Mpg @55?

[mikeb535]

I've noticed that when driving 55mph which equals 2000rpm I get better mileage than when I drive locally between 35-45mph. At the lower speeds I am at a lower rpm range (OD on) but can do no better than 27-28mpg. If I have OD off would I get better mpg even though I may have a little higher rpm?

[MetroMPG]

How are you measuring your fuel consumption? (Instrumentation?)

It's possible you're so far out of the ideal BSFC zone for that engine at low RPM in the highest gear that fuel consumption takes a hit (vs. same gear, higher RPM).

I've yet to see that in any instrumented car I've driven though, where the "lowest RPM in the highest gear" rule of thumb always seems to return the best fuel economy.

[FastPlastic]

My Jeep works this way as well. Mines an automatic which I think is part of the problem. At 55mph on level ground the ScanGauge reads somewhere around 27-29mpg at around 2100rpm. At 35mph it reads at most 20-22mpg but around 1400rpm. Push the speed up to 38-39mph the rpms go up to 1500-1600 and you get about 1-2mph more.

At idle my Jeep pulls around 0.75gph. Is it just you have to get to a certain speed to counter act the shear amount of gas the engine needs just to spin?

All the more reason to encourage people to pick up a ScanGauge I guess.

[Tourigjm]

most vehicles operate most efficiently around speeds of 50-55 miles an hour.

you may be using less fuel traveling at 35mph, but at the same time you're traversing less ground for that fuel

[SVOboy]

I get better mileage all the way down to 30 mph, but with an automatic, if you're below the threshold for o/d engagement you prolly won't see this same sort of thing.

[xbUser]

If you are driving at 30mph for 10 miles and return 35 mpg, it still saves more gas than driving at 60mph for 10 miles and return 30mpg. Mpg is miles per gallon, no time factor. But in reality, we will put the time factor in consideration, which is save money vs save time. That's it!!

Engine has its own power curve. Usually, lower rpm saves more gas because it rev less = less cycle per min = less # of gas injected cycle per min. However, less gas = less power. If the engine doesn't produce enough power at low rev, you need to burn more gas in each cycle. In this situation, lower rpm may burn more gas than higher rpm. What makes this situation happen is controlled by your right foot. If your foot is light enough and the gas injected is just enough to maintain the speed, then you save gas. However, if you have a heavy foot, more gas injected, but may not have enough power to reviv up the rpm, it looks like cruising at the same speed, but you are burning more gas.

In some situations, we have to shift down to maintain the low speed, like going uphill at low speed. I won't drive 25 mph with top gear. Engine can't produce enough torque at such low rpm. Driving with that combination won't save you gas and probably bad for the engine. In that situation, use 3rd gear or 4th gear, whatever gives you the best mpg. That is our ultimate goal, right?!

[mikeb535]

Originally Posted by MetroMPG

How are you measuring your fuel consumption? (Instrumentation?)

It's possible you're so far out of the ideal BSFC zone for that engine at low RPM in the highest gear that fuel consumption takes a hit (vs. same gear, higher RPM).

I've yet to see that in any instrumented car I've driven though, where the "lowest RPM in the highest gear" rule of thumb always seems to return the best fuel economy.

Have no gauge as yet. Ordered Plex KIWI. I drove to Ohio last year at speeds of 60mph and avg'd 37mpg. I normally get 80 miles per 1/4 tnk driving between 35-45mph which equals about 27-29mpg. My travel is 20 mile round trips over hilly terrain (10ea way) in OD with rpm's between 1500-1700. The automatic will kick-down when driving uphill with rpm reaching 2250-2500 rpm's and speed drops down to about 25mph. I drive with a lightfoot watching tach hit 2150rpm when trans shifts thru each gear. No extra weight in trunk. Right now with recent tankful I'm driving with OD off when travelling between 35-45mph to see if I can see any difference in mpg. Maybe engine is lugging in OD at low speeds. Sorry I sound so long winded.

[MetroMPG]

Ah. Without instrumentation that lets you track trip mpg, I'd hold off on making an absolute judgment.

You're the first person I know who's ordered the KIWI. Please let us know how you like it when it comes.