After being involved in the "how fast" thread, I would like to raise an issue. How accurate are the late 1980's 911 Speedometers? As I mentioned in the other thread, I have a consistant 5%-7% discrepancy between my speedo and my GPS. I have tried the GPS in other cars & boats with better (more accurate) results. Is this a typical problem with late 80's 911's or just mine? I see lots of you claim speeds in excess of 130mph. Are these possibly affected by poor accuracy of the speedo?

I don't doubt the ability for most of these cars to reach these speeds but I have driven at these speeds and it is often nerve wrecking. I believe few drivers are comfortable at speeds over 120mph.

 

CN, All speedometers have errors, regardless of the car. It's just the nature of the beast. I rely on tach info to get my top speeds. I would say that a car that is properly corner balanced, aligned, lowered and with good rubber should be very stable at these higher speeds. I felt no problem with mine at these speeds in regard to handling. Visually though, it feels like you are in the Millennuim Falcon as it enters warp speed

James

 

My speeds in the "how fast" thread were based on the tachometer, tire circumference, etc. since I still have the infamous 85mph speedo. Of course, electronic tachometers are sure to be prone to the same kind of errors as electronic speedometers. From my experience, German speedometers are usually just as optimistic as the American and Japanese ones - 5 to 7% high seems about right. To my surprise, however, when mine came back from being serviced by VDO, checking it against a measured mile several times indicated an error of only about 1mph at 70 - possibly because I'm running tires one size larger than the originals.

 

I've noticed the speedo in my '86 to be 7-10% high,I've checked displayed speed vs rpm/gear chart in the manual.

I'm assuming you have stock tire/wheels as a change in overall diameter would affect the reading.

Discrepancy seems to be the same all the way up, but I'm busier when travelling in triple digits.

If I would ever be comfortable at 130mph it would be in my Carrera., the car doesn't even wake up unti you reach 90.

 

I have no doubt that my speedo is off. As with the others I use the tach and the speed/rpm/tire dia/gear ratio formula to figure the actual speed. Oddly enough before I tilted my speedo I thought it was way off, of course I couldn't easily see anything over about 55-60 because of the steering wheel. After I tilted the speedo so that 0 was at 6 o-clock I could see all of the speeds and now oddly enough the speedo is pretty close to what my calculations indicate it should be for a given speed. I can't imagine why/how tilting it could affect its accuracy, but I still look at both and they agree as much as you can tell from the graduations on the speedo.
BTW, at top speed I'm just looking at the tach.