Historically vehicle speedometers and odometers are all over the map when it comes to accuracy, digital or not.
Our two Prius vehicles read speed as at least 2 MPH faster than GPS, and distance at least as bad. The 2012 Tacoma reads speed about 1.5-2 MPH LOW depending on tire wear. The 2002 Tacoma reads about 2 MPH high.
If I was looking for accuracy, I'm not going to find it on vehicle displays, obviously.
Feel free to skip the rest of the post...engineer hat on...
The problem is the difference between 'ACCURACY" and 'PRECISION' when you are talking measuring stuff.
Accuracy refers to displaying information that is correct based on some standard. In other words, if you put exactly 12 volts into some multimeter, it will display exactly 12 volts every time if it is properly calibrated.
Precision is just the ability of a display, given the same input to measure, to give the exact same output. It does not speak to accuracy--just repeatability under the same conditions of input and output.
So if you put exactly 12 volts into a precise digital meter, and it displays 12.1 volts every time you apply 12 volts to it, it is precise, but it sure isn't accurate or, for that mater, calibrated--its just precise and repeatable...
Unless you spend a lot of time (and $$) calibrating your measuring instruments, you can only assume they give a reference measurement, not an accurate measurement. For most of us humans, we tend to believe digital readouts, but if you carefully look at the specifications, they will be listed as Accuracy +- 3%, +- one digit. In other words, it is neither accurate or precise.
For casual use troubleshooting Airstream trailer electrical systems, a cheap Harbor Freight meter is 'just fine'. If you are calibrating industrial process equipment, you use a relatively expensive Fluke meter with an unexpired calibration sticker on it from the last time it was sent to the Metrology Lab and checked against the NIST-tracable and documented hugely expensive standards they used to calibrate it against every freaking six months....
So now you know more about this subject than you wanted to, assuming you read this far...bottom line, use the cheap meter. At least your measurements will be consistent until the next time you overload it or drop it off the roof of the trailer...(grin) Then just toss it and buy a new one. They run on smoke, by the way--that's why when the smoke leaks out they all quit working...
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.