U.S. CitiesCity-Data Forum Index
Go Back   City-Data Forum > General Forums > Automotive
 [Register]
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
View detailed profile (Advanced) or search
site with Google Custom Search

Search Forums  (Advanced)
Reply Start New Thread
 
Old 11-15-2014, 08:03 AM
 
Location: Central Texas
13,720 posts, read 24,632,366 times
Reputation: 9178

Advertisements

Quote:
Originally Posted by BostonMike7 View Post
Depends on many factors. Engine load, gearing, weight of the car, etc etc. RPM alone doesn't affect MPG. Chugging a car at 1.5K RPM under load could be worse for MPG than if the car was crusing at 3K RPM under little load.

I have an Infiniti G35, and at 80MPH it's turning 3200 RPM, but I still get 25MPG or so at this RPM. The Nissan 3.5L isn't exactly a fuel efficient V6 either. I'm lucky to get 20MPG mixed with it
RPM absolutely affects MPG. It isn't the only thing, but the more RPM, the more fuel is used. Every RPM causes ignition, which requires fuel.

If you had a manual shift car, your MPG at 60 MPH will be instantly worse if you drove in 4th gear instead of 5th.

Nissan could have chosen a taller gear for the car in question. But they didn't - probably because they want the car to be more responsive at the expense of MPG.
Reply With Quote Quick reply to this message

 
Old 11-15-2014, 09:27 AM
 
10,873 posts, read 41,184,197 times
Reputation: 14019
Quote:
Originally Posted by hoffdano View Post
RPM absolutely affects MPG. It isn't the only thing, but the more RPM, the more fuel is used. Every RPM causes ignition, which requires fuel.
Sorry, but this ... and similar lines of RPM = more fuel consumption ... is completely WRONG. You are assuming that there's a directly linear relationship between RPM/HP and fuel consumption. It's not so ...

What you're not taking into account is the fuel efficiency curve of a given engine; ie, the specific fuel consumption per horsepower hour.

All engines will have such a performance curve, and it's directly related to the fuel economy for the vehicle. The final on the road mpg result is a combination of optimization of the engine gearing to place the intended cruise speed in that best use of fuel by the engine.

This is why there is a fallacy that going faster always results in lower fuel economy mpg. Yes, aerodynamics come into play as the speed increases, but again ... it's a case of optimization of the fuel efficiency of the engine. Increased road speed requiring more HP can be met by an engine running more efficiently at the higher RPM to the extent that the net fuel consumed to deliver the HP is LESS.

For example, I get better fuel mileage in some of my BMW's and MB's at 68-70 mph than I do at 55 mph. In an extreme example, my old 1972 BMW 2002, box stock ... was geared to cruise efficiently at 72 mph, delivering 28 mpg. If I drove it at 55 mph, as we were required to do for a few years, the fuel economy dropped to 24-25 mpg. Why? because the 4th gear ratio put the engine RPM at a lower point where the camshaft/intake/exhaust breathing was not as efficient and volumetric efficiency was down so it required a higher throttle opening (ie, lower manifold vacuum reading) to be able to get down the road. I could accelerate in 4th gear from 55 mph to 70 mph and lift my foot pressure on the throttle as the power came in.

Similarly, I can cruise my current Subaru OBW's ... fuel injected modern engines ... at the same fuel MPG at 70-ish mph as I get at 55 mph. I've demonstrated this several times where I did a couple hundred miles on the same freeway using cruise control to maintain a steady cruise speed over the same trip and ambient conditions. Got 25 mpg either trip. Did the same thing with my 1993 Dodge 3500 Roadtrek RV wih a 5.2 Magnum engine; got 17 mpg at 55 mph, and 17 mpg at 65 (also tried this at 70 mph, and that's when the fuel economy dropped to 15.5 mpg ... aerodynamics of a box get fierce at that speed and the engine is running at a lesser efficient RPM).

The same mpg performance results can be seen with many other naturally aspirated gasoline fueled engines. My 750 cc and 850 cc motorcycles deliver 55-58 mpg whether cruised at 55 mph or 75 mph.

