Speedo accuracy for '85
 

I took my gps on a drive yesterday and surprised to see that my indicated 70mph was only 65mph on my gps.

Anyone else notice speedo inaccuracies?

My '85 is on stock wheels with stock tire sizes...

Oh yeah!!

Mine is in KPH but is still off about 5 miles per hour. I like it that way as it builds in a little protection. Unless a person was going WAY over.:)

Our BMW was off as well. At 72 indicated we were only doing 65. They are notorious for being that way from the factory. I wondered why everyone was passing us on the freeway.

The speedo on my '85 is also off by 5mph.

My '86 reads out about five over actual, too. No matter how fast I go, I know it ain't for real. Had a cop stop me not long ago in a "construction" zone (NOT, noone working, improper signage, etc., even the citation had discrepancies!) and he had me at 37 in a 35 (20, so he tried to say, in the "zone") and I was seeing 42. It seems to be about 5 over from 20 to "illegal". :) Yes, I beat the rap..... :D

My 86 is the same way.

My 80SC speedo is almost 10mph higher than my GPS, but I have 17" turbo twist wheels with huge tires, which certainly screws things up. Only problem with the higher than real indicted speed is the mileage on the odometer is the same percentage higher!!

My 86 is 3-4 mph off, reads higher than actual. I attribute it to the lowering using the low profile tires on the extra wide wheels.

Until a trip to North Hollywood - my speedometer was also off a good 10% 60 miles per hour was 54

It's now right on the money - Of course it was nice to tell people I was hitting 140 on the back straight at Sebring when in reality it was 126...

My 75 was way off - smaller than stock tires on the original 15" rims - stock tire size no longer available and all. I was reading a good 10% off, which led to much paranioa while driving.

Its better now that I've upgraded to 16"s (15"s were bent... much sadness) moved closer to stock size. I'd say I'm still 3-5% slower than indicated based on GPS and various radar signs at different speeds. Way better than the other way around where the speedo says 60 and you are really doing 65+.

My speedo is just about exactly 10% off. I use RPM to judge my speed. I know that in 5th gear, every 1000 RPM equals 25 mph, and every 200 RPM tick mark equals 5 mph. Therefore 3000 RPM is really 75 mph, 2800 RPM is 70, 3200 is 80, etc. I've been doing it that way for so long that I don't ever look at my speedo any more. The RPM is my speedo, afaiak.

I know at least one other guy who likes to quote his speedo's speed on the track (not me, btw). :rolleyes:

Ditto here too. When on the interstate at an indicated 80mph and everyone else around me is doing the same rate I knew something was awry.

As JeremyD (indirectly) mentioned, it depends on how fast you're going. At 40, mine was spot on, per GPS. I didn't check at highway speeds (not my GPS), but I feel that I read a little higher than I'm actually going.

They're all off 10% - part of the experience - makes you think you're going faster than you really are.

Joking aside, there's an adjustable trimpot inside the speedometer than you can adjust to calibration. Just tweek until it matches your GPS. Once you adjust it, you'll realize that the car isn't giving you the illusion of speed you once though it was.

I was able to adjust the pot on my stock speedo and posted the routine a few years ago.

Warren said that there were 3 different elec speedos and not all have the pot.

even after I did the adjust my odo is still as before afai know.

Yep, 5 mph off or more, when my odometer broke I took it to North Hollywood Speedo and they fixed, cleaned, calibrated and repainted the face.

My '83 SC speedo reads 5 mph faster than actual as well. Does that mean that my mileage/odo is 5-10% higher than it should be?

My SC speedo is off by about 4 percent, on the high side, like most everyones'.

My theory is that they purposefully do (did) it from the factory. Think of all the issues with speeding tickets if the fretful owner found out his speedo read low from the factory, and decided to sue Porsche.

I found it irrating when it over clocked.