The fuel consumption curve is nearly always a mirror image of the torque curve for most engines but this relates only to the engine on a test stand.
Out there in the real world it all gets impacted upon by a heap of additional factors so the two frequently fail to match up.
In the long run the only thing which matters is the fuel used by the car and driver and in practice usually the lower engine the engine speed and vehicle the better the fuel economy will be.
By the way I am also driving a variation of the same engine and find the best fuel economy is around 90 kph (about 55 mph) on the highway. The car is a four door sedan with auto GM 4L60E and weights about 3200 lbs.
TC Lockup FTW! With my SuperMID, the lower the engine speed, the better the fuel economy. Once Lockup engages, the economy goes right up. 50mph (lockup speed) gets 42MPG (imp), prior to that (49mph) it gets 35mpg (and still gets 35mpg at 30mph etc).
__________________ Team GasMisers5 - #1 for first three rounds of the original GS Fuel Economy Challenge
Miles displaced by e-bike since 1 Jan 2008: 62.6 (0 kWh used)