Incorrect Speedometer reading

I've passed a few of those "unmanned" radar devices, the kind that
tell you how fast your going, and they have been telling me my
speedometer is lying about my speed. Specifically, my speedometer is
saying I'm going 4mph faster then I am.

I'm wondering how accurate those radar devices are?

How hard is it to get my speedometer back to the proper reading? And
what would cause it to be off? (My tires are factory spec, along with
the original rims/wheels.)

Thanks

Randall Fox

 

IMLE, most newer cars seem to read 2-4% high. Depending on the speed you
were travelling when "checked", yours may be a little more off.
ASSuming you were doing 60 MPH, it'd be closer to 7%.
You might pay attention to the "odometer check" zones and see how far the
odometer is off in a 5 mile stretch. If it's 7%, I'd be back at the
dealer. If nothing else, I would want my warranty to expire 7% early!
BTW, I seen check zones vary from each other by .1 mile+.

 

I'm sure their accuracy can vary a lot depending on the manufacturer. The
one that I drive by everyday corresponds exactly to my speedometer, down to
the MPH.

 

I've used my GPS before to check my speed. My speedometer is usually
around 5 MPH over what the GPS says, at highway speeds.

 

Thanks to all that responded.

My car is not new at all, but its the first time I noticed it being
off..

I guess I should just be happy that it is over and not under..
(assuming the radar devices are accurate)

Randall Fox

 

http://tinyurl.com/4tt7r

 

that's interesting, but if that's the case, then all of my cars that I've
ever owned have been off by 5 mph. The unmanned police unit has always told
me that I am going slower than I am. I look at it as a game each time I
approach one to guess how fast it will tell me I am going.

For confirmation, I asked my neighbor, Mr. deputy sheriff, and he told me
that they are calibrated to read slower than actual (around here anyway).
He said that they put them out a week or two before to see what are the true
speeds. If the average is way too fast, they will run some radar there.

Unless you've changed wheels/tire diameter (i.e. bought larger wheels),
don't worry about the difference, the vehicle is 99% correct.

 

Manufacturers deliberately set the speedometers to read a little high.
The odometer is usually more accurate than the speedometer, i.e. if you
set your cruise control to 60 mph (according to the speedometer) and
drive that way for 10 hours, the odometer will increment less than the
expected 600 miles.

Also, the deliberate odometer error is both a gain and an offset error.
If true speed is X, the speedometer will read (x + a%) + b mph. where a
is somewhere less than 10 % and b usually is somewhere less than 5 mph.

Honda does not want people demanding to get reimbursed for speeding
tickets because the speedometer reads too low, so they start with a
fairly accurate speedo and tweak it up so that even when all tolerances
stack up the wrong way, the speedo still does not read too low.

 

I've owned quite a few cars and motorcycles from three continents, and
amazingly, they have all indicated within 1 or 2% of the correct speed.

I have done calculations of tachometer and tire size and gearing and
compared that to indicated speed. I have verified the odometer against the
conveniently accurate roadside "mile markers" that are all over California.

I have used my GPS to compare to the indicated speed.

Changing tire size gives a noticeable error.

I found this one to be interesting reading, with some good links.
http://truckandbarter.com/mt/archives/000099.html

If the accuracy bugs you, you can adjust it with this widget:
http://www.ricks2k.com/products/yellowbox.php

 

I tested my '04 Accord EX's speedometer against my hand-held GPS, and the
speedometer is spot-on.

My '97 Civic read too fast, with the gap in readings increasing as the speed
increased. If the speedometer said 50, I was really doing about 47 or so. By
the time it said 80, I was really doing 75.

 

my friend has a dodge ram. i imagine the speedo read correctly when new,
or maybe a bit fast. after installing a lift kit and big tires, it now
reads about 10mph slow, and he has the speeding ticket to prove it.

 

So the $99 electronic speedometer adjuster might be a good investment.
http://www.ricks2k.com/products/yellowbox.php

 

I've owned a few cars in my life, but I don't recall the speedo error
being noticeable until I got my first Honda a few years ago. (and I
guess you were talking to me. Otherwise,....nevermind.

 

very cool!

 

I had a VW Rabbit which used to indicate F on the fuel tank at 80mph...
with the speedo needle of course.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

My 04 Civic is also right on the button with the GPS, but my 91 Civic is
5mph slower.

The newer electronic sensors are more accurate than the old analog.

 

They are easier to adjust, as well.