I use my Garmin for directions and based on the speed it shows the original speedo is dead on. What I miss is an odometer and a fuel gauge. I get nervous running 10 hours before fueling and then I only need 85 gallons. Someday I'll be pulling a crosswind and be needing more than that! I could probably run another 4 hours but then I would be frettin and stewing those last 4 hours!
I ran out of fuel last year because of high winds. My fuel gauge has never worked and I had made the same run with no issues in the past. I also found that my tank labeled as 150 gallons on the plate really only has 130 usable gallons. Had I had one or two more usable gallons I would have made it to my planned fuel stop.