The speedometer works well. Here is the finished unit, which has been shipped to the customer. Built for a show car, this one displays speed, heading, GPS time, and has several show/demo modes. I have a kit of parts on the way for a second customer's unit, which will have a pulse output for a cruise control.
This is a large 0.8 inch non-multiplexed display: each digit has its own 74AC164 driver chip. It is bright enough for daylight use. The display brightness is continuously variable, and the decimal point goes away when you exceed 100 mph. With a minor firmware modification, it could have separate day and night brightness settings determined by a headlight input.
The device requires an ATMEGA88 or larger MCU, 2KBytes of program memory, and 100 - 200 ma of 12-volt power.
GPS speedometers in general read slightly low on turns and grades.
There is a video demonstration here, originally prepared for the customer:http://www.opengpstracker.org/counter_demo.avi