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Old 11-22-2014, 01:40 AM
 
Location: H-town, TX.
3,400 posts, read 5,484,563 times
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Quote:
Originally Posted by sunsprit View Post
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.

This is true to a good extent. I just made a round tripper to Gainesville for my final exam in my '08 SuperCrew and got 18.5 MPGs, more or less, and I was doing ~75-80 the whole time when allowable.

I did the same trip to just to Tallahassee in the v6 F150 back in 2010 and got about 19.2 doing the same speed.

You wanna know something? Doing 55 really won't net me any better MPGs--maybe on an EPA dyno, but I don't drive on those too often. ~1500 RPMs in either truck at that mileage is even farther out of the powerband (the 4.6 peaks at 4000 RPMS, the 4.2L v6 peaks at 3400 RPMs) than ~2200 RPMS is at ~80. Them 3.55 gears, FTW. If I was doing 55 and had to climb those hills and wait for a two-gear shift just to keep speed, well, that would just make the point even clearer. I did tow a trailer with all my cousin's stuff in it to Orlando and back last fall (w/out trailer going home) and lost an MPG doing it, but a 5x8 U-Haul trailer isn't very aerodynamic.

I guess it would be something if either engine was struggling at that higher speed, but they aren't.

OP, are you using premium juice in your ride? I am pretty sure that's what the 3.5L calls for in the cars for best efficiency.
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Old 11-23-2014, 09:57 AM
 
2,776 posts, read 1,231,468 times
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Quote:
Originally Posted by AlfredB1979 View Post

OP, are you using premium juice in your ride? I am pretty sure that's what the 3.5L calls for in the cars for best efficiency.

No I use regular unleaded and 87 always. The manual says unleaded and a higher octane number though.
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Old 11-24-2014, 08:53 AM
 
Location: Massachusetts
8,770 posts, read 9,007,245 times
Reputation: 12238
Quote:
Originally Posted by shanv3 View Post
No I use regular unleaded and 87 always. The manual says unleaded and a higher octane number though.
Most gas sold in this country is unleaded that it's pretty much assumed that any gas you put in your car is unleaded. Really tough to find leaded gas and it's usually for special applications and not available at the standard fuel pump.

But you should really run the minimum octane required as indiciated by the owners manual. If it says to run 91+ octane, then that's what the engine was designed to run on to achieve the advertised horsepower and fuel economy numbers.

Running a lower octane causes the engine's knock sensor to adjust timing to "save" the engine. If the knock sensor ever fails, you could really damage the engine as detonation is not kind to engines.
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Old 11-24-2014, 09:20 AM
 
Location: East TX
2,085 posts, read 1,833,675 times
Reputation: 3175
Default Know your vehicle

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 .
Sorry if this will sound particularly harsh, but you have demonstrated for everyone the need to familiarize yourself with a vehicle when you make a change. Over the years I have watched an incredible number of people damage their vehicles or cost themselves money through improper use of their vehicles because they do not take the time to read the manual, ask questions, or learn how the vehicle operates correctly.

Take the time to read through the manual. It only takes an hour or so to familiarize yourself with the controls and basic maintenance requirements so you can protect your investment and potentially prevent some serious safety issues. If there is something you cannot find, understand, or seems unclear, most dealerships would be happy to have a service adviser or other staff member familiar with the vehicle help ensure you are comfortable with the features and controls to enjoy the vehicle safely.
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Old 11-24-2014, 11:01 AM
 
Location: NWA/SWMO
2,749 posts, read 2,625,560 times
Reputation: 2659
Quote:
Originally Posted by shanv3 View Post
Hi,
I have an Infiniti I35 2003 . It spits 240 hp and it effortlessly passes any vehicle. But when I drive in freeways, the engine RPM goes as high as 3.5 to 4. And even within city for 35 mph, the RPM is around 2-2.5.

I remember getting the same speeds with 1.5 rpm with corolla, elantra etc and they were only around 140 hp..

Does this impact the mpg, as I get only around 18 on an avg..
BSFC...brake specific fuel consumption.

This is determined by cam and valve lift and airflow characteristics, as well as combustion chamber shape, etc.

Each motor has a "sweet spot".

Each motor will be more efficient in that "sweet spot".

Compounding this is gearing, weight, and aerodynamics of what it is powering.

You have asked a very complex question that involves a lot of math. So why don't you just drive at 60,65,70, and 75, and re-set your EVIC/DIC read-out each time and see what the car is telling you is happening. Lacking this, calculate it manually during a long road trip.
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