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trying to figure out what rpm is best for mpg on my 04 gt all stock except for a tune and cai and exhaust same 3.27 gears with 245/45/17
 

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He of Long Wind
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Mathematically, the best fuel mileage occurs at 1,313 rpm for a V8, the point at which horsepower and and torque curves converge divided by the number of cylinders in each bank (5252/4). For a V6 it's 1,751 rpm.
 

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Poo tee weet?
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Mathematically, the best fuel mileage occurs at 1,313 rpm for a V8, the point at which horsepower and and torque curves converge divided by the number of cylinders in each bank (5252/4). For a V6 it's 1,751 rpm.
That seems overly simplistic and complicated at the same time.
 

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US Air Force (retired)
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Mathematically, the best fuel mileage occurs at 1,313 rpm for a V8, the point at which horsepower and and torque curves converge divided by the number of cylinders in each bank (5252/4). For a V6 it's 1,751 rpm.
Reference? I like the the ideal but I'd like more information because on the surface this simple formula will not work for all motors. You are correct in that horsepower and torque always cross at 5252. That is because we measure torque and calculate horsepower. The formula is HP = TQ x RPM/5252. It doesn't matter if the motor is 4 cylinder or 10 cylinder torque and horsepower will cross at 5252 rpm. It doesn't matter if it has 200 hp or 1000 hp they will still cross at 5252. But, if the motor was a V10 then the ideal rpm would be 1050. And if the motor was an inline 6 the ideal rpm would be 875. Basically idle. I don't think so.
 

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King Trashmouth
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Mathematically, the best fuel mileage occurs at 1,313 rpm for a V8, the point at which horsepower and and torque curves converge divided by the number of cylinders in each bank (5252/4). For a V6 it's 1,751 rpm.
I don't think that's true at all.

Look at some of the diesel 6 cylinders. They're all I6 engines. Your ideal cruise speed on the tiny ISB6.7 is way different than it is on the mammoth ISX 15. Both are I6s, and have entirely different operating ranges.

To truley tell the most efficient point you would need a fuel map, and your ideal cruise point would be a function of torque and engine speed.

A general rule of thumb in the automotive world is that the most efficient point on the map is usually around torque peak. Then it becomes a series of isobars that radiate out from there. That's the rule of thumb, and some engines have very crazy patterns which would need to be adjusted for.
 

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US Air Force (retired)
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Peak torque might be the most efficient point for the motor but I don't see that as being most efficient for a car. Peak torque is around 4500 rpms. I lost 2 mpg when I put in 3.73 gears which moved my interstate rpm from around 2150 to almost 2500.

OP, lugging your motor down is running at too low an rpm. This is almost impossible with an automatic unless you are pulling a heavy load uphill. But with a manual transmission you might experience the car lunging up and down as the motor tries to keep running. New drivers do this all the time. You also are lugging the motor down if the car cannot maintain its current speed. This is often seen going uphill in the mountains. I have even had to downshift an automatic under those conditions.
 

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OK fine. It's not true. I just made that up ... but I had a couple people going. :)
Lol had me believing you. But I don't know any better. I don't know much on how motors work. Just add fuel right? :dunce:


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He of Long Wind
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Lol had me believing you. But I don't know any better. I don't know much on how motors work. Just add fuel right? :dunce:


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LOL! No hard feelings?

Truthfully, predicting an optimal rpm for fuel mileage can surely be accomplished in theory, but only with a known set of conditions. Assuming we knew precise vehicle mass, road conditions, angle of incline/decline, frictional data from tires, wind resistance, drivetrain frictional losses, etc. ... it might be possible to predict how much horsepower is needed to maintain any particular cruise speed - and then extrapolate the amount of fuel needed to sustain that horsepower level, but this would be more a test suited to laboratory conditions and teams of number-crunching nerds.

As a practical matter, a fuel flow monitor and a good speedometer would be all you need to figure out where best mpg occurs. Some factory trip computers have this function built in. If you know fuel flow and vehicle speed, you can easily compute mpg for any given moment - any rpm - any gear - any set of conditions. But ... with all that said, there have been a number of studies on this topic already. See Fuel economy in automobiles - Wikipedia, the free encyclopedia While air resistance is the dominant limiting factor, it is generally believed that best vehicle efficiency tends to occur in the 25-55 mph range.
 

