The zoom function just sets the maximum display speed on the gauge, for example if you have a crawler doing 5mph max there no point having a speed gauge going up to 60mph.
The roll out function is just a factor that converts the rpm of the thing you are measuring with the sensor to speed in mph of the model.
From the manual:-
Method A
• Mark the clutch bell that the sensor is reading from
with a small reference mark. A marker works well.
• Set the car next to a ruler and at 0” then roll the car
forward by hand, counting each revolution of the
reference mark. At exactly 10 revolutions stop the
car.
• Measure the exact distance that the car traveled in
ten revolutions and divide this distance by 10 (e.g.
12.0” divided by 10 = 1.20”).
• Adjust the Roll Out value until 1.20 appears on the
screen. Now all the rpm related functions will be
displayed in mph or km/h.
Method B
For this method you either need to know the internal gear
ratio (normally provided in the vehicle’s manual) or be
able to calculate the ratio via the number of teeth on the
gears. It is also necessary to calculate the circumference
(distance around) the tire. Once the internal ratio is
known, and the circumference in inches has been
determined, simply divide the circumference by the
internal ratio and use this value as the conversion.
To calculate circumference—multiply 3.14 x the tire’s
diameter in inches.
To calculate internal gear ratio—divide the larger gear
by the small gear. With multiple gear transmissions, it is
necessary to multiply each of the larger to smaller gear
reduction ratios to arrive at the final ratio.
Zoom - The Zoom setting sets the maximum range or
boundary of the Speed unit.
Note: The Telemetry screen displays the
maximum recorded speed from the point
that the receiver was turned on. To reset the
maximum recorded speed, it is necessary to
turn off the receiver, then back on.