Calibrate my Speedometer

RSWiser · 05-14-2011, 04:59 PM

I have owned my 911SC for two years. Last year on a road trip I noticed that my speedometer/odometer is off.

When it says I am going 60 I think I am about 54-55. At 100 I think I am about 90mph. When I measure miles with the Mile markers it is off there also. I am going to test it more exactly with a GPS in the next few weeks.

It is adjustable. Can this be corrected?

If it has been this way for ever then my car has about 10% fewer miles than it says. Not concerned about that but would like an accurate Speedometer.

Joe Bob · Joe Bob's Garage

North Hollywood or Palo Alto Speedometer.....and yes....

donporfi · 05-14-2011, 05:32 PM

What size tires do you have?
That could be one of the problems.You cannot have a totally accurate speedometer, many factors affect it.
If it is showing a faster speed no problem, less speeding tickets.

DRACO A5OG · DRACO A5OG's Garage

Mine is about 5 mph less than actual. Going to try to calibrate by adjusting the side adjuster as recommended by some here.

Need to locate and tap a hole on the side for the small screw driver

If I fail, it's going to NHS.

dshepp806 · dshepp806's Garage

This (higher than normal speedos) is quite common. I've been searching for the actual calibration procedure, as I've been told by someone who works on these that a signal generator is needed to to it properly (this is NOT to say that others can't just wing it with needle positioning and land in the right spot==I guess). Should someone here have access to the procedure, I'd sure like to see it. Otherwise, mine is to be sent off to NHW to get it right. I would note that I've yet to see the actual circuit to make any observation.

Again, I heard it's a cal procedure,..not necc'y a physical "needle movement".....we'll see.

BEST!

Doyle

porschenut · 05-15-2011, 02:07 PM

How fast is the turnaround at NHS this time of year?

hcoles · 05-15-2011, 02:29 PM

if you have the time, borrow a friend with an iphone and run the Speed Box app with the iphone on the dash, record several points, this should give you a good idea if the offset is linear or other. I think Speed Box has a free version for trial. They are nice people having answered a number of emails from myself.

nesslar · 05-15-2011, 02:52 PM

For a speedo that reads high, pulling the needle and repositioning it a bit "left" corrects the problem. Not that easy, involves taking the thing apart, about an hour job or less, removing needle can break it. Fixed mine, and others without incident. I used to read 65 at actual 60, now it's right on.

E Sully · E Sully's Garage

Some of the speedometers have an adjustable potentiometer, as shown by the arrow, that can be used to adjust the speed. Some have drilled a hole just big enough to insert a small screwdriver in the can to adjust it.

RSWiser · 05-15-2011, 04:07 PM

Just downloaded the Free version of Speed Box.

Headed out of town in the morning. I'll test the Speedo against Speed Box on Thursday.

Weather permitting, headed to a DE day on Saturday.

nesslar · 05-15-2011, 04:51 PM

Quote:

Originally Posted by E Sully

Some of the speedometers have an adjustable potentiometer, as shown by the arrow, that can be used to adjust the speed. Some have drilled a hole just big enough to insert a small screwdriver in the can to adjust it.

Interesting (and sensible). I don't recognize the circuit board and wiring configuration/connections, relative to the SC and Carrera speedos I've dealt with.

dshepp806 · dshepp806's Garage

Now, we're talking!!!!!!!!!!! Now,..the procedure!

Thanks for those photos. These are from what?

THANKS!

Doyle

DRACO A5OG · DRACO A5OG's Garage

That is what I had mentioned above, the trick is tapping the hole in the correct spot.

nesslar · 05-16-2011, 06:17 AM

The large holes in the circuit board correspond to the locations of the three screws on the back of the body/shell of the speedo. The innards of the speeedo that hold all the gears and such fit into those holes, so it should be "fairly" easy to estimate the right spot to drill. The photo above is a good guide, but as Doyle asked, what is it from? It's surely not from a Carrera or SC 160 MPH model...
However, I am not sure I would want little bits of metal lying around inside a speedo after drilling through, though it might not actually present a problem. The circuit board rests on washers that keep it away from the metal body, so I guess the drilled out metal would just lie harmlessly in the gauge. A wrap of electrical tape around the gauge and covering the hole would suffice to cover. Still, it looks like this circuit board only represents "some" speedos. Here is a Carrera/SC type, obviously different; any accessible potentiometer in these? I don't know....it might be necessary to take this type apart even if it has a pot.....

jtook · 12-24-2013, 10:50 AM

One alternative to drilling if you have the unit apart already (probably to replace the crumbling odometer gear) is to leave the guts out of the casing and "hotwire" it to the wires coming out of the dash. I built three wire extenders (don't care about the bulbs) and put the speedometer on the seat with my GPS to calibrate it. Hint: if you have someone to help you that would be useful. Here's a pic of the setup:

kjchristopher · kjchristopher's Garage

I adjusted mine a few months ago after a tire change. Interestingly enough, it is dead on till 50mph, then begins to read slow, in increasing amounts as expected. At an indicated 70, I'm going 75.

jtook · 12-27-2013, 09:31 PM

@kjchristopher: Most people have reported that the speedos are pretty linear. Mine is slightly as you say (a little faster at 30 than at 70), but only by 1-2 mph. I would suggest you make sure your speedo needle is on the zero mark at rest and calibrate from there. I'm not sure that will make a difference, but it's worth a shot.