Prior to adding the OD
you could try a few 'real world' experiments
by adding a vacuum gauge connected to the intake manifold
then driving in top gear at 1700 rpm at whatever mph the current ratios give,
up a progression of ever steeper hills,
to increase load on the engine.
When the vacuum drops below
the 6 to 8 inches of Mercury level,
you are getting out of the operating region of good fuel economy.
You can estimate your horsepower load
at different speeds and grades of hills
using the stuff found in this article;
http://web.archive.org/web/20061123075351/http://www.etrucker.com/content/downloads/ccj0302.pdfnotice it is in pdf format
Although the article is focusing on big trucks, the same applies to cars climbing hills
This admittedly old (1960s)
but 'universal'
BSFC graph from the Taylors at MIT for engines
shows the 'island' of good fuel economy:
Notice the bottom line of the graph is not rpm
but 'average piston speed' in feet per minute
which you get by converting your stroke from inches to feet, doubling it since it goes up and then down once per each revolution of the engine shaft,
then multiplying by rpm
