Speedometer 10% low
11-14-2018, 02:09 PM
#1
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Speedometer 10% low
Ever since purchase my 80 US has seemed to be going faster than indicated. Also, higher indicated RPM vs speed as compared to my 83 ROW. After a different issue (50% low reading after cleaning up the differential, which was solved by moving the wire coming out of the speedometer sender, went back to original reading) I replaced the sender with a new one and cleaned the connection in the spare tire well.
This didn’t seem to change things.
I just went out with our local chief of police and had him monitor my speed with his radar.
It consistently 10% under,33 at indicated 30, 60 at indicated 55, 77 at indicated 70.
Tires diameter is within 1/2% of spec
Has anyone run into this before?
Could this be as simple as a miss mounted speedometer needle?
This didn’t seem to change things.
I just went out with our local chief of police and had him monitor my speed with his radar.
It consistently 10% under,33 at indicated 30, 60 at indicated 55, 77 at indicated 70.
Tires diameter is within 1/2% of spec
Has anyone run into this before?
Could this be as simple as a miss mounted speedometer needle?
11-15-2018, 12:23 PM
#3
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if the needle clocking were off that would always read he same MPH over and not a %. If it was 5 mph at 30 it would be 5 mph at 100 !! . The system uses magnets in an aluminum wheel inside the differential. The sender mounted outside he diff counts those pulses and that is the signal to the instrument. It works more like a tachometer than a conventional speedometer. I believe there is an adjustment to recalibrate but I have never done that....Used to send them out to North Hollywood Speedometer for repair, refacing from 85 mph to 170 , etc.
11-15-2018, 01:46 PM
#4
Mine had the constant speed issue. Was 8 mph off at all speeds. Simple fix to remove the speedometer and gently push the needle down the appropriate amount. Sound like yous is not that simple unfortunately.
11-15-2018, 02:48 PM
#5
Rennlist Member
Ya, definitely sound like a rotation/diameter issue. if it was a bad sensor, or mis matched speedo to ring ratio in the diff, you would get a variable in percent as you go faster.
But -always- 10%, has to be a revs per mile change at the wheel.
But -always- 10%, has to be a revs per mile change at the wheel.
11-15-2018, 04:23 PM
#6
Rennlist Member
Two thoughts:
1. Think about what others have spent for a 10% increase in top speed that you have just gotten for nothing!
2. Buy and install one of these and adjust to fix your problem:
1. Think about what others have spent for a 10% increase in top speed that you have just gotten for nothing!
2. Buy and install one of these and adjust to fix your problem:
11-15-2018, 10:06 PM
#7
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I have the same issue on my ‘80 and have not found a solution. However, mine reads 10% higher. I’ve monitored mine with a GPS not a radar gun. Its a good way to keep your math skills sharp.
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11-16-2018, 11:03 PM
#8
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Not sure which is worse - 10% under, or my 10% over. Here we have speed cameras all over the place randomly, as well as fixed on freeway overpass bridges, instantaneous and elapsed cameras on interstates etc, 10% over is a bit safer, but still irritating. You can get speedo correctors that tap into the pulses and can correct over a decent range, but just havent got around to installing one so far.
jp 83 Euro S AT 57k
jp 83 Euro S AT 57k