DodgeTalk Forum banner

Status
Not open for further replies.
1 - 20 of 35 Posts

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #1 ·
Ok... Seeing how Fuel economy is such a hot issue, I decided to do a "scientific test" of my own to see just how well my truck could do. This is as accurate of a test as I could perform and is in no way skewed, or padded.. My truck is basically stock with a few bolt on Mods as listed below.

Test Subject: 2004 Ram SC/SWB
Mileage: 35,000
Tires: Stock 20"
Gears: 3.92 LSD

Mods:
K&N CAI
=AIR=RAM=> IAT Mod
Flowmaster 50 Series (Dumped)

Outside Conditions:
96 Degrees / 90% Humidity

Fuel: Shell 89 Octane

Test Factors / Guidelines:
The test was performed this weekend on my 1500 mile trip to North Carolina and back. The terrain was fairly consistent, with stretches of both flats and hills. No serious mountains or stretches of flat baron desert. Starting @ 50MPH, I drove 100 Miles at each MPH setting shown on the bar graph. I Used my Cruise Control to make the test as accurate as possible and did not disengage the Cruise during the test unless I was about to be run over, or was about to run over someone.

I calculated speed and distance using my Garmin Quest GPS. The RPM Readings were estimated based on the Tachometer and rounded up or down to the nearest logical unit of five. Since the RPM reading was not a critical factor in computing my MPG, I found this method sufficient so please don’t flame me if they seem a bit out of whack. You have NO idea how hard it is to monitor the Overhead computer, a GPS, the Dash Panel, record entries into your Palm Pilot, AND DRIVE...

The Results:
The results as shown here were not a huge surprise. However, My initial suspicions of better MPG between 60 an 70 were confirmed.. It seems my truck performs best @ 68 MPH. And yes, I know the numbers are higher @ 50-55 MPH... But seriously... I cannot drive THAT slow... 100 Miles of test and I was going insane. So reasonably, 65-70 is where I performed best.
 

Attachments

·
Registered
Joined
·
2,860 Posts
wow, nice test. I wish I could drive 50 mph and get 24.4 mpg, but I can't drive that slow either. I usually stay around 85 on the interstate just because time is money, and I like to get places quick. I would like to stay around 65-70 to conserve gas, but I just can't do it.
 

·
HEMI Power
Joined
·
993 Posts
Nice job on the testing Dane. Those are some interesting results. I don't have the overhead computer display in my truck to give me mileage per gallon, so your chart gives me a good idea of what a comparable truck to mine will get as far as gas mileage.

I'm guessing that mine will be a little (not much) better because of the fact that I have the 17's and the 3.55 gears (which I have been wanting to change) on my truck.
 

·
C HEMI GO
Joined
·
1,739 Posts
Now put some new gears in and drive back! : )
 

·
HEMI Power
Joined
·
993 Posts
RammingDodge said:
I usually stay around 85 on the interstate just because time is money...
In this case less time equals more money spent for gas. LOL
 

·
C HEMI GO
Joined
·
1,739 Posts
page329 said:
Nice job on the testing Dane. Those are some interesting results. I don't have the overhead computer display in my truck to give me mileage per gallon, so your chart gives me a good idea of what a comparable truck to mine will get as far as gas mileage.

I'm guessing that mine will be a little (not much) better because of the fact that I have the 17's and the 3.55 gears (which I have been wanting to change) on my truck.
I have 17's and 3.55 and mine is worse. I just drove to arkansas hill country and back and I got better gas mileage on moderate hills than flat highways because I was driving 50-55 and in 3rd or 4th gear. On the flat HW I was in 5th and dumping a 1/4 tank everytime I hit a good breeze.
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #7 ·
sgrizzle said:
Now put some new gears in and drive back! : )

Yea, This is partly the main reason I did this test... Im about to install gears and wanted a "stock" base to compare against.

:rck:
 

·
Registered
Joined
·
2,860 Posts
page329 said:
In this case less time equals more money spent for gas. LOL
lol, very true
 

·
C HEMI GO
Joined
·
1,739 Posts
Driving 3 hours at 50 (150miles) is 6.15 gallons. The same distance at 75 takes 2 hours and 8.24 gallons. If it costs $4 to cut an hour off my trip, that's a no-brainer.
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #10 ·
RammingDodge said:
wow, nice test. I wish I could drive 50 mph and get 24.4 mpg, but I can't drive that slow either. I usually stay around 85 on the interstate just because time is money, and I like to get places quick. I would like to stay around 65-70 to conserve gas, but I just can't do it.

Funny you should mention that.. The whole time I was driving 50-60, I was sitting there thinking... "Now I KNOW Im gonna see higher MPG numbers, but I bill out at $120 per hour..." so I was wondering how in the world I could derive a formula where I could plug in an "Hour amount" and find out what it actually actually costs me in Gas, Billable time, etc per mile that I drive based on Speed and calculating my MPG... ARRRRRRRRRRG!!!! Makes my brain hurt thinking about how to do it!!!!
 

·
Registered
Joined
·
1,733 Posts
That is a good effort - but I would guess that the 'hump' in your results from 62 mph to 72 mph is due to a shift in the wind.

