I don't have a name for it. Just a trip computer thingamajig...
And tapping into a vacuum line? How would that work???
In t'good old days, when everyone drove petrol engined cars and there were no turbos, at one end of the inlet manifold sat the engine, at the other end was the carb with a throttle valve.
Imagine driving along on the flat with 2000rpm, with the throttle half open and you come to a downward hill, in order to maintain the same speed yoiu have to close the throttle a bit to stop the car from running off down the hill.
The engine is still turning at 2000rpm, trying to suck the same amount of air down the intake system as before, but you just closed the throttle valve a little bit which has increased the amount of vacuum in the intake system.
At that point, a vacuum gauge measuring the vacuum would indicate a greater vacuum when you're going down the hill and the needle would rise into the green 'GOOD' area of the gauge.
At the bottom of the hill yoiu have to go up the other side so you need to give it more beans just to maintain the same speed and yoiu open the throttle. Because more air/fuel is filling the intake system (the engine is still trying to pump the same swept volume as before) the pressure comes up (decreasing vacuum) and the needle on the gauge drops to the red 'POOR' area of it's scale.
A very crude device, but many peple were able to use it to an advantage, as an indicator of ballpark economy.
For illustration, here's one on ebay right now http://www.ebay.co.uk/itm/Smiths-Vac...item35c1c276db
Obviously useless on cars fitted with turbos which pressurise the intake system
And in those days all you had were brim to brim calculations.