There are a lot of discussions on the ideal RPM range for the mileage junkies... . Anywhere from 1300+ to 2100+ is said to be the best. Well, with our low winding engines that is a big gap. Can anybody squash the subject for me so I know where to run my engine at when I want to check mileage.
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2nd Gen Non-Engine/Transmission Ideal RPM's.....Is it true?
- Thread starter RussW1911
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This debate has been hashed around quite a bit lately. I had posted a while back about the US gear OD units which "lower" rpm by changing the final drive ratio and thus... . better mileage. Guess what. MOST of the members that responded did NOT report an increase in mileage or only a slight increase.
That being said, I try to stay between 1600 and 2200, especially while towing. I've found mileage drops rapidly above 2200, and is best around 2000. But having 4:10's, 2000rpm doesn't get you but about 60mph. So... ... ..... I spend alot of time around 2400.
One noteworthy comment that has been made before, is that our trucks are as aerodynamic as a brick. SLOWING down at any RPM seems to increase mileage the most.
Kev
That being said, I try to stay between 1600 and 2200, especially while towing. I've found mileage drops rapidly above 2200, and is best around 2000. But having 4:10's, 2000rpm doesn't get you but about 60mph. So... ... ..... I spend alot of time around 2400.
One noteworthy comment that has been made before, is that our trucks are as aerodynamic as a brick. SLOWING down at any RPM seems to increase mileage the most.
Kev
Wiredawg
TDR MEMBER
2100 is sweet spot
2100 was the sweet spot on my 00 2500 QC LB 4X4 w/ 3. 54s and is on my 02 3500 QC 4X4 w/ 4. 10s. The 2500 weighed 8860 lbs and the 3500 weighs 10,100 lbs and the 2500 got over 17 MPG and the 3500 gets 16 mpg.
The dyno also seemed to point to 2100 rpms being the ideal RPM.
Wiredawg
2100 was the sweet spot on my 00 2500 QC LB 4X4 w/ 3. 54s and is on my 02 3500 QC 4X4 w/ 4. 10s. The 2500 weighed 8860 lbs and the 3500 weighs 10,100 lbs and the 2500 got over 17 MPG and the 3500 gets 16 mpg.
The dyno also seemed to point to 2100 rpms being the ideal RPM.
Wiredawg
Juz have to give her a try....
I appreciate the info... . I will have to give it a try. My usual speed is never over 62 to 65 because that was what I was told was best... . I will try 70 at 2000 RPM's. When I change my tires I will see about running around 80 with the taller tires. Just goes against what I thought. . slower is better. I am young I can relearn!
I appreciate the info... . I will have to give it a try. My usual speed is never over 62 to 65 because that was what I was told was best... . I will try 70 at 2000 RPM's. When I change my tires I will see about running around 80 with the taller tires. Just goes against what I thought. . slower is better. I am young I can relearn!
Remember ... the higher the boost poundage the lower the mileage..... It takes Fuel to spool. Some of the modded rigs out there can have a tough time with it but on a otherwise stock truck you can drive downtown with lil' or no boost at all. Keep your foot out of it and let the truck catch up to the speed ... . not visaversa.
I'd say vehicle speed is the deciding factor in obtaining the best fuel milage. At speeds above 50 mph, aerodynamics (or lack thereof) really come into play. The infamous "sweet spot" on our engines may apply to loaded conditions at lower speeds, but empty, I find the best fuel milage to be around 20mpg keeping her at 55mph. This is not always easy (or safe on the L. I. E. ).
I think for the transmission you need at least 1600 rpm unloaded and 1800 rpm loaded.
Prior to doing the upgrades I played some with the overhead computer and determined one of the inputs is the TPS (throttle position sensor). If I accelerated briskly then let off slowly to the same rpm on a level road I could change the mpg from accelerating slowly up to the desired rpm. Same rpm but the TPS was in a different position apparently.
