Mpg

[hitman72]

I think i saw a list of the most efficient RPM to run in each gear, can someone help me find it. Thanks.

[Eagle]

2250 RPM, for any gear.

 

The most efficient engine speed is the torque peak.

[onlyinajeep726]

Here you go...

 

[gogmorgo]

2250 RPM, for any gear.

 

The most efficient engine speed is the torque peak.

Not true. There's an efficiency point that moves around depending on load and a few other variables. Usually it's significantly below peak torque. Best mpg I've ever had was cruising at around 1800 rpm. If I go much faster, my fuel economy makes a steep downward curve.

 

onlyinajeep726, where did you get that list and what exactly are the numbers? As in where do they come from / how were they determined?

[onlyinajeep726]

onlyinajeep726, where did you get that list and what exactly are the numbers? As in where do they come from / how were they determined?

 

 

Honestly, I Googled it a while back when I was trying to determine what gears I needed to run 31's or 32's.  I couldn't tell you where they derived these numbers, but I cross referenced it with others that I found and they all were the same or very close.  This particular one came from Super Lift's website IIRC.

[onlyinajeep726]

Here, I found the link to the page that contained the chart I posted...

 

http://www.superlift.com/library/gears.asp

 

It seems to go more in depth as to how they derived the numbers seen in the chart.

[hitman72]

so onlyinajeep726, if i read the table right you want to be in the yellow correct?

[Biotex]

That table shows you what your rpm will be with any given tire size at what gear ratio you have.  It is set up for a 1:1 final drive ratio, meaning it isn't going to tell you anything about what your asking. Perhaps I'm mis-understanding your original question.

 

Every engine is different as far as what rpm is the most efficient.  It depends on a lot of factors such as elevation, bore & stroke, internal friction, torque, wind load, etc... so the best you can do is to generalize.  Over the years, I have come to learn that there is a fairly wide operating band on these engines. In other words, the point where the engine is lugging vs. the rpm where you are wasting gas to the max.

 

In my experience, the best economy is in the 1900 rpm range and I try to shift in the 2200 rpm range. These rpm numbers appear to me to need to go up with lift and tire size.

[cruiser54]

Install a vacuum gauge in your dash. Drive so that you maintain the highest vacuum. Works for EVERY kind of vehicle, gear ratio, tire size etc.

[Biotex]

Very good advice!

[cruiser54]

Very good advice!

It's also a permanently mounted diagnostic tool. Cheap and easy to install. No more charts, discussion, guesswork. Just mount it in plain view so you can see it at a glance.

 

Here's the instructions:

 

http://www.ehow.com/how_8098066_drive-vacuum-gauge-save-gasoline.html

[gogmorgo]

Here, I found the link to the page that contained the chart I posted...

 

http://www.superlift.com/library/gears.asp

 

It seems to go more in depth as to how they derived the numbers seen in the chart.

Yeah, like biotex said, that chart only shows what rpm you'll be running at 65mph. Useful for doing gear ratios, but not really for figuring out you best mpg.

[cruiser54]

Vacuum gauge tells no lies.

[onlyinajeep726]

Yellow is best RPM's for fuel conservation, blue is for best pulling/towing power, and yellow is the mix of both.

[cruiser54]

But, does that chart take into account a lift, the weight of larger tires etc?

[onlyinajeep726]

I can't say for sure, but I doubt it.

[Biotex]

I'm going to look into a vacuum guage mounted in the A pillar. As cruiser said, it is a great diagnostic tool.

[hitman72]

Is there an a pillar plastic piece that is molded for a gauge like you see the diesels have, but for us?

[cruiser54]

Is there an a pillar plastic piece that is molded for a gauge like you see the diesels have, but for us?

That would be cool, wouldn't it?

 

This might help you choose. I'd rather have a vacuum gauge than a clock....

 

[HOrnbrod]

Adding a vacuum gauge is on my to-do list. I have the gauge but still need to purchase a dual pod A-pillar gauge shell.

 

[hitman72]

Adding a vacuum gauge is on my to-do list. I have the gauge but still need to purchase a dual pod A-pillar gauge shell.

 

where did this one come from and what does that gauge tell you?

[HOrnbrod]

Autometer Air/Fuel meter. better than a vacuum gauge for monitoring fuel consumption.

[hitman72]

Is it harder to install? just figure if i am going to put a gauge in i might as well get the better one, or i guess i can do both. haha

[Eagle]

 

2250 RPM, for any gear.

 

The most efficient engine speed is the torque peak.

Not true. There's an efficiency point that moves around depending on load and a few other variables. Usually it's significantly below peak torque. Best mpg I've ever had was cruising at around 1800 rpm. If I go much faster, my fuel economy makes a steep downward curve.

 

onlyinajeep726, where did you get that list and what exactly are the numbers? As in where do they come from / how were they determined?

For the engine, the torque peak RPM is absolutely the most efficient operating speed. Torque is mundanely defined as "horsepower per revolution." so the engine speed that produces the most power per revolution is the most efficient engine speed.

 

In a vehicle, of course, outside forces come into play. Most importantly, aerodynamic drag and parasitic losses due to friction increase exponentially with vehicle speed, so there is a point where the direct correlation breaks down. I'll get to that in a minute.

 

I have experimented with this mostly with the '88 Cherokee, which is a 4.0L 5-speed. I live in Connecticut, where most of the highways still have 55 MPH speed limits, and a couple of stretches of interstate allow 65. I only use 5th gear on the highway -- 55 MPH in 5th gear is only 1540 RPM, and 65 MPH is only 1820 RPM. Below 55 MPH, 5th gear is totally useless with the 3.07 gears. I have on several occasions driven the same route in 5th gear and then again using only 4th gear, and every time the gas mileage is equal or better using only 4th gear. Using 4th as the cruising gear, 55 MPH is at 2055 RPM and 65 MPH is at 2430 RPM. So in 4th gear, 60 MPH is sitting right on the torque peak, and delivers the best gas mileage.

 

I had another proof of this a number of years earlier. When I was in the Army, I drove a Rambler American, with an earlier version of the engine that became the 4.0L Jeep engine. That car was a 3-speed manual with 3.07 axle gears. The gearing worked out such that 2500 RPM was exactly 60 MPH. Running at that speed, I got 28 MPG -- the dealer asked me to never mention that in front of other customers, because they'd all complain that their Rambler wasn't doing as well.

 

A 6-cylinder Rambler American wasn't exactly a race car, but I wanted to optimize the performance. I pt on a straight-thru glass-pack muffler, tweaked the ignition timing (we could set it, in those days), and made up a home-brewed open air cleaner to replace the fully-enclosed can that was standard from the factory. I took it out on I-95 near Edgewood Arsenal, MD, late at night to test my tweaks. I finally broke 100 MPH -- got it up to (IIRC) an honest 104 MPH, after correcting for speedometer error. Then came Winter. I needed snow tires, and I couldn't afford them. My brother had switched from bias-ply tires to Michelin radials, so he gave me his snow tires. They were from a larger, heavier car and were probably four sizes larger than my tires. But they fit inside the wheelwells, so I used them.

 

One night I took the car back out on I-95 with the snow tires on to check the top speed. It dropped, from 104 to 80. Why? Because the larger tires reduced the effective final drive ratio enough that the drag curve crossed the power curve, and above 80 MPH (which should have been 3333 RPM but dropped well below that with the bigger tires) there just wasn't enough horsepower available to overcome the aerodynamic drag and parasitic losses above that speed.

[Biotex]

So where can I get one of those A pillar guage mounts? Looks good.