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Priest of the Car Gods
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Peak torque might be the most efficient point for the motor but I don't see that as being most efficient for a car.
Basically this. Peak torque is when the motor is the most efficiently using the fuel it's consuming, but due to the rpm it's still consuming more fuel per minute than it would be a lower rpm. If you knew or could calculate fuel consumed per revolution at a specific rpm (say, by knowing the percentage efficiency compared to peak at each rpm... say, by looking at the torque produced at that rpm vs peak), you could chart that out multiplied by rpm and find a minimum consumption per minute.

I did have a car (AE86 SR5) where that was the case; peak torque was right at 3k, and cruising right at 3k most definitely netted a signficant increase in fuel economy over even just a few hundred rpm higher or lower. Of course, 3k was only 60mph, which wasn't very helpful for a 20 mile commute on a 65mph highway.
 

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King Trashmouth
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Basically this. Peak torque is when the motor is the most efficiently using the fuel it's consuming, but due to the rpm it's still consuming more fuel per minute than it would be a lower rpm. If you knew or could calculate fuel consumed per revolution at a specific rpm (say, by knowing the percentage efficiency compared to peak at each rpm... say, by looking at the torque produced at that rpm vs peak), you could chart that out multiplied by rpm and find a minimum consumption per minute.

I did have a car (AE86 SR5) where that was the case; peak torque was right at 3k, and cruising right at 3k most definitely netted a signficant increase in fuel economy over even just a few hundred rpm higher or lower. Of course, 3k was only 60mph, which wasn't very helpful for a 20 mile commute on a 65mph highway.
Peak torque on my stang is around 2500, and peak torque is flat from about 1500 - 2200 RPM on my truck. :tomato

That said, I didn't say "cruise at peak torque speed all the time." I said that operating at peak torque, which is to say on the torque curve at that point, not less torque at that speed. It is very much a function of load (torque) and speed.

With each gear and speed change you change both load and engine speed. You have to optimize your load (through gear selection) as well as speed. Generally as you increase load (higher gear, higher torque) you move up the map, across the isobars, toward peak torque, with better efficiency. Spin your engine up higher, you're getting a lower load, which pushes you down to the bottom of the map where fuel economy sucks.

Or think of it this way. Upshift, and your point on the map goes to lower speed, higher load, and generally into a more efficient region. Downshift and you move to higher speed, lower load, across the isobars into a less efficient region.

Anyone getting my drift? This would be abundantly clear but I'd need to draw a diagram, write a small essay, and to be honest I think that's way beyond the scope of the question.
 

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how does it matter where your peak tq is? are you guys cruising at wot??

personally found with stock gears 2k is a good spot. enough rpm to get up the hill and not be pushing too much air. otherwise a hill takes more throttle or a downshift, and more speed begins to really push the mass of air around the car.. just my experience.. my 4.30s are a whole other ball game.. nothing works haha
 

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I was a little concerned with load and peak torque when I bought my last truck. I have pulled a two horse trailer across country many times with an old Ford 4.9L I6. If I had a strong head I'd sometimes have to go to third to maintain speed. (Use of overdrive is limited.) I researched the Ford 4.2L V6 before buying my current truck. It's peak torque comes at a higher rpm than the old I6 and was above normal driving rpm. I wondered how it would do. I bought the truck and it drives about the same but I'm now pulling an aluminum trailer instead of steel so that helped. Down shifting is given. I've crawled up many a hill in second gear with the truck and horsetrailer.

If you are interested in gas mileage than pedal movement is probably more important than a few rpm. I've read that in gas mileage competitions they ease away from stops slowly accelerating. They let the car slow down going up hill and let it gain speed going down hill. They start stopping early coasting and using as little brake as possible. That doesn't sound like very much fun and its not worth it to me for a couple of miles per gallon.
 

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Priest of the Car Gods
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how does it matter where your peak tq is?
Because peak torque is the rpm that your engine most efficient; here efficiency is defined as consuming the least fuel relative to the amount of force it's putting out.

However, cars are not that simple, and they use a slightly different definition of efficiency. Between gearing, aerodynamics, and safe driving (inc. speed limits), minimizing consumption of fuel per distance driven is more complex than just running your engine at it's maximum efficiency.
 

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I think what he means is the difference between wot peak torque vs actual cruising speed partial throttle tq numbers.

Catch my drift?
 

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My peak torque comes at around 5500 rpms (not a typo)!

I don't think my gas mileage would be that great running at 5500 rpms all the time!
 

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US Air Force (retired)
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It's not that complicated. Lugging down a motor simply means you are running it at a lower rpm than required for the load you are carrying. Haven't you ever seen the car hop that sometimes occurs with new drivers or lost speed in the mountains?
 
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