With cruise on (ie no other inputs), level, I can turn the Comp off and on and get different numbers by 1 mpg time after time. With the defueling supposedly in 1x1 I think I am seeing valid results.
I totally agree that wind resistance is probably THE biggest variable. Does anyone have any real testing data for tail gate up / down? Supposedly that makes some sort of difference.
I have long stretches of flat florida on the way to work and set 1600 rpm to keep the transmission happy and don't worry about the speed.
Bob Weis
Prior to doing the upgrades I played some with the overhead computer and determined one of the inputs is the TPS (throttle position sensor). If I accelerated briskly then let off slowly to the same rpm on a level road I could change the mpg from accelerating slowly up to the desired rpm. Same rpm but the TPS was in a different position apparently.
With cruise on (ie no other inputs), level, I can turn the Comp off and on and get different numbers by 1 mpg time after time. With the defueling supposedly in 1x1 I think I am seeing valid results.
I totally agree that wind resistance is probably THE biggest variable. Does anyone have any real testing data for tail gate up / down? Supposedly that makes some sort of difference.
I have long stretches of flat florida on the way to work and set 1600 rpm to keep the transmission happy and don't worry about the speed.
Bob Weis
Yes ... In fact TDR issue 3X? Had a write up on this very subject. There is data that proves that the Tailgate in the up position provides the best aerodynamics. When the gate is up it creates a low pressure voticy that helps airflow over the cab tward the rear of the truck. when th egate is down there is positive pressure created against the rear of the cab.
WMonroe
TDR MEMBER
I know that 2600-2700 rpm's is not the best for mileage 
. Over the weekend I did about 125 highway miles, 100 mountain road miles, and 50 city miles and only averaged 16. 0 mpg. Although staying around 2400 rpms I jump up to about 18-19 mpg and if I take it real easy I have seen 20, but its too hard to go that easy. Love to hear the turbo scream 
.
Will
. Over the weekend I did about 125 highway miles, 100 mountain road miles, and 50 city miles and only averaged 16. 0 mpg. Although staying around 2400 rpms I jump up to about 18-19 mpg and if I take it real easy I have seen 20, but its too hard to go that easy. Love to hear the turbo scream . Will
Mileage Sweet Spot?
At one of the May Madness events a couple years ago, someone gave a talk on this subject. If I remember correctly, you will achieve the best fuel economy if you keep your boost and EGTs as low as possible, for the speed you are traveling.
Seems to make sense, since boost and EGTs are directly related to the amount of fuel being used. I've tried it and it seems to be true, however, it takes some "paying attention" to the gauges.
Good Luck,
Frank Dz
At one of the May Madness events a couple years ago, someone gave a talk on this subject. If I remember correctly, you will achieve the best fuel economy if you keep your boost and EGTs as low as possible, for the speed you are traveling.
Seems to make sense, since boost and EGTs are directly related to the amount of fuel being used. I've tried it and it seems to be true, however, it takes some "paying attention" to the gauges.
Good Luck,
Frank Dz
Yeah that topic got beat all around and got into BSFC and all kinds of stuff. They should never have posted those charts because people took one look at them and Thought hey i can look at this number and it will tell me best RPM for Mileage. WRONG! I'm with the lowest rpm without lugging and also the slower is better for mileage camp. I'll bet almost any truck will see a gain of 5MPG just by slowing from 70 to 50. I bet that truck that was running 80 would gain 7-8MPG by slowing to 50.
My 2001 Auto with 3. 54's @2100rpm on our last trip got 21. 2
mpg. It was all highway miles with the air on. I wasn't towing,
just a load of lumber and tools going over to our property on the western slope in Colorado. But that is climbing over two passes
also. Eveything is stock except a PNP and a K&N filter.
It is a lot better than my buddies Powersmoke!!
Chuck
mpg. It was all highway miles with the air on. I wasn't towing,
just a load of lumber and tools going over to our property on the western slope in Colorado. But that is climbing over two passes
also. Eveything is stock except a PNP and a K&N filter.
