I use my old GPS unit to tell me the speed I'm driving. The maps in it are out dated but the speed reading is dead on. Save a few bucks and go with the GPS unit until you get exactly the part you need or want.
Just my 2 cents...
I do the same thing actualy.
I also use the trip time and milage feature for paid hauling jobs.
As far as repairing the existing diaphram with an innertube section, you CAN do it, but it should be a lighter type of tube like a bicycle tube (I used one to repair a 1940's oxygen regulator from a Mustang fighter plane, it held up to 50,000 feet in the test chamber too). Just have to find the right thickness of material and dont make it too tight so it has room to throw the mechanism.
