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Originally Posted by Peter7307
Quite an impressive running economy.
For a comparison my Commodore (2003 V6 Acclaim auto with a/c) manages around 5.5 / 100 km ( actual not simulated ) at 90kph.
Engine speed is around 1750 RPM.
Interestingly dropping the road speed to 80 actually increases the fuel used.
I am not sure why but I think the engine is off the fuel economy "island" on the fuel map and efficiency suffers as a result.
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Do you have your A/C on at this speed? A huge sound system?
I simulated it using kerb weight 1573kg, frontal area = 2.7m^2 (couldn't find, used a recent falcon), Cd = 0.319, rpm @ 100kph = 1944rpm, Pmax = 152kW. I get 5.2l/100kph @90kph. I get lower at 80kph.
If I assume 1kW of power is used by accessories (e.g. a/c), I still get less power used at 80kph, 5 l/100kph. Although 5.5 l/100kph @90kph.
Traveling at constant speed at those sorts of speeds, almost any car, especially with a 3.8l engine, is never anywhere near the fuel economy "island". However, the low revs help a lot. A smaller engine at the same revs would of course use significantly less fuel.
Maybe there is some sort of lean burn mode triggered above 80kph, or perhaps the lockup converter drops out at 80kph?
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Big engine (3.8 litres) rev slowly = good economy.
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It's just the "rev slowly" bit that does it. There is not a car on the road with an undersized engine for any Australian highway speed, at least from the point of view that switching to a larger engine at the same revs would improve fuel economy. (Assuming constant speed travel, not going up a hill of course).