The volt meter is marked at 13 volts, low/high red zones at 8/18 - what values do the lines in between 8--|--13 and 13--|--18 indicate?

Math says 10.5 and 15.5 if they are "halfway between" but for the life of me cannot find anywhere that confirms this in the FSM, this site, or the rest of the internet.

 

11.5 13.5

 

Just remember ;

The gauges in our cars are by no means scientific quality measuring instruments that will read down to the thousandth of a volt , in fact the astounding rate of inaccuracy when measured against even a service grade "Fluke" digital multimeter is just this side of comical .

 

Also gauges are not always linear so half way on the gauge may not be half way between the indicated values.