North American Turbocoupe Organization

Speedometer caibration info (long)
Jeff K Offline
The calibration on the original JY speedo I converted to read 140 mph sucked at speeds below 40 mph, being off by as much as 5 to 9 mph at real low speeds, so I pulled the original speedo out and converted it to 140 mph. Calibration was MUCH better, within +/- 2 mph between 25 and 100 mph.

Had a few free hours over the weekend, so I took apart the poorly calibrated JY speedo to see if I could figure out why calibration was so far off. IT TURNS OUT THAT THERE ARE MECHANICAL CALIBRATION ADJUSTMENTS INSIDE THE SPEEDOMETER MOVEMENT!! If the speedo face is removed (2 black acrews), and the odometer assy / stepper motor drive is removed, the actual speedo movement is pretty much exposed. There are 2 thin, about 1/16" wide, 1/2" long aluminum tabs sticking out of the rotating armature. There is a small brass slider on each tab. There is also a larger brass slider on the armature perpendicular to the 2 tabs. The larger slider has a significant effect on calibration at low speeds, and the 2 smaller sliders seem to "trim out" the calibration at different speeds.

I put the 140 mph face and indicator needle on without the odometer assy in place which allowed access to the brass sliders using a small screwdriver while monitoring the speed reading. Note I was driving the speedo with a signal generator to mimic VSS input, and monitoring drive frequency with a frequency meter. 2.22 Hz input per mph.... i.e., 222 Hz = 100 mph, 133 Hz = 60 mph, etc. After about 45 minutes of playing with the positions of the sliders, I was able to get the calibration withing +/- 2 mph between 25 and 100 mph, which is a HUGE improvement over the original calibration. BTW, calibration varies with the front / back tilt of the speedo, so I made all measurements with the speedo sitting at about the same "tilted back" angle it sits at when mounted in the car.

Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 21 psi, forced air intercooler, water injection, bypass valve, Ranger roller cam, subframes, etc., etc.. // 86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP nitrous, ignition, gears, suspension, etc., etc.... // 91 Escort: Bone stock winter car // 02 Taurus Vulcan(wifes car)
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.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized

Walsted Offline
Posting Freak
Thanks, Jeff.

Now maybe I can get my speedometer and odometer to agree without assuming 67 minutes to an hour.

Mike Walsted - NATO member
1986 5-speed TurboCoupe and 1985 5-speed XR7
Mike Walsted - Sold my 1986 5-speed TurboCoupe

fred k Offline
Senior Member

Really good work. The effort is appreciated.
It's great when you have the equipment and time to work on these things.

Thanks again,

fred l kennedy

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