GPS Speedometer
Posted: Tue Oct 31, 2017 6:15 pm
After a number of speedometer failures due to snapped cables and angle drive failure I stumbled across another approach: a GPS based unit that drives the original Smiths speedometer. While I am not particularly concerned about the speedometer it's nice to have everything working.
In the process of a complete rebuild of my S4 I had purchased a new speedo cable as well as a new angle drive(not cheap). With these new units the speedo worked for about 25 miles when the cable snapped. I think the cable failure in this case was probably my fault as I had managed to pull the crimped end off of the cable (speedo side) during installation. I had put the cable back together but obviously not good enough.
Rather than get another cable I chanced upon a story about a GPS unit for classic cars in Classic Motorsports magazine. This GPS approach would also potentially yield a more accurate speedo as well, as I have changed tire sizes from original. The unit is not dependent on tire size or diff ratio.
I thought I would share my installation. (I'm not promoting this, just a customer).
The "Classic Speed" kit from Classic Auto Innovations comes with the following (motor unit, gps antenna, calibration dongle). I also purchased a custom 36" speedo cable (this can be ordered to length). The cable is standard so you can use your existing one if you can.
The install was quite easy. I chose a location for the motor unit just under the bonnet (hood) bracket on the left side (I have a LHD car). This allows a relatively straight run for the cable. The unit is quite hidden, for the purists.
I drew power off of the fan thermostat (ignition switched). I affixed the GPS antenna out of sight on the underside of the nose body. A nice advantage of the Elan is the fiberglass body which allows placement of the antenna virtually anywhere internally.
Calibration of the unit was very easy, just temporarily plug the dongle in. My unit was already very close and only required a minor adjustment.
Removed the existing cable. Capped off for now the angle drive (using cap supplied).
When I next change the transmission oil I will remove the angle drive and speedo gear and use the aluminum plug below (from TTR):
What I have found in use is that the GPS unit responds faster than the phone speedo apps. At a steady speed both the CAI unit and the app are the same. The CAI unit apparently has a 5 hz sample rate vs 1 hz for a cell phone. Advanced, accurate units used for motorsports and testing have 20 hz rates. The advantage of the CAI unit is that it drives the original speedo.
In the process of a complete rebuild of my S4 I had purchased a new speedo cable as well as a new angle drive(not cheap). With these new units the speedo worked for about 25 miles when the cable snapped. I think the cable failure in this case was probably my fault as I had managed to pull the crimped end off of the cable (speedo side) during installation. I had put the cable back together but obviously not good enough.
Rather than get another cable I chanced upon a story about a GPS unit for classic cars in Classic Motorsports magazine. This GPS approach would also potentially yield a more accurate speedo as well, as I have changed tire sizes from original. The unit is not dependent on tire size or diff ratio.
I thought I would share my installation. (I'm not promoting this, just a customer).
The "Classic Speed" kit from Classic Auto Innovations comes with the following (motor unit, gps antenna, calibration dongle). I also purchased a custom 36" speedo cable (this can be ordered to length). The cable is standard so you can use your existing one if you can.
The install was quite easy. I chose a location for the motor unit just under the bonnet (hood) bracket on the left side (I have a LHD car). This allows a relatively straight run for the cable. The unit is quite hidden, for the purists.
I drew power off of the fan thermostat (ignition switched). I affixed the GPS antenna out of sight on the underside of the nose body. A nice advantage of the Elan is the fiberglass body which allows placement of the antenna virtually anywhere internally.
Calibration of the unit was very easy, just temporarily plug the dongle in. My unit was already very close and only required a minor adjustment.
Removed the existing cable. Capped off for now the angle drive (using cap supplied).
When I next change the transmission oil I will remove the angle drive and speedo gear and use the aluminum plug below (from TTR):
What I have found in use is that the GPS unit responds faster than the phone speedo apps. At a steady speed both the CAI unit and the app are the same. The CAI unit apparently has a 5 hz sample rate vs 1 hz for a cell phone. Advanced, accurate units used for motorsports and testing have 20 hz rates. The advantage of the CAI unit is that it drives the original speedo.