We've found the same results with our 1995 Ford Powerstroke diesel. I can cruise it at 55 mph and get no better fuel economy than cruising it at 62 mph. It's a very dramatic "sweet spot" at that 62 mph; driving at 65 mph will see a reduction in fuel economy. My wife can consistently get 22 mpg at 62 mph with the truck lightly loaded (but at volume capacity with stuff for her craft shows). She doesn't get any better fuel economy staying at 55 mph.

Again, it's all revealed in the specific fuel consumption per horsepower hour, usually expressed as pounds of fuel per HP. The curves can be dramatic, and you also need to look at the peak torque and HP figures and how they are related to the overall gearing of the vehicle. The engineers can optimize these relationships for targeted cruise speeds.

In latest engines, they try to level out the efficiency curves with variable valve timing, variable intake volumes, and similar external devices to enhance the performance curves. There are times where these tactics have yielded good results in delivering improved fuel economy.

BostonMike7, posting above, observes these latest fuel economy results in his G35 where the engine performance curves yield comparable fuel mpg at faster road speeds using higher RPM.

Last edited by sunsprit; 11-15-2014 at 10:02 AM..
Reply With Quote Quick reply to this message
 
Old 11-16-2014, 03:56 PM
 
2,754 posts, read 1,218,047 times
Reputation: 2098
Reading all your suggestions, I tried to implement them. Same road, same load but with some slight variation on the pedal I could cruise steadily at 60 around 2.9 RPM and could maintain 35 around 1.8 RPM. ALso observed the gear shift, it reached 30 in 2nd gear and could reach almost 60 in the 3rd.
1) Not sure if it really shifted to 4th today, because I could pass and accelerate pretty easily .
2) Previously when it did reach 4 , was it due to pushing it harder with the 3rd gear itself?
Reply With Quote Quick reply to this message
 
Old 11-20-2014, 01:45 PM
 
8,735 posts, read 8,940,444 times
Reputation: 12208
The trans should always upshift to 4th gear. Even if you only maintained 30MPH, the trans would shift 1-4 gear and stay in 4th. Now if you encounter a hill and apply a little gas, the trans should downshift to an appropriate gear.

Also, the MPH will vary, so you really can't say at what MPH the trans shifts to which gear unless you had the pedal to the floor. Then the trans will shift at engine redline. But at less than full throttle, the shift point will depend on a variety of factors.

FInd a road where you can stop and accelerate to 65MPH safely. From a stop, count the number of times the transmission shifts gears. You should count 3 shift points getting you into 4th gear. If you cruise at 65, you should be in 4th gear (overdrive), but if you wanted to pass a car, you would apply a little throttle and the trans should downshift to 3rd gear.
Reply With Quote Quick reply to this message
 
Old 11-20-2014, 08:59 PM
 
Location: Not far from Fairbanks, AK
16,176 posts, read 27,454,946 times
Reputation: 11838
Quote:
Originally Posted by BostonMike7 View Post
The I35 shifter is gated and has a slot for "3D"



So the OP is in the right gear. I do not know if there is an overdrive off button though.
It means that it has a four speed transmission. The same on the 2012 Toyota Corolla LE, in that it has the following: 1, 2, 3D. With the shifter set on the left side of the notch (away from the passenger seat, and toward the driver's seat), the transmission is on third gear. But if the shifter is moved to the right end of the notch (away from the driver's seat, and toward the passenger), then the transmission is on 4th gear or overdrive. Somewhere in the gage cluster the display will show 1, 2, 3, 3, and D depending on where the shifter is positioned. I just don't know if the Infiniti gets into the 4th gear the same way as the Corolla. All I know is that around 60MPH the RPM should be around 2,100 if the transmission is on 4th.

