Speedometer showing 6 - 7 mph too much

[Classic 911]

As the heading states my brand new original 911 speedometer shows 6 - 7 mph too much. Checked with GPS.

This speedometer is for 1976-77 3.0 Carrera and 3.0 Turbo cars with 225/50-15 tires: 0 - 300km/h rated. Part number is the correct 930 641 501 00

My car is running a 1980-1983 SC 915/62 gearbox and 225/50-15 Yokohama A052 tires.

So, WHAT can be wrong ?

[dw1]

Yo don't say at what speed you see this error. Is it a constant offset at all speeds, or does the error go up as speed goes up?

I ask because my '87 factory speedometer is quite accurate until it gets to extra-legal speeds and then it reads too fast, by approximately 5 mph depending on speed.

[Classic 911]

Originally Posted by dw1

Yo don't say at what speed you see this error. Is it a constant offset at all speeds, or does the error go up as speed goes up?

I ask because my '87 factory speedometer is quite accurate until it gets to extra-legal speeds and then it reads too fast, by approximately 5 mph depending on speed.

Hi,

Yes this is a constant offset at all speeds


Sent from my iPhone using Tapatalk

[EC900]

GPS on cellphones is not as precise as many think and is only a guide due to so many things and factors can interfere with signals. Though these electronic speedos being (bench) calibrated instruments, they could be out of whack from shipping. Speed may be correct but the analog needle isn’t ?? You don’t mention if the sensor cable is mated for that speedo model/year configuration…if that might be a factor?

What I would do is enlist the aid of the local law enforcement agency and ask for the favor of a LIDAR check. Even those highway road construction speed checks have a calibrated sensor and seem spot-on accurate too.

Used to be 10 mph over the posted limit was the threshold, as towns seek more revenue, 5 mph over the limit is ripe for a citation and/or a court date. So the local PD would be my first choice, then the place that sold the speedometer.

[dw1]

There have been several threads on speedometer issues (the search tool can be your friend) and from your description it sounds like it might be a "needle offset" problem.

A good thread on this (and there are others): https://forums.pelicanparts.com/porsche-911-technical-forum/778994-how-do-i-fix-my-speedo-error-2.html

[PeteKz]

I always check my speedo and odo against the highway mileage markers, especially when they set up a 5-mile measured course. The regular mileage markers can be off a little bit, depending on where they had to put the marker due to off ramps, etc., so if you use the regular markers, average the measurements over a 10 mile course (or more). When you do that, your results are extremely accurate.

And dw1 provided a good link. If the offset is the same 5MPH from 20 MPH to 80MPH, then the needle is offset.

[spuggy]

Originally Posted by EC900

GPS on cellphones is not as precise as many think

You're a funny guy. Tip your waitress!



Seriously, most people aren't using cellphones or GPS units from the early 2000's - some of which had trouble seeing 3 satellites overhead with clear sky visible if you were holding the antennae up to a window - and forget about urban canyons or high tree cover...

With a 3-4 year old smartphone, in a concrete basement (that does a pretty darn good job of killing cell coverage). I can see 31 satellites overhead. It takes 3-4 seconds to first fix, and it will use 20 of the best, if I allow all 6 of the best-known constellations to be used.

That class of GPS is likely going to provide far better accuracy than any analog speedometer - certainly in a straight line, where calculated speed between points, given the reliability of those estimated positions, is going to be most accurate - typically around 01 MPH, or 0.2 KMH can be expected. Yes, a windy road will throw off these calculations somewhat.

Reasons for the other inaccuracies (some of which are deliberately introduced) are left as an exercise for the interested to research.

Even the (much, much, older tech) GPS module for my Motec is more accurate than a speedometer - even the one I adjusted with a Yellow Box, a device I would heartily recommend to correct for legally-mandated speedo optimism, or changed tire sizes throwing everything off (even fitting a different manufacturer's tire of the supposed same size can give different results): Speedometer correction!