Headwinds obviously hurt, and tailwinds help - but keep in mind that SIDEWINDS also hurt.
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #12 ·
HankL said:
That is a good effort - but I would guess that the 'hump' in your results from 62 mph to 72 mph is due to a shift in the wind.

Headwinds obviously hurt, and tailwinds help - but keep in mind that SIDEWINDS also hurt.
I dont think so Hank.. I was somewhat suprised to see that shift as well, so I tried it again on my trip home... Same results @ 65 and 70... plus or minus .2

I think it has to do with the relativity of speed and time (MPH) compared to the Engine RPM. I havent had time to do another test to verify that, but I will. I will have to look at my notes, but I figured out a way to possibly confirm this...
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #14 ·
<-------- No trailer, No gear, No passenger, no weather.... I guess being a SWB and 2WD and SC makes a bit of difference too... I was actiually suprised at my average MPG @ 70.... Not bad at all
 

·
C HEMI GO
Joined
·
1,739 Posts
dskirtech said:
I dont think so Hank.. I was somewhat suprised to see that shift as well, so I tried it again on my trip home... Same results @ 65 and 70... plus or minus .2

I think it has to do with the relativity of speed and time (MPH) compared to the Engine RPM. I havent had time to do another test to verify that, but I will. I will have to look at my notes, but I figured out a way to possibly confirm this...
The mileage goes down in the chart initially because the faster you go, the more gas you use. The dip is where the engine is barely able to maintain speed. A few more RPMS and the engine is able to maintain speed easier, thus using less gas. The hump is what the gear guys have been proclaiming for a long time, a few more RPM's can HELP mileage.
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #16 ·
Makes sense Scott, Guess I never really thought about it... But you are exactly dead on.. We apply this same theory @ cruise RPM in an aircraft... the engine works most efficiently at a set RPM... many times its a bit faster than one would think.
 

·
Registered
Joined
·
1,733 Posts
When picking an aircraft cruise speed, it is not just the rpm, but also that the throttle is around 80% open.

Nearly all spark ignited engines, from 2 hp lawn mowers to 20,000 hp natural gas compressor engine achieve their best fuel economy when the throttle is about 80% open and the piston speed is in the range of 1100 to 1300 feet per minute.

The 5.7Hemi engine in the test above does not have a 'sweet spot' that depends on rpm like some poster are saying here - that is a myth from people that have read a few hot rod magazines and know just enough to be dangerous.

At part throttle engine operation, like in those tests above, the best fuel economy would be if the transmission had 6th, 7th, 8th gears so that as the speed dropped you could upshift and keep the throttle near 80%

If you could work out the hp used to go the faster speeds, then you could calculate the better value for 'true' fuel economy - what engine testers call 'Brake Specific Fuel Consumption' or BSFC for short. The BSFC would get better as the speed increased and the throttle got near 80% open, but did not go above that to the point where the PCM begins 'fuel enrichment' and takes the air to fuel ratio from 14.7 down to about 11.5

The most likely explanation for the 'hump' in those test results is a shift in the wind.
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #18 ·
a consistant 100 mile stretch of wind? enough to rob you of over 100 miles total on the fuel consuption for that particular 100 mile stretch (Taking into consideration that the hump Difference is about 1 MPG)??? Thats a little hard to believe. I understand the wind will effect the outcome, but not THAT much.. and also.. how is it the following day I was seeing the same average MPGs? That would mean the wind would have had to either a) Equally effected the readings @ ALL MPH, or in some striking coincidence, the wind kicked back up at the very moment I was testing the 65-75 for the second time?

I carefully recorded every aspect of the test in my PDA... and suprisingly the results were very consistant the second time around with the first round of results...

Hell, I dunno....
 

·
C HEMI GO
Joined
·
1,739 Posts
HankL said:
When picking an aircraft cruise speed, it is not just the rpm, but also that the throttle is around 80% open.

Nearly all spark ignited engines, from 2 hp lawn mowers to 20,000 hp natural gas compressor engine achieve their best fuel economy when the throttle is about 80% open and the piston speed is in the range of 1100 to 1300 feet per minute.

The 5.7Hemi engine in the test above does not have a 'sweet spot' that depends on rpm like some poster are saying here - that is a myth from people that have read a few hot rod magazines and know just enough to be dangerous.

At part throttle engine operation, like in those tests above, the best fuel economy would be if the transmission had 6th, 7th, 8th gears so that as the speed dropped you could upshift and keep the throttle near 80%

If you could work out the hp used to go the faster speeds, then you could calculate the better value for 'true' fuel economy - what engine testers call 'Brake Specific Fuel Consumption' or BSFC for short. The BSFC would get better as the speed increased and the throttle got near 80% open, but did not go above that to the point where the PCM begins 'fuel enrichment' and takes the air to fuel ratio from 14.7 down to about 11.5

The most likely explanation for the 'hump' in those test results is a shift in the wind.
So your are saying that aircraft engines have a sweet spot at 80% but the truck may not have a sweet spot at 30%?
 

·
Registered
Joined
·
10,637 Posts
Discussion Starter · #20 ·
And yes, I agree.. if we could have an accurate Horsepower reading for the above test, it would be allot more easy to tell exactly what was going on, but have you ever tried to take a 1400 mile trip towing a Dyno behind you?

:) :) :) :) :)
 
1 - 20 of 35 Posts
Status
Not open for further replies.
Top