North American Turbocoupe Organization



Speedometer different calibration in automatic versus standard?
vegas_ss Offline
Senior Member
#1
Swapped in a different instrument cluster and it seems the speedometer is reading lower than actual speed. Noticed on one of the radar speed limit signs for 35 mph saying driving too fast, showed 44 and I was closer to 40. I went for a quick drive with a garmin gps that was showing the speedometer was reading ~ 4-7 mph lower than what the gps was showing when traveling ~ 45-65 MPH.  I was wondering if the speedometer for an automatic would be different than what is used for a manual?  The old speedometer was quite accurate but on occasion I would see the needle bounce.  The cluster is from a 87/88 as the tach has the firm ride light (installed in a 87).  Does the IVR play any role with the speedometer or just the temp/oil/fuel gauges?  I do notice the gauges seem to have 2 different readings (higher oil/temp/fuel) at times.  This also occurred with the old cluster (2 different IVR's).
1987 TC, 5sp, Boport Stage 3 Head/2.1 Cam
1996 Impala SS
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JT Offline
Posting Freak
#2
(07-21-2021, 04:50 AM)vegas_ss Wrote: Swapped in a different instrument cluster and it seems the speedometer is reading lower than actual speed. Noticed on one of the radar speed limit signs for 35 mph saying driving too fast, showed 44 and I was closer to 40. I went for a quick drive with a garmin gps that was showing the speedometer was reading ~ 4-7 mph lower than what the gps was showing when traveling ~ 45-65 MPH.  I was wondering if the speedometer for an automatic would be different than what is used for a manual?  The old speedometer was quite accurate but on occasion I would see the needle bounce.  The cluster is from a 87/88 as the tach has the firm ride light (installed in a 87).  Does the IVR play any role with the speedometer or just the temp/oil/fuel gauges?  I do notice the gauges seem to have 2 different readings (higher oil/temp/fuel) at times.  This also occurred with the old cluster (2 different IVR's).


There is no difference between the automatic and manual transmission speedometers.

The IVR is not used for the speedometer or tachometer.

The speedometer jumping around, such as at idle, is an indication of noise in the electrical system. It could be a connection or ground issue, or an alternator issue.
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Kuch Offline
Senior Member
#3
I wonder if maybe you have a bad capacitor or something on the speedo giving a different reading. Not sure how these actually work, but there are a few capacitors on the back of them, and these can go bad after so many years.
1988 Turbo Coupe, Black/Black, 5 Speed, Moonroof,  T3/T4, ported E6, 255LPH, Kirban, Stinger Exhaust, MGW shifter, K&N, Gillis valve, BP1.5, PIMPx, Koni's
1964 Ford Galaxie 500XL, 390 6V, Big Solid cam, Headers,3.89's, 4 Speed, Vast and fast
1960 Ford Starliner, 292 Y Block, 312 4bbl intake, headers, 3 Speed, slow and low
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Jeff K Offline
Administrator
#4
Speedometer is just a voltmeter (and voltmeters are really ammeters, but that is beside the point). The speedometer can be calibrated with a 12 V DC power supply to power up the speedo and a sine wave signal generator. The input frequency to speed transfer function is 2.22 Hz per MPH. For example, if a 111 Hz sine wave signal is applied to the speedo, it will read 50 MPH. If you look closely at the speedo mechanism you will see 2 brass colored flat arms with a brass slider on each arm attached to the shaft that holds the needle. These sliders can be moved on the arms to get the speedo to read correctly at nearly all speeds. This involves LOTS of trail and error. I was able to get my home made 140 MPH speedo to be accurate within +/- 1 MPH from 25 MPH to 130 MPH with a lot of trail and error. Under 25 or above 130 MPH, accuracy is +/- 2 MPH This was verified with GPS. If doing this, the speedo should be set at the same angle as it sits on the cluster, as the angle the speedo sits at has a minor effect on the indicated speed. If the speedo reads high or low by a couple MPH across the range, the needle can be moved to correct for this.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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vegas_ss Offline
Senior Member
#5
Thanks for the info... it looks quite delicate! I'll have to verify the error over a wider range of speed and distance. It may be the needle is just off. I did notice it sits below zero when stopped however I noticed the same with the old speedometer. It may be a little more below zero though so I'll definitely look there first now that I know there is no difference between the automatic and manual transmission calibrations.

Now how exactly would you move the needle if that is what is off? Does it come off and then reposition it or???
1987 TC, 5sp, Boport Stage 3 Head/2.1 Cam
1996 Impala SS
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vegas_ss Offline
Senior Member
#6
Went for a longer drive with the GPS to verify the speedometer and found it off (reading lower) by ~ 4 mph at speeds up to ~35-40, then ~3 mph from 50 - 65. Gets within 1 mph at 70 and stays pretty accurate up to ~ 90.

Will probably swap the old one back in as I'm pretty sure that was accurate but never checked.

Did find the key didn't want to turn in the ignition, seem that the release button mechanism that you need to press to remove the key may be problematic. Always something!
1987 TC, 5sp, Boport Stage 3 Head/2.1 Cam
1996 Impala SS
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Jeff K Offline
Administrator
#7
Needle just pulls off and pushes back on. Done this many times back when I used to sell home made 140 speedos.

FYI, once the speedo is apart, it is pretty easy to reset the odometer mileage to whatever you want. Only takes 5 minutes.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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vegas_ss Offline
Senior Member
#8
Does the speedometer needle rest on a stop or would you, in my case, just pull off the needle and position it back on a little closer to zero? The mileage on this speedometer is 46,xxxx, assuming that it wasn't changed was the main reason for using this one over the original since I thought it would be newer as the one I had was 212,xxx miles.

The needles on the temp an fuel seemed to be a little bent outward so I swapped those over from the old cluster. No telling what this one has been through since I bought it 10+ years ago and partially disassembled it and left it in storage. After I installed it I found the firm light on the tach worked, but was very dim and you couldn't really see it. Also the overlay with the check engine, antilock brake and high beam indicators didn't look too good when lit up so I swapped that back. The plexiglass cover is basically flawless after polishing with some mequires plastic polish. Changed all the 194 bulbs and it looks very nice! Went ahead and ordered a solid state ivr from NPD and once I get that I will either swap the speedo back or try moving the needle.

Thanks
1987 TC, 5sp, Boport Stage 3 Head/2.1 Cam
1996 Impala SS
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Jeff K Offline
Administrator
#9
You could put the cluster in the car with the clear plastic bezel off and go for a drive and with the bezel off which would let you easily pull the needle off and put it back on until the speedo reads correctly.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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vegas_ss Offline
Senior Member
#10
Yes, that was what I was thinking of trying. Would like to get it pretty accurate for 25 - 55 mph. Will try and gauge ~ 3-4 mph difference.
1987 TC, 5sp, Boport Stage 3 Head/2.1 Cam
1996 Impala SS
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