Electronic GPS Speedometer Conversion

Started By: William Unger on 2017-02-27 00:00:00

The Problem: I rebuilt my 1969 Jaguar E-type a number of years ago and replaced the original 3.54:1 differential with a 2.88:1 rear axle. I have been driving with a significant speedometer error ever since and making mental corrections to the readings. The only alternative until recently has been to remove the speedometer head, roll the car in gear for a premeasured distance, count the rotations of the cable core, and send off the speedometer to be recalibrated for about $250 plus shipping.

Alternative Solution: Classic Automotive Innovations (https://www.classicautoinnovations.com) markets a cost effective alternative that also allows for future adjustments for any wheel or tire changes. It is an electronic GPS conversion unit that can be owner calibrated regardless of what rear axle ratio or tires are used. By disconnecting the speedometer cable from the gearbox and screwing it into the unit’s black box, an acquired GPS signal controls the speed of an electric motor that rotates the cable core, thus transmitting a corrected mechanical signal to the speedometer head. A separate remote plug-in “dongle” is used to dial in an initial calibration reference point after which the car can be driven to verify the speedometer reading with a GPS navigator or smart phone.

Initial Checks: My speedometer gauge worked well, having previously disassembled the head and checked and cleaned the internals, and the speedometer cable had been replaced. Checking these items is essential to assure that the car’s speedometer system works well and that any preexisting problems will not be blamed on the GPS conversion unit. To test feasibility before ordering the conversion kit, I made a cardboard mockup sized to the GPS unit and searched for an accessible location. The box fit neatly into a large empty cavity in the cowl behind the glove box inside the car. Although water resistant, I wanted the black box mounted away from harmful elements outside in the engine compartment.

How to Do It: I removed the center and radio consoles, folded down the tunnel carpets, and removed the gearbox access cover to disconnect the speedometer cable. Jaguar has a starter bolt access cover in the passenger foot well that I removed, drilled a hole in the cover, added a cable grommet, rerouted the cable through the grommet, and then reattached the access cover. The conversion kit provides a plastic cover to screw onto the gearbox to seal in the lubricant, but I elected to use an aluminum cap with an internal rubber seal that I had. The carpets and center and radio consoles were then reattached.

The rerouted speedometer cable’s stock length was satisfactory, however Classic offers custom made speedometer cables of various lengths if needed. Because of tight quarters in the gearbox tunnel, Jaguar used a right angle speedometer drive which has a 3/4"x26 fine thread screw cap that attaches to the gearbox. (Note that the cable end has a 1/2"x26 fine thread cap which many other Brit cars screw directly into the gearbox without the right angle drive. Be sure to order the correct size GPS unit). Using the right angle drive on the GPS black box allowed for the cable to remain flat against the cowling and have the unit installed perpendicularly in the cowl cavity to avoid having to make any severe bends in the cable.

The GPS antenna base is magnetic and was mounted in the center of the dash cap with the cable tucked in between the windscreen and dash cap, down under the right side corner, behind the glove box, and through an opening into the cowl cavity. The unit’s black ground wire was attached to an existing ground screw at the starter solenoid relay on the A pillar while the red hot wire was fitted with a Lucas bullet connector and slipped into a 4-way connector from fuse # 7 feeding the choke and reverse switches and lamps (all wires solid green). Excess wiring and antenna cable were bundled and tucked into a cavity in the side of the cowling.

The Results: After making the initial calibration according to the manufacturer’s instructions, I prepared to test drive the car. Turning on the ignition, the unit made a grinding noise (similar to road-tire noise) and high pitched sound as the unit initialized for about 15 seconds, and then the sounds stop. This is a normal. As soon as the car began moving, the speedometer needle registers the speed. Using a separate GPS navigator unit to verify speed, the speedometer’s calibration accuracy was about +2 mph at 70 mph. I tweaked the calibration a couple of times until I was satisfied with it.

I fit a foam box around the GPS unit and reinstalled the trim panels which quieted the GPS unit’s sounds which actually are only noticeable at startup in the garage before the unit has acquired satellites. Once the car is moving, there are enough squeaks and groans, wind, engine, gearbox, axle, exhaust, and tire noise … you know, the typical noises of vintage cars … that the unit’s sounds are not noticeable. And when I crank up The Eagles or Mo-Town on the stereo, I cannot hear anything but the ‘70’s.