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Old 12-04-2007, 04:39 PM   #1 (permalink)
MetroMPG
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 53.56 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 57.1 mpg (US)
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MetroMPG.com mailbag: is lowest RPM really best for max MPG with big displacement?

Got this question recently from a MetroMPG.com visitor:



Quote:
I've been recently wondering about the whole rpm's and speed thing and I've tried it myself, I know that if I go 50 in my Jeep that's where it shifts, then I can slow down to about 47 without it downshifting.

I Have a 95' Jeep Grand Cherokee (ZJ) and It runs the 242 4.0L I6 And when going 47 I achieve around 1400 RPM. At 50 I run around 1475 RPM at 55 I do around 1530 and at 60 I do 1690. I've driven at all these speeds and I think that my best mpg is achieved at around 60mph.

I don't know why, but it seems that after experiencing driving with other vehicles that, vehicles with larger engines get better MPG at higher speeds.

I don't know why this is true, but it might have to do with the large engine displacement, and that a lot of fuel is being wasted at lower rpm's because maybe there is almost the same amount of fuel being put into the engine at those speeds.
This isn't the first time I've read a comment like this about larger displacement motors.

I can say definitively that in all the vehicles I've driven with fuel economy instrumentation, the best cruising efficiency is achieved in top gear at the the lowest RPM. (Level road, constant speed, no other traffic creating aerodynamic effects.)

That said, most of the vehicles I've driven have had small displacement 3 & 4-cyl motors. I've only tried a couple of V6's with instrumentation.

I'm not saying it's not possible that much bigger 6- and 8-cyl motors may behave differently. I just don't have experience with them.

The only way to know for sure is to get instrumentation and try it.

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