Last edited by RayinAK; 11-20-2014 at 09:25 PM..
Reply With Quote Quick reply to this message
 
Old 11-20-2014, 09:35 PM
 
Location: Earth
797 posts, read 376,169 times
Reputation: 775
Yes it matters. That's what sucks about autos.
Reply With Quote Quick reply to this message
 
Old 11-21-2014, 02:54 PM
 
2,754 posts, read 1,218,047 times
Reputation: 2098
Quote:
Originally Posted by RayinAK View Post
But if the shifter is moved to the right end of the notch (away from the driver's seat, and toward the passenger), then the transmission is on 4th gear or overdrive. Somewhere in the gage cluster the display will show 1, 2, 3, 3, and D depending on where the shifter is positioned. .
Thanks, got it. 3 and D can be interchanged. I always drove on 3 till today. Will experiment with D .
Reply With Quote Quick reply to this message
 
Old 11-21-2014, 06:42 PM
 
Location: Not far from Fairbanks, AK
16,176 posts, read 27,454,946 times
Reputation: 11838
Quote:
Originally Posted by shanv3 View Post
Thanks, got it. 3 and D can be interchanged. I always drove on 3 till today. Will experiment with D .
You use 3 when you need more power for going uphill, for example. But when you do so, the RPM is going to be higher. For cruising or just driving on a flat rood or highway, put it on D. As I mentioned before, on D your car motor's RPM will be around 2,100 at 60 MPH. Using D on any flat or level road, regardless of speed, will save you some gasoline since the RPM will be much lower.

If you are driving uphill on D, the transmission will automatically shift down to 3rd, and the RPM will jump to 3,000+, then switches to 4th when it does not need the extra power at the top of the hill. if you search on the Internet you will find PDF owner's manuals for your car. I saw one for the 2004 model.
Reply With Quote Quick reply to this message
 
Old 11-21-2014, 09:22 PM
 
Location: Minneapolis
1,617 posts, read 4,531,059 times
Reputation: 1203
Quote:
Originally Posted by hoffdano View Post
RPM absolutely affects MPG. It isn't the only thing, but the more RPM, the more fuel is used. Every RPM causes ignition, which requires fuel.

If you had a manual shift car, your MPG at 60 MPH will be instantly worse if you drove in 4th gear instead of 5th.

Nissan could have chosen a taller gear for the car in question. But they didn't - probably because they want the car to be more responsive at the expense of MPG.
Gas was still $1.50 per gallon when that car's development was finalized. Nissan didn't give a damn about fuel economy back then.
Reply With Quote Quick reply to this message
 
Old 11-21-2014, 09:48 PM
 
2,754 posts, read 1,218,047 times
Reputation: 2098
Quote:
Originally Posted by RayinAK View Post
You use 3 when you need more power for going uphill, for example. But when you do so, the RPM is going to be higher. For cruising or just driving on a flat rood or highway, put it on D. As I mentioned before, on D your car motor's RPM will be around 2,100 at 60 MPH. Using D on any flat or level road, regardless of speed, will save you some gasoline since the RPM will be much lower.

If you are driving uphill on D, the transmission will automatically shift down to 3rd, and the RPM will jump to 3,000+, then switches to 4th when it does not need the extra power at the top of the hill. if you search on the Internet you will find PDF owner's manuals for your car. I saw one for the 2004 model.
Yes, already got some fuel savings by trying to maintain lower RPM . This should help even further.

I do have the hard copy of the manual from the owner .
Reply With Quote Quick reply to this message
Please register to post and access all features of our very popular forum. It is free and quick. Over $68,000 in prizes has already been given out to active posters on our forum. Additional giveaways are planned.

Detailed information about all U.S. cities, counties, and zip codes on our site: City-data.com.


Reply
Please update this thread with any new information or opinions. This open thread is still read by thousands of people, so we encourage all additional points of view.

Quick Reply
Message:

Over $104,000 in prizes was already given out to active posters on our forum and additional giveaways are planned!

Go Back   City-Data Forum > General Forums > Automotive
Similar Threads
Follow City-Data.com founder on our Forum or

All times are GMT -6.

2005-2018, Advameg, Inc.

City-Data.com - Archive 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35 - Top