Gren71
Well-Known Member
- Thread starter
- #1
After talking with a couple other forum members, and reading a good bit, and seeing a TON of opinions...ive decided to do an experiment.
When I first got my JTmt I cut down the air dams on the bumper within the first week. Then did a mess of other things, much like many of you. After towing with the JT I decided to do a different experiment. I ultimately ended up back at a more OEM tire size to preserve my towing power / capability and my daily driver mpg. Here is a link to that little life lesson incase anyone want to thumb through it "Worth it to go back"
Now, in the same vein of MPG for daily driving, ive wondered if the air dams really did anything significant. And if they did, was it significant enough to keep the not so jeep look. Ive leaned more function over form with the JT..so it wouldn't be the end of the world if they had to stay on.
So the experiment.
Question:
+Do the plastic air dams on the OEM bumper increase MPG over the OEM bumper without them?
+ If there is any gain, at what point does it offset the cost to "go back" to the OEM bumper?
Research:
+A lot of opinion on the forum believe they help a little, in the area of 0.5-1mpg max
+jeep added them intentionally, which lends to them being useful since most all design features they did on the jt and jl are actually pretty impressive. (ex: the exhaust loop)
+after talking with an engineer, i was introduced to the drag formula. Now I dont know the coefficient of the JT, but what I glean from that formula is that it takes the entire frontal area of the JT into account for the drag of the vehicle. And the air dams make up a very small portion of that surface area. ( paraphrased from the engineer I spoke with). There is a school of though that the interior area of the wheel well creates a significant extra drag since it is open to the air movement now and sort of traps it. Certainly not something I can test, but interesting to think about. finding the drag coefficient of a vehicle
+MPG is largely determined by driving style, which is a variable I should be able to mostly isolate. interesting info on fuel economy
+I know what I already get for my regular travels.
-20.5 mpg average overall driving.
-22 mpg average for the test trip "track", given the same variables listed below
Hypothesis:
+The air dams will increase my MPG with daily driving
+The air dams will decrease drag, allowing me to get into 8th sooner
+The increase in MPG will not be significant enough to make the cost worth while for most of us
experiment:
I will drive to and from the same place during my regular work week.
-control variables
+fixed distance to and from work
+fixed time of day for travel
+consistent fuel source
+no further vehicle changes until end of experiment
-variables
+wind
-if wind is over 10mph, and/or primarily from the north or south the day will not be counted
-driving habits
-I will be using cruise control for the entire travel distance, barring unforeseen need to slow down
+inclement weather
-if the weather effects the road conditions, the day will not count
+acceleration
-while accelerating onto the highway there is a specific marker that ive chosen at the beginning / middle end of the on ramp. I will accelerate at a (relatively) fixed rate between these points until I hit highway speed (65mph) by the last marker.
+road type
- I will only record the data during the highway portion of my driving in order to keep this variable, and others, as controlled as possible.
Test.
Time to test and see!
Anecdotal findings worth note:
AF1- The Jt seems to need fewer steering inputs with normal driving. I use the right lane only for the entire trip, unless I need to pass a driver going slower than 65mph. In which case I merge and....slowly....pass them with out increasing my test speed.
AF2- The Jt seems to hold 8th longer on inclines during the trip.
AF3- The Jt shifts back into 8th sooner after automatically down shifting into 7th while in cruise at 65mph.
Im NOT a scientist and...im sure....i probably botched some of the basic writeup. So feel free to nitpick me friends.
*+*+*My setup for those wondering*+*+*
i currently have the iron rock off road 2in spacer lift with…
Core 4x4 track bar
Fox 2.0 steering stabilizer
MPP front and rear sway bar ends
MPP front LCAs
Front geometry correction brackets
Rear Drive shaft drop block(can’t remember brand)
Rear bump stop extensions (axle side)
Sumo springs bump stops all around
Rear track bar bracket
Air lift bags
255/75/17 Bridgestone dueler a/t (bronco take offs)
Jeep JT 17in steelies (for now)
Data:
Day 1
Trip 1
24.5mpg average for the trip
-no notable increase in how fast the JT shifts into 8th
-interesting finding. While traveling up a bridge I remained in 8th longer than usual. Routinely the JT will shift into 7th before a specific sign. Today I stayed in 8th for an additional 3/10th of a mile. Seems small but points to a possible decrease in wind resistance. Interesting enough that Ill keep an eye on when the JT shifts on most inclines now and maybe add that as a data point.
-anecdotal observation...the jeep seems to track straighter with fewer steering inputs on straight line, right lane, driving.
Trip 2
24.1mpg
-had winds that varied in direction, nothing sustained or enough to rule out the trip.
-the jt shifted down to 7th at about the same point as usual on an off ramp. But it did seem to shift back to 8th very quickly compared to w/o the air dams.
-repeat of the same anecdotal finding from trip 1. Noticeably fewer small steering inputs needed with regular driving.
Day 2
Trip 1
22.9mpg
-notably colder this morning. Day 1 AM temp was 29d, today it was 21d. Not sure if that is a contributing factor or not. but there are no other changes Im aware of that would change the trip 1 average so much from day 1. Time will tell.
-anecdotal finding relating to shifting on an incline (now going to call this AF2) was not repeated today. The JT down shifted into 7th IAO the same place it typically would.
-anecdotal finding relating steering inputs (now going to call this AF1) seemed consistent with day 1. Fewer small steering inputs were needed during normal driving conditions.
Trip 2
23.1 MPG
-af 1 was present
-af 2 didn’t happen
-af 3 was present
- minor wind and 26d outside, not enough wind to rule out the day. The temperature has me wondering if its a contributing factor or not. Ex: maybe the lower air temp @ the intake changes the fuel use requirements in a negative way. If I remember (Cadbury egg brain thanks to the army) ill use the jscan app the see what the intake temp is.
Day 3
Trip 1
23.6mpg
Mild wind, 29d ambiant temp.
-af 1 present
-af 2 present, traveled an additional 0.3 miles before shift
-af 3 present
Day 4
Trip 1
23.6Mpg
-30d, no wind
-af 1 present
-af 2 didn’t occur
-af 3 present
Trip 2 - RULED OUT
22.6mpg
-Trip ruled out due to 10+ mph sustained head wind combined with inclement weather.
-Interesting to me that, despite the weather factor, I still had a MPG average in the same range as one of the days in the data pool.
-AF 1 present. In addition to fewer steering inputs, I noticed the JT