GPS computer accuracy

DC Rainmaker is a good website to check for this kind of info - whenever he reviews a GPS device (and he's covered most common cycling computers) he does some testing of GPS accuracy, including in "tricky" situations (buildings/trees).

Each of my fleet has a computer with a wheel magnet and sensor. I'll occasionally ride with a garmin forerunner 945 and/or garmin Edge Explore 2. Never do the 945 or EE2 match and are often 5-10% less than the computers, regardless of the bike ridden. The more curves in the route, the greater the disparity between the devices because gps measures a series of straight lines, whereas the computer is based on wheel revolutions. The greater the frequency of the gps points, the more accurate it will be, but it won't match the accuracy of wheel sensor computers properly calibrated to wheel circumference. Manufacturers generally claim that the wheel sensor computers have an error level less than 1%. My experience with the fleet is that is probably the case.

If you just want an accurate record of your trip and mileage, Strava on a phone in your pocket should do the job well for you as, unlike most (all?) GPS computers, it will determine your position not only from GPS but also from all available mobile masts and known wireless networks. If you want the most accurate live speed information a simple and inexpensive speed sensor will do a good job.

It makes you wonder how much the device even needs to depend on GPS accuracy. If you have a mileometer (speed sensor) strapped around the wheel axle, presumably for road use the device really only needs occasional GPS fixes, and can interpolate accurately using the map and the mileometer. Obviously for off-road it can't make so many assumptions.

Look for a GPS computer that has multiband, for example the Garmin Edge x40 series (1040, 840, 540). They can track more than one frequency from each individual satellite, improving accuracy. This combined with the option to use more than one system simultaneously (e.g. GPS, GLONASS, GALILEO) will give the best chance of getting a fix from the overhead satellites, although nothing will be totally infallible as ultimately, you are receiving a weak signal from over 12500 miles away, it's weak enough for wet leaves to block this. 

https://support.garmin.com/en-GB/?faq=9NWiPDU4gM0JWMfdWFol7A