I attached a Garmin GPS to the windshield to check out how accurate my speedometer is. At 25-45 mph it was consistently one MPH off. At 35 mph on the speedometer the GPS showed 36 mph. above 45 it was about 2 mph slow. At 53 mph on the Speedometer it showed 55 mph on the GPS. Don't like to take it any faster than that. Usually drive about 40-45 mph on the county roads.
A few years back I was advised to add a modern oil pressure gauge as the old gauges my not be accurate anymore. I added an oil gauge from the Autoparts store. It lasted about three years and broke. Put another one on it and it too lasted about three years and this past weekend it too broke. I will stay with just the original, seems to be working good enough.