These wireless units suffer from at least 2 problems:

1. Lost pickup pulses. This results in low speed indication and lower than actual mileage. If your indications are consistently low, then that is what is happeining in your case.
2. Interference. Your display will be all over the place, high, low, crazy.

Causes:
1. a) Low battery in pickup unit leading to weak signals. Replace battery.
b) Low temperature leading to weak signals. Move the 2 units closer to each other.
c) Units spaced too far apart leading to intermittent pickup. Move the 2 units closer to each other.

2. Most likely from power lines or cars. You can't do anything about it except to get another brand or model.