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Old 03-27-2018, 10:24 AM   #1 (permalink)
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speed vs rpm scangauge?

Greetings Ive recently picked up a scangauge and have been monitoring my mpg. It appears to favor higher speeds over a lower rpm, example being i could be going 18mph at 1100rpm vs 22mph at 1400rpm and it'll always read the higher mpg for faster speed. Why is that? I assumed the lowest rpm it can go would yield best mpg. Same when it kicks into 5th gear at 35ish and has the rpms really low, it always reads lower mpg than if i was going faster. Sorry if this is a dumb question im new to all this. veichle if it matters is an 07 civic thanks!

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Old 03-27-2018, 04:06 PM   #2 (permalink)
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As a rule of thumb, lower rpm tends to be more efficient, but a rule of thumb is all I take that as. There are tons and tons of variables such as airflow through the engine, throttle needed to keep your speed, and many many more which make it vastly more complicated than any rule of thumb. Every engine will have its sweet spot(s) where it burns the least amount of fuel per hp produced (the guys with lots of university degrees hanging in their offices will call this Brake Specific Fuel Consumption). If low rpm were always the key, we'd have engines which were capable of merely idling along at the speed limit. Sorry I can't speak specifically to your car, but that's the general reason why you indeed might not always see top MPG when you're at your lowest rpm.
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Old 05-29-2018, 04:55 PM   #3 (permalink)
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While RPM is one-factor playing role in MPG, there are some other factors that impact mpg. Major ones are:
Air to fuel ratio
Compression ratio
Engine load factor
Spark timing
Temperature
Furthermore, the RPM vs Efficiency curve is not linear.
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Old 05-30-2018, 06:45 AM   #4 (permalink)
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It's usually the case that the lowest speed you can use top gear with will deliver the best fuel economy. That's basically true of my Insight, though fuel economy is very roughly about the same from ~30mph to 45mph in 5th. In all other vehicles I've driven (especially automatics), best economy came at the lowest speed I could stay in top gear with.
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Old 10-01-2018, 09:26 AM   #5 (permalink)
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Less speed - less fuel.
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Old 10-01-2018, 12:58 PM   #6 (permalink)
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Quote:
Originally Posted by MistyLJ View Post
Less speed - less fuel.
Not always true

Lowest speed in top gear where your engine isn't lugging, will be very hard to beat in any lower gear at lower speed
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Old 10-01-2018, 03:35 PM   #7 (permalink)
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Quote:
Originally Posted by euromodder View Post
Not always true
Especially when you're talking about speeds in the 18-22 mph range (which a lot of us only see in passing, as it were :-)), where air resistance is not much of a factor.

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