It is a lot better than my buddies Powersmoke!!
Chuck
Lately, I have had an inquiering mind....
I find myself in a lot of different configurations (speed/grade/load/etc) and try different gears to see if moving up of down in gearing, while maintaining the same speed, affects the EGT's much. There is a large difference in EGT and I have found a number of times that selecting a lower gear reduces EGT's by as much as 250deg. That has got to relate to better mileage somehow, IMO.
I find myself in a lot of different configurations (speed/grade/load/etc) and try different gears to see if moving up of down in gearing, while maintaining the same speed, affects the EGT's much. There is a large difference in EGT and I have found a number of times that selecting a lower gear reduces EGT's by as much as 250deg. That has got to relate to better mileage somehow, IMO.
Some college guys did a test on a Dodge pickup a few years ago related to tailgates and covers. Here's what they found (complete with photos of the air flow):
http://mars.wnec.edu/~ehaffner/did.htm
Their results run counter to what others claim, they found a decrease in drag with the tailgate down but an increase with it off.
http://mars.wnec.edu/~ehaffner/did.htm
Their results run counter to what others claim, they found a decrease in drag with the tailgate down but an increase with it off.
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Note: I got a bit carried away, and the length ran up on me. If you don't want the technical details, read the summary at the end, but the justification's in the center.
--Snip--
Being a newly-graduated aerospace engineer myself, I can vouch for the theory behind their experiments. The best configuration for reducing Cd would be an open tailgate with a cap that started at the roofline, and sloped down to meet the tailgate at the very end. The more flat vertical area at the rear of the vehicle, the more drag (wind resistance)
In practice, real-world results tend to differ a bit from experimental modeling, and using scale models always introduces some error. Their figures for drag coefficient, for example, are with the mirrors removed. I'd be interested to see a full-size truck placed in one of the big wind tunnels in Tullahoma to see how close their figures are.
But I digress. At any rate, I can say with certainty that reducing your speed is the best way to gain mileage. To a point. I'm an airplane guy, so the calculations are a bit different, but if you make a plot of drag versus velocity it will increase exponentially with increasing speed. The very well-known formula for equating drag and velocity (I'll spare you the specifics) indicates that drag increases with the square of velocity. Basically, if you go twice as fast, you get four times the drag, three times as fast, nine times, four times as fast, sixteen times.
There are more considerations for drag produced as a consequence of lift that are more applicable to airplanes than autos, but they do factor in. This message is already overlong, though, and I'll not bore you with more.
Basically, the amount of drag is what determines how much horsepower the engine needs to produce to sustain a given speed, and the BHP determines the fuel consumption (that's where the BSFC comes in to play).
Given the charts for BSFC vs. RPM, we can determine the RPMs where the engine runs most efficiently. BSFC is in units of lbm/hp*hr. That's how many pounds of diesel per hour are required for each horsepower produced--that's the sticky part. Yeah, you might have the best engine efficiency at a certain RPM, but if you get a 10% decrease in BSFC from some other RPM, but the increase in speed requires 50% more power, you're actually less efficient--fuel flow rate will increase. If we also had a chart for drag force vs. speed, we could relate the two, and a direct correlation of vehicle speed to fuel consumption.
Personally, I'd love to throw something like this together (yeah, some geeks drive big trucks, too ), but I don't think Chrysler would give me their aerodynamic data. And booking an hour in a vehicle-size incompressible flow wind tunnel makes a week or ten on the dyno look cheap.
-------
Okay, this is way long now. In summary:
BSFC relates how much fuel is burned *per horsepower* at a given RPM.
Drag increases with the *square* of velocity.
Without knowing the drag/velocity performance of the vehicle, it is impossible to get any real benefit from the BSFC figures.
In general, going slower will *almost always* give you better mileage.
