Speedometer Calibration

[Yousuf Khan]

Same here, 2005 Forester XS, 72 mph indicated, 68 mph actual speed.
Service writer at my dealer said nothing they could do. Mine has been
off since day one. I drive a different rental car each week with my gps,
and most new cars are dead on with their speedometers. Seems Japanese
cars are the most off. I think they have issues with kilometer to miles
conversions, or a problem with using different tire sizes in their
domestic market vs. the U.S. as the 2005 Forester had 17" rims in Japan
vs. 16" in the U.S.

That's a possibility. I have a Canadian 2000 Subaru OBW, and it's
speedometer is primarily km/h, with mph the secondary scale. I find my
km/h matches within 1 km/h of the GPS reading (also in km/h). Try
switching over to km/h on the GPS and compare it to the km/h markers on
your US Subarus. Maybe the km/h ratings are more accurate?

Yousuf Khan

 

[Yousuf Khan]

I appeqars that the speedometer is calibrated to exaggerate the speed
a bit, in order to comply with (almost) world wide regulation.

I never heard about there being an (almost) world-wide vehicle
regulation to overstate the speedometer speed reading. Doesn't even seem
to be followed universally if there was such a standard.

Yousuf Khan

 

[Blair Baucom]

I did this today, indicated 100 km/h, speed on GPS: 96 km/h, so it is closer
than mph.

The idea I had was in the software, if it was based on metric. On Siemens
840D controls in the machine tool industry, all calculations are done in
metric. So if program is input in inch, it converts to metric and processes
the program. Under certain scenarios, this conversion of 1 inch = 2.54 mm
causes a cumulative error to take place and parts are no longer in a nice
grid, the further from 0 the are, the more they spread apart. There is a
program we use to correct for this, or it is implemented in the post.

Blair Baucom
2005 Subaru Forester XS

 

[Ingo Menger]

I never heard about there being an (almost) world-wide vehicle
regulation to overstate the speedometer speed reading.

Vehicle models will not be allowed by the authorities, if the
speedometer shows *less*.
(Look at the WIkipedia article for speedometer.)
The only possible technical response to make sure this does not happen
is to make sure it shows a bit more even in the most unfavorable
circumstances.

 

[weelliott]

I had chimed in earlier on this thread, and had commented that my 95
subaru has an error on the speedometer. However, this past weekend I
was driving my newly acquired 98 legacy, and had different results. My
95 has slightly smaller than OEM tires on it. My 98 has OEM size but
not OEM brand tires on it. Compared to my aftermarket GPS, the
speedometer was nearly spot on. The speedometer always indicated
between actual speed and 1mph over actual. It never indicated under my
speed.

I've noticed that different tires of the same size are not actually
the same size. I got some Toyo tires in the 195/50-15 size for my
miata and they were wider than other 205 tires, and also a larger
diameter than a Sumitomo 195/60, which it should not be. Then again
that Toyo might come a little larger because they anticipate people
shaving the tread down since it's a racing tire. I wonder how much
this variability comes into play.

 

[Carl 1 Lucky Texan]

I had chimed in earlier on this thread, and had commented that my 95
subaru has an error on the speedometer. However, this past weekend I
was driving my newly acquired 98 legacy, and had different results. My
95 has slightly smaller than OEM tires on it. My 98 has OEM size but
not OEM brand tires on it. Compared to my aftermarket GPS, the
speedometer was nearly spot on. The speedometer always indicated
between actual speed and 1mph over actual. It never indicated under my
speed.

I've noticed that different tires of the same size are not actually
the same size. I got some Toyo tires in the 195/50-15 size for my
miata and they were wider than other 205 tires, and also a larger
diameter than a Sumitomo 195/60, which it should not be. Then again
that Toyo might come a little larger because they anticipate people
shaving the tread down since it's a racing tire. I wonder how much
this variability comes into play.

I also wonder if the g'mint insists on the = or < reading and max
sidewall inflation pressure?

meh - I could live without a speedometer just fine. Least valuable gauge
IMHO.

Carl

 

[XR650L_Dave]

I have a 2003 Subaru Outback that has a 4 mph error on the speedometer.  It
is constant and I have verified it with radar, stopwatch, and a gps, so I
know it's not my imagination.  I could deal with 1 or 2 mph, but 4 is
getting annoying.

I'm hoping at least one of you Subaru guys knows how or even if I can adjust
the speedometer to read correctly.

Thanks.

Crap-and-a-half, just saw this, my '03 H6 OBW is exactly the same way,
speedo reads +4MPH all the time, odometer seems to be correct.


Dave