that and so on. i'm currently running stock spec. tires. now i'm just wondering what are the ramification of this info. did toyota do this purposefully to inflate mpg claims? any thoughts

[Tom_06]

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

Since I got my nav system, i've noticed that my speedometer seemed to be reading 2mph faster than what my nav was stating. so, i used the 5 mile speedometer checkpoints to see if my miles were calculated correctly and noticed that the first mile i was off by about 25ft(as in, i reached 1 mile on my car and wasn't at the 1 mile checkpoint yet) and the second mile was about double that and so on. i'm currently running stock spec. tires. now i'm just wondering what are the ramification of this info. did toyota do this purposefully to inflate mpg claims? any thoughts
[/b]

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

[galaxee]

toyota cars read about 3 mph fast.

[hobbit]

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*

[Tom_06]

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

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*
[/b]

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

[Begreen]

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.

[saechaka]

thx for all the info guys. i was a bit worried. i'll see if inflating the tires can help.

[Sarge]

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

Since I got my nav system, i've noticed that my speedometer seemed to be reading 2mph faster than what my nav was stating.
[/b]

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

[TonyPSchaefer]

toyota cars read about 3 mph fast.[/b]

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.[/b]

Hmmmmmm.

[Doutt It]

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