Manual Transmission Fuel Economy Question

My question is this: Are RPM's always indicative of fuel efficiency, even if the even is lugging? For example, I am on level pavement, traveling at 20mph. 3rd gear will produce lower RPM's, but the engine will lug. 2nd gear will feel great but produce higher RPM's. Am I burning more fuel in 2nd gear in this scenario? Is the amount of fuel supplied to engine equal in all gears with the same amount of throttle? Or does, for example, 4th gear supply more fuel to the engine than 2nd gear even if the same amount of throttle is given?

 

In general, keeping the RPMs down will definitely save fuel. However if you're in such a high gear at such a low speed that the engine is really lugging against the gear choice, that's not particularly good for the engine or gear. But I don't think it uses more gas. It just stresses components some.

I've heard of guys with the 6-speed who will go all the way up to 5th gear as low as 35mph and even 6th by 40-45. I can only imagine how it lugs on anything by perfectly flat or downhill runs. But they claim they save fuel by doing this. Some will shift 1-3-5-6 and skip 2 & 4 entirely.

But if you really want to know, get an Ultra Gauge .. a bunch us have them, and they're a super, inexpensive alternative to the Scanguage. Go here - www.ultra-gauge.com. The Ultra Gauge gives you instantaneous gas mileage, and is especially useful with a manual because it will definitely help you learn to shift better.

 

On a manual lower RPM = better efficiency untill you reach the low 1000's (1100-1200 RPM min). At lower RPM you get a higher effective pressure into the cylinder during combustion, because of the slower down travel of the piston (think of it like if the piston goes too fast, the explosion does not have time to reach it to give it downward pressure).

Also, you gain on friction.

When the engine lugs (lower than 1000-1100 on my 4 cyl), you are just stressing your engine bearings.

 

I shift before I hit 2k RPM and I also skip 5th. Using this technique, I managed to hit 19.5mpg. I'll be doing this from here on, even though its super boring.

 

I've been trying to do similar and you're right, it is super boring, but it produces good mpg!

 

I went through a skip shifting phase 2-4-6 and then gave it up because even with all that careful shifting, I could only get 19-19.5. There aren't too many MPG gains to be had with the V6 6speed no matter what you do.

 

heh...sweet!!

What a great driver he was!

PC

 

I shift around 2.5k to about 2.8k, and cruise around at about 1.8k to 2k. I get about 23 in the 4cyl doing this. I also do city type driving to commute to work, with a short stint on the highway.

It seems like that "happy engine point" is around 1.8k

Edit: I have not updated Fuelly in awhile

 

Sounds like a great tool, thanks.

 

You are just wasting gas by doing 1.8k cruise city driving. My automatic 6cyl, at 30mph, cruise at about 950 RPM. I had the 4 cyl and a scangauge, the highest gear you can go without lugging will give you the best fuel economy.

 

I ordered an ultra gauge right after I read your post, I had never seen the ultra gauge.

 

You can't compare the RPM's on the 2.7 and the 4.0 V6. Obviously the I4 is going to rev higher. But you're using gas in 6 different cylinders as opposed to 4. So in actuality, you may be using more gas at 950 RPM's than the 4 banger is using at 1.8k.

 

I had a scangauge on my 2.7 and studied the thing enough to know that you will burn more fuel in 4th gear cruising at 1.8K than in fifth at, what, 1.3K RPM.

At 950 RPM, I go through approx 7.5 liters/100Km in my 6 cyl at 30MPH.
In my 4 cyl at same speed, in fifth gear, I would have gotten into the 6's liters/100KM. Cruising at low speed in the I4 M5, you loose approx 1L/100KM by using 4TH gear instead of 5TH gear. About the same thing in the V6.

FYI

70 KM/H V6= approx 8-8.5liters/100 KM
70 KM/H I4= approx 7liters/100 KM.

Sorry for the units, but metrics are way easier to work it and analyse!

 

my 4cyl blows with gas i drive it easy shifting around 2k at the most 2.5k and it burns through more gas than i could have ever imagined. i'm actually shocked my 2002 frontier 6cyl with a surper charger did better lol. tire choice pressure and over all driving habits and preferences come into play, but i for sure had more aggressive tires on the nissan and my old ford and they both got better or the same mpg. oh yeah did i mention that my truck is slow as balls lol even if i wanted to rev the piss out of it its horrible haha. toyota fell off hard the first gen 4cyl was way better imho.

 

I can't stand the liter per 100 km system you guys use...

 

http://lmgtfy.com/?q=liters+100km+to+mpg

 

I usually don't go that crazy with it but in normal driving I'm cruising in 5th at 40-50mph and 6th anything over 50-55mph. This allows me to keep the RPMs between 2-3k, but closer to 2k. With doing this I have been able to consistently maintain 19-22 MPH depending on highway/city mix. I am soon going to be getting a scangauge or ultragauge so I can keep an even closer eye on it. All in all I am very pleased as the lowest MPG I have gotten off the V6/6-speed setup (save for towing) was 17.6 when in the city.