Brian_77_3.6

Drove my 77 911 today which is equiped with 16" wheels. My son was following me in his Exterra and called me on the cell phone and asked why I was driving so slowly in a 55 MPH zone. Told him according to my speedo I was doing 68..he said according to his I was doing 58. So I seem to have a 10 MPH error around 60MPH. How can I get this calibrated properly?

r911

your tire sizes are not stock are they?

N Hollywood or Palo Alto Speedometer shops can re-cal your speedo

Brian_77_3.6

Tire sizes are not stock. Speedo and odometer where fixed last year and this is only since the fix. Only problem then was that the odometer did not work.

Bill Gregory

When I changed my 81SC speedo face from the 85 mph one, North Hollywood had to recalibrate it to the new face, and it was spot on. 30 was 30 and 50 was 50, with 16"tires (205's/225's and 225's/245's).

Rod.

I'm running 15" wheels on my '77 911S. I checked my speedo against a GPS and it consistently indicates 10 kph low between 30 kph and 170 kph. I've heard this is "factory standard".

Rod.
'77 911S

Dan Cobb

It may well have saved my tail a few times!
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Brian_77_3.6

I have suspected for some time that it records over the actual speed and I find this nerve racking because when you are just flowing with traffic on the interstate and you see a cop and slow down to the posted speed limit everything on the road passes you. Guess I will have to send it in to be calibrated again. Any one know how they calibrate it? Is it something you could do yourself?

Jay Laifman

When you calibrate it, they will need to know the number of rotations of the speedometer cable per a certain distance - ie 5,280 feet. The basic way everyone says to do it is as follows, I'm just adding the garbage can part: You pull your cable out and put a piece of tape on the tip, then drive your car 17.60 feet (or 35.2, or some other convenient fraction of 5,280 feet), counting the revolutions of the cable. Here is just a tip on how to measure it perfectly. It's easy to mark 17.6 feet on the road (do it in safe place of course). But, you can't see the second mark as you approach it. What I did was put a large plastic garbage can in front of my car just touching the bumper. I then marked the ground where the bottom edge of the garbage can was. Then I measured the 17.6 feet from that mark. Then I moved the bottom edge of the garbage can to the second mark. Then I drove/rolled the car forward, counting the revolutions, until I just touched the garbage can again. I did it a few times just to be sure.

Note this of course does not take into account expansion of the tire at speed. Short of putting the car on a rolling dyno type set up, or some calculation on what to over inflate the tires to, that's the best you can do.

Also, the final revolution of the cable was a partial revolution. I made a flat disc out of paper that I taped perpendicular to the cable housing, behind the tape on the cable. I marked where the tape pointed at the start, then marked where it was when I stopped for that last partial revolution. However, once you do it, you will see that even without this extra step, it's pretty easy to figure out if it's a 1/4, 1/3, 1/2, 2/3s, etc. turn, and any more accuracy is not meaningful.

Jay Laifman

Whoops, I meant pull the speedo out, and detach the cable housing. I did not mean to pull the cable out from inside of the housing.

Haxle

I am getting suspicious my speedo is a little optimistic. This is not suprising because I have a '79 SC and the speedo is out of an '85 (give or take a year or two). The old one didn't have km and the odo didn't work. The price was good, too. I.e. I seem to have the first two dials off the North Hollywood "Dial Options" web page: http://www.nhspeedometer.com/911/dial_options/1.htm
So, do I need to recalibrate? I was thinking that both cars these speedos came out of were producing the same number of revs of speedo/odo cable per mile. Is this not correct?

Dan Cobb

Rotation of the cable results from a combination of the gearing in the box AND tire diameter.
Once this is established, the correct gearing inside the speedo can be determined.
It would be optimistic to assume that all speedo cables would rotate the same with the same tires. It may well be pretty close to true, but I would not assume it to be concrete.

This would have no impact on cars with electronic speedometers such as mine. I am unceratin as to which model year Porsche made the change to electronic, and even if there was a 'split year' of both electronic and mechanical.
I would guess that SC models (up to/including 1983) had mechanical, or possibly then the SC was introduced it had electronic (I don't think so).
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