Speedo question

[MAS]

My 77 911S is pretty much stock (915 tranny, etc), with the exception of wider 15" phone dial wheels.

The electronic speedometer "appears" to be functioning smoothly and normally, without any sticking or bouncing.

OK... I've been suspecting over the last few weeks of ("new" 911) ownership, that my speedometer was reading a little low. I've now had that confirmed several times when I drove the car past a "This is Your Speed" roadside radar device. It seems like my speedo is reading 50kph (50kph = about 30 mph) when I'm only doing about 44kph.

However, recently when I was with a friend driving fast late night on the highway... my friend commented that we seemed to be going much faster than the speedo indicated. The speedo was reading about 130 kph, but he was right, it seemed much faster (160 kph = 100 mph). I pretty much put it down to the fact that Targa's feel like they are going faster than they are.

I still had the nagging feeling that he was right, since I'd opened the car up several times on the highway and never got past 150 kph ( about 94 mph), even though I was obviously going way faster than the regular traffic... much of which cruise at about 120k anyway.

So I decided to check my apparent highway speedometer readings against my tach. And sure enough, even though I'd taken the car up to almost 6,000 rpm in 4th gear, my speedometer was still reading under 160 kph ( 100mph), when the real speed should have been almost 115 mph, according to my owner's manual. Same story in 5th gear too, where 100 mph is at around 4,400 rpm.. and my speedo was reading well under this.

Also, it seems that my speedo is almost exactly "on" at about normal highway speeds (80-90kph)... reading high at lower speeds and reading low at higher speeds. So it cannot be a question of tire sizes or gear ratios, but the speedo itself. As I said, it's not jerking or sticking, and "appears" normal, but it obviously is not. Any ideas what to do, short of shelling out $$$ for a new one?

-MAS

[Formerly Steve Wilkinson]

Send it to Helmut at North Hollywood Speedometer (ad in any Porsche-oriented publication). It'll cost you about $115 (as I remember) to have it cleaned, lubricated and recalibrated for the wheel and tire sizes you're running. Quick, easy, convenient and a comparative bargain. I had it done for my '83 SC when I put larger wheels and tires on, and I also had him change the mileage to zero, since I'd toatally restored the car. (No possibility of there being anything fraudulent about that, since the car is obviously 20 years old and will never have "believeable" mileage, and I logged the change inthe maintenance manual I keep with the car.)

Stephan

[RoninLB]

Originally posted by Formerly Steve Wilkinson

Send it to Helmut at North Hollywood Speedometer. Quick, easy, convenient and a comparative bargain.

gotta agree on this. I believe they also do the florescence on the numbers.. although I did a different route.

Speedometer Error. Need some help

'86 Carrera Speedometer Calibration??

[Formerly Steve Wilkinson]

Oh, and by the way, MAS, since you say you're new to 911s, one speedo caution: never reset the trip odo while the car is moving. It will eventually (or maybe immediately) strip the plastic gear that drives it.

Stephan

[Jim Sims]

I seem to recall that the 911 VDO speedometers can only be adjusted to be "right on" at a single speed. When new, your car had up to 5% speedometer error and if the tire effective radius has changed that will add or subtract to it. Jim

[RoninLB]

Originally posted by Jim Sims
I seem to recall that the 911 VDO speedometers can only be adjusted to be "right on" at a single speed.

I kinda thought so also.. I've looked for hard info, but can't find it.. and NH wanted me to clock the rpms at 30 & 60 mph. so my tach is 100 rpms under/ whatever .............Ron

[MAS]

"Oh, and by the way, MAS, since you say you're new to 911s, one speedo caution: never reset the trip odo while the car is moving. It will eventually (or maybe immediately) strip the plastic gear that drives it."

Thanks... actually I already know this, since I used to own an 87 924S.

-MAS