[gone-ot]

Q: if I know the maximum, aero-limited vehicle speed (124 mph) and the engine RPM at that speed (4,800 rpm), and the calculated horsepower output at that engine speed (135hp = 148lbft × 4800rpm/5252), shouldn't I be able to "back-into" other such variables as:

1) Engine efficiency (~0.23)
2) Drivetrain efficiency (~0.95)
3) Coefficient of Drag (Cd ~0.33)

...so as to obtain a "better" estimated MPG, using the EM calculator?

...I just plugged in the following numbers for our 2011 LTZ and got a calculated 110 hp value at 124 mph, which is too low:

• Vehicle weight = 3500 lbs (my guess: 3177 lbs curb + 323 lbs load)
• Crr = 0.009 (my guess)
• Cd = 0.33 (working backwards from 0.298/0.9 from GM)
• A - Frontal Area = 24 sg.ft. (from brochure dimensions)
• Fuel Density = 33557 (summer blend)
• Engine efficiency = 0.23 (a guess)
• Drivetrain efficiency = 0.95 (default value)
• Parasitic overhead = 0 (default value, what for A/C being "on"?)
• Air Density = 1.164 @ 86ºF (yes, this is the desert)

...both C&D and R&T magazines have published 124 mph as the 'tested' aero-limited top speed for the 2011 Cruze LTZ 1.4LT with 6-speed automatic.

...comments or suggestions or "±" critiques?


NOTE BENE -- the "DIM LAMP" reference is a tongue-in-cheek nod to "...light's ON, but only DIMMLY lit..." meaning I've got a half-baked idea, so do I keep cooking or turn the light full out?