thats what i did too. I took a phone with velcro and used a GPS app and calculated the percentage. When going 40 my GPS read 34 therefore 15%. I'll reinstall it tomorrow and try again. I'll go 1 day without it and see if it shorts or not, just to help rule out the speedo. Then on tuesday i'll put it back in and test it.
Just went through their FAQ and ran by this.. You could try this out and see if it is a reason why your actual speed differed from your set speed by as much as you said. I dont remember who told me, but it was to use 60MPH as the mark to set this thing up.. I guess the info below explains.
Also, IMO cell phone GPS are not as accurate as an actual GPS , I.E. Garmin, Tom Tom. Cell GPS tend to run off of cell towers and not actual satellites which does make them more inefficient. Unless your phone has the option to disable using cell towers for GPS and to run off true GPS then I would try another. I suction cupped my Garmin next to my speedo.
From 12oClockLabs FAQ:
Will my Speedometer be accurate at all speeds using the SpeedoDRD?
Yes, the SpeedoDRD corrects your speed based on a percentage of error, you may have noticed your speedometer error increases the faster you are traveling. Example: if your GPS reads 54mph while your Speedometer reads 60mph, that's a difference of 6mph and 10%, now when your GPS reads 108mph, your speedometer will read 120mph, your now off 12mph, but it's still 10% of error. For this reason SpeedoDRD uses percentages to correct your speedometer error. There are some rare circumstances where you may notice your speedometer itself has some slight linearity error, speedometers usually have no more than 1% to 2% linearity error, if any. Other things to consider when calibrating your SpeedoDRD is the accuracy of your GPS device, most GPS devices are accurate to about 1% or better at speeds of 55mph or higher, at lower speeds your GPS device can start to have in-accuracy anywhere from 1 to 14%, with higher in-accuracy the slower you are moving, and also higher in-accuracy depending on satellite signal strength. Also, another thing to consider is that most vehicles speedometers usually only give whole number readouts, as well as most GPS devices, thus when your GPS or vehicle speedometer reads 55mph, you could be traveling 55.0mph or 55.9mph, when calculating your error percentage at multiple speeds remember to take this into consideration, this situation as well as possible GPS accuracies, may very well account for any suspected speedometer dis-linearity.