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Pretty much every car gets bad gas mileage above 80 miles per hour. That is a pointless comparison, IMO. I am skeptical that the Prius will do that much better at such high speeds, especially going over mountains and maintaining 70+. Probably the Insight would do the best, simply due to aerodynamics and the fact it is so light. But still, why drive in a non-fuel efficient manner and then complain about bad gas mileage?
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His point is that the
relative mileage at that speed is bad. While I generally won't drive as fast as 88, there are times when driving 78-80 is just a lot more practical than driving 60. An example is with long road trips with multipel peopel. It's worth an extra $5 in gas to save an hour of driving time in my opinion. When you're only driving 4 miles on the freeway though, it's probably not worth accelerating all the way up to 80 and braking back down, as the time savings would be minimal. Faster cruising speed is defintiely a major plus on very long drives though.
While the extra aerodynamic drag is there no matter what, meaning absolute MPG will never be as good as driving slower, good engineering can reduce a lot of the powertrain waste associated with those speeds in certain engine designs. In the case of the Civic, the engine is actaulyl pushed out of its fairly narrow efficient powerband, making it achieve worse mileage than even some non-hybrids of similar size, driven in similar ways. This is also I believe why things like accessories, better handling tires, and other modifications have a much more pronounced effect on hybrids than non-hybrids.
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Also, while a more powerful engine may do better at high speeds, they do worse at low speeds, so the trade-off is most likely going to be for the worse. People with the HAH seem to be struggling to get 30+ MPG in the first place, so in that light your mileage numbers don't look that bad.
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This is why I think someone should come out with a turbocharged miller cycle hybrid that is tuned for better highway mileage. Boost pressure could be designed to kick in mostly at 2000rpm+, and the electric motors could be used to mask the acceleration lag. Things like the transmission, the turbo, and the electrics could all be controlled to save gas in the city, while still providing a more powerful, sustained engine load on the freeway without needing to rev the engine as fast.
Also, the Camry hybrid from all indiciation seems to be a very good balance between engine and electric power as well, as most magazine tests that have driven it in side-by-side tests have shown it achieves only a couple MPG lower than the Civic Hybrid, much closer to EPA, despite being a much larger car with much higher peak power. The Toyota HSD design can also minimize the larger engine's low-speed inefficiency by shutting it off more and using more electrics, and still benefit from the slightly larger engine size on the freeway. The V6 used on the Accord however is probably a bit too much engine for that car, by comparison.