Gonna elaborate on this a bit.
Above is a BSFC curve for a DOHC 1.9L Saturn engine. Close enough to our KAs and out of boost SRs.
You want to minimize BSFC, as it is a measure of fuel consumed per unit of power. In this case, maximum efficiency is 2500RPM and 124N-m of torque. At this engine speed and power output, 250 grams per kw of fuel are consumed per hour.
For reference:
250 grams ~= 1/10 of a gallon
124n-m ~= 90lb-ft
1kw ~= 1.3hp
and, 90lb-ft at 2500RPM is about 45hp. Which is about where you'd require in cruise at 70mph. Funny how that works. Good job, auto engineers.
Anyway, moving on. Downshifting is like moving along a line parallel to the two i drew above. Those lines may not be exact, they're pretty arbitrary, and their actual slope is dependent on the transmission ratios.
Anyway. You'll see that downshifting ALWAYS puts you in a worse BSFC category, given that you're over like 2500rshared_pm.
FYI, part of why this is:
Gasoline engines thermodynamically are not very efficient at partial throttle. The engine experiences signifigant pumping losses from a partially closed throttle. Diesel engines don't have this problem, and so their torque peak is typically right at best BSFC. Which is why the euro guys have gotten such good city MPG out of diesels.
95% of TR probably doesn't care about anything I just said. Here's a sweet S13 dropped on its nuts: