I use my iPhone mounted on a
Steelie Car Mount it will also neatly mount a GPS device. The ball is in the centre of the black panel and the iPhone is rotated landscape so it does not impede the other dials or heater switches. I have a retractable USB to iPhone cable stuck under the dash using double sided number plate tape so no trailing wires.
The phone GPS apps I have tried are Speedometer, MotionX and the MPH readout of Co-Pilot, the most accurate I have found to be MotionX, closely followed by Co-Pilot, Speedometer is quite accurate but can loose tri-angulation for short periods, however it can be adjusted to reflect a true speed + or -. I find Co-Pilot to be the best to use as it audibly warns if exceeding the prevailing road limit, you can set this to be higher by 5mph increments, and also warns of safety cameras. I used a combination of these to check the calibration of the speedo on my 3.7 Roadster when trialling the ECU upgrade for MMC and my speedo is now within 1MPH at 70mph. Originally it was reading 62-63mph at a true 70MPH (this will vary with tyre wear). I would suspect that most 3.7 Roadsters will be similar with the original ECU as it is locked down by Ford.
I doubt Peter's suggestion of a reader will work with 3.7 Roadster ECU but would be interested to know if it does.
