Speedometer error

My 2000 Accord speedometer is off by about 8 mph according to my gps.
It's kind of a good thing, it'll read 70 but I'm actually doing about
62. Is there a fix for this?

TIA

 

GPS's are not definitively accurate. There is a fairly large built-in error
in all GPS's. GPS satellites are the property of the US Department of
Defense, and the military doesn't want you having the same thing they've
got for reasons of national security. This is outlined in all the new-car
PDI docs issued by Honda for vehicles that have NAVI.

In addition to that, speedometers are designed to be "fast" on account of
liability laws. Add the two together, and it could well be 8mph at 62 or
70.

If you really want to know your car's speedo/odo accuracy, you need to
spend some time with a piece of chalk and a long tape measure.

 

Surely you jest!!!

Certainly GPS's are not "definitively" accurate with locational
information, but the time traveled between too "not-so-perfect"
locations should be dead on. Speed indications should be very accurate
compared to most automotive speedometers.

Also, I am sure car makers would not design in inaccuracy to "account
for liability laws", leaving themselves open to lawsuits for short
changing owners of warranty limits. If the meter is reading too high,
the odometer is approaching the warranty limit too fast.

Lastly. A very easy method of determining speed is by the time is takes
to go between mileage markers on most highways. At 60 MPH one mile
should take exactly 60 seconds.

 

And Honda lost a class action suit for exactly that reason several years
ago. The warranted mileage on my '06 CR-V was extended as part of that
settlement.

 

Or visit a country that has discovered the mile or kilometer marker.

 

Bill Clinton ended that silliness in 2000.
http://www.navcen.uscg.gov/GPS/selective_availability.htm

Any current GPS will have a steady state speedometer accuracy of .1mph.

 

Are the tires currently on the vehicle the same size as those specified by
Honda?

I did a calculation on my (off-size) tires. Honda specified 195/60 but I'm
running 195/65. The calculation involves the static loaded radius and the
rolling circumference. For my configuration the error is 7 percent at 70
miles per hour. I just add 5 mph to the dashboard reading.

My old Prelude had a recall on my speedometer that I never took advantage
of. That would have corrected some of the error but I just do a mental
adjustment and live with it.

 

I'm running 195/60's.

I know it's an easy thing to deal with but it's one of those things that
aren't right and I'd like to make it right if I can.

 

What does Honda specify for the tire size? The internal settings of the
speedometer are matched to that dimension. Note that if you match the Honda
specification your speedometer will probably read a few miles per hour
higher than actual.

You can adjust the tire size to correct the error. Start with a calculator
such as http://www.csgnetwork.com/speedocalibcalc.html

 

10+% is an awful lot of error. You'd need to run several sizes over
before tire diameter accounted for all of it.
See: http://www.miata.net/garage/tirecalc.html

 

I'd have to go to 225/65 to compensate with tires.

Isn't there a gear I could replace or are they electronic?

 

Typically, they are electronic although you should confirm that with the
service manual. Honda is not likely to provide you with the schematic for
the speedometer, so you'll have to pull it out and reverse-engineer it. If
you have lived a pure and wholesome life, you'll find a set of switches or
jumpers in there clearly marked with the divider values. Often the design
engineers build in some adjustability for the various tire sizes likely to
be delivered with the vehicle.

On another point, as Joe Sixpack points out, your readings are off more than
the usual amount of a few miles per hour. Is that normal for that model? If
possible, find another vehicle identical to yours and determine whether
_that_ vehicle is off as much. I suspect that yours has another problem.
Usually there are magnets on the drive shaft that pass a reed switch. As the
drive shaft turns, a series of electrical pulses is sent to the speedometer
which counts them and converts that number into miles per hour. Perhaps one
of the magnets is missing or not quite in the right position. Perhaps the
switch or sensor is askew. Maybe there is a defect in the speedometer so
that it is missing some of the incoming pulses.

It wouldn't hurt to confirm the speedometer reading with a measured mile. I
went so far as to borrow a police radar gun to verify my speedometer. If the
GPS, the stopwatch and the radar agree, look at the pulse circuitry.

 

Way incorrect. SA/CA went out long ago. During Desert Storm they had
to disable it because the DoD couldn't get enough MilSpec GPS units
and was buying civilian units from Magellan.

Hell, my iPhone regularly puts me not only at the right address, but
at the right location in my house. I can observe it track me from the
back door to the front door.

If the FAA is happy with GPS, so am I. And with the added accuracy of
WAAS, cat 3C landings are possible at some airports.

Are you sure? I've been told that's a myth. I've had three different
GPS units agree to within about 1 mph on my speedo, along with two
different radar guns hitting me at the same time (voluntarilly). For
clarity, one gun was used in conjunction with one test, another with
anohter.

Even better, get a metering wheel (fifth wheel).

--

- dillon I am not invalid

"Get a shot off fast. This upsets him long enough to
let you make your second shot perfect."

-- Lazurus Long

 

Then how come all the 2010 PDI docs say this?:

"The GPS (global positioning system) satellites used by the navigation
system are operated by the U.S. Department of Defense. For security
reasons, certain inaccuracies are built into the GPS. This can cause
occasional positioning errors of up to several hundred feet. If the
navigation system indicates your position incorrectly, wait several seconds
until it corrects itself. The system may also correct itself after you make
a turn or cross a road."

If your reported location is incorrect, then it stands to reason that
reported distances may be incorrect as well.

 

Considering that my GPS's both know exactly where my house is, and tell
me to turn the wrong way at the end of my driveway onto the north/south
road that runs in front of my house, I gotta think there's some merit to
this.

 

Thanks for the link.

 

An odometer that over reports the mileage makes the gas mileage look
good also.

However, speedodometer and odometer errors are not always directly
linked. I check all mine versus measured miles, the GPS, know
distances, and the interstate mile markes. The last five Fords I owned
all under reported the mileage traveled (by 1% to 3%) and over
reported the speed (by 1 to 3 mph).

I beleive the GPS is very accurate for both speed, and measuring
longer distances. I checked the GPS over the same course multiple
tiems, and there is no significant variation in the distance reported.
If, you leave the GPS in one position for a long period of time, it
will report slight movement, but this doesn't affect the accuracy for
a single trip over any significant distance.

Ed