Okay. I'm done. If anyone actually read this far, and wants clarification on any of this, let me know. And I should offer a disclaimer:
No, I don't drive slow--it takes too long, so I get 15-16 mpg. And just because I've got an engineering degree doesn't necessarily mean I know what I'm doing. :-laf . I've got the theory down, but the thought of doing something like changing my injectors still scares the hell out of me. You guys are helping with that, though--thanks.
--Ty
--Snip--
Being a newly-graduated aerospace engineer myself, I can vouch for the theory behind their experiments. The best configuration for reducing Cd would be an open tailgate with a cap that started at the roofline, and sloped down to meet the tailgate at the very end. The more flat vertical area at the rear of the vehicle, the more drag (wind resistance)
In practice, real-world results tend to differ a bit from experimental modeling, and using scale models always introduces some error. Their figures for drag coefficient, for example, are with the mirrors removed. I'd be interested to see a full-size truck placed in one of the big wind tunnels in Tullahoma to see how close their figures are.
But I digress. At any rate, I can say with certainty that reducing your speed is the best way to gain mileage. To a point. I'm an airplane guy, so the calculations are a bit different, but if you make a plot of drag versus velocity it will increase exponentially with increasing speed. The very well-known formula for equating drag and velocity (I'll spare you the specifics) indicates that drag increases with the square of velocity. Basically, if you go twice as fast, you get four times the drag, three times as fast, nine times, four times as fast, sixteen times.
There are more considerations for drag produced as a consequence of lift that are more applicable to airplanes than autos, but they do factor in. This message is already overlong, though, and I'll not bore you with more.
Basically, the amount of drag is what determines how much horsepower the engine needs to produce to sustain a given speed, and the BHP determines the fuel consumption (that's where the BSFC comes in to play).
Given the charts for BSFC vs. RPM, we can determine the RPMs where the engine runs most efficiently. BSFC is in units of lbm/hp*hr. That's how many pounds of diesel per hour are required for each horsepower produced--that's the sticky part. Yeah, you might have the best engine efficiency at a certain RPM, but if you get a 10% decrease in BSFC from some other RPM, but the increase in speed requires 50% more power, you're actually less efficient--fuel flow rate will increase. If we also had a chart for drag force vs. speed, we could relate the two, and a direct correlation of vehicle speed to fuel consumption.
Personally, I'd love to throw something like this together (yeah, some geeks drive big trucks, too
), but I don't think Chrysler would give me their aerodynamic data. And booking an hour in a vehicle-size incompressible flow wind tunnel makes a week or ten on the dyno look cheap. -------
Okay, this is way long now. In summary:
BSFC relates how much fuel is burned *per horsepower* at a given RPM.
Drag increases with the *square* of velocity.
Without knowing the drag/velocity performance of the vehicle, it is impossible to get any real benefit from the BSFC figures.
In general, going slower will *almost always* give you better mileage.
Okay. I'm done. If anyone actually read this far, and wants clarification on any of this, let me know. And I should offer a disclaimer:
No, I don't drive slow--it takes too long, so I get 15-16 mpg. And just because I've got an engineering degree doesn't necessarily mean I know what I'm doing. :-laf . I've got the theory down, but the thought of doing something like changing my injectors still scares the hell out of me. You guys are helping with that, though--thanks.
--Ty
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I've found that with 265-75x16's, a 3. 55 axle, 2wd, with the tach reading 2000, my speedo shows 71 - 72, and on long highway runs I get a shade more than 20mpg. I've very happy with that since my previous pickup, an old '85 Ford with the 300 cid I6 with the overdrive manual 4-speed, would consistently give me 13 mpg.
Wayne
Wayne
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Issue 128 – Digital Version
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FREE!
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Renew
Subscribe
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Back Issues
Subscription Status
Address Change Form
Buyer's Guides
Ram Diagnostic Trouble Codes
The Perfect Collection
The Perfect Collection Vol. II
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What Makes Us Tick?
Product Showcase
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