Determining most efficient speed

[Meph]

Hello again,

I have been searching though topics and posts lookin for an answer to my question but I havnt found a complete answer yet.

So, Im wondering how can you determin your cars most efficient speed? Ive herd anything from 55mph to 25 mph. From what I gathered the most effient speed is when you can first shift into top gear. My car is geared very high, so 100km/h is 3200 rpm in 5th, and 2500 is 80km/h and 1900 @ 60km/h ect. This theory would put my high MPG speed at about 50 km/h when im at about 1450 rpm?

Ive herd the mention of a vacumme guage, is someone who can explain this technique, or any other way of determining the most effiecient speed?

Oh and if you have already run a test on a car with similar gearing please let me in on the results!
Thanks

[PaleMelanesian]

Those numbers sound correct. For steady-state cruising, the lowest rpm possible while not lugging should give you the best mpg. I haven't done a controlled test, though.
http://metrompg.com/posts/speed-vs-mpg.htm
http://www.metrompg.com/posts/rpm-mpg.htm
Neither gives you a real answer, though. Maybe combining them and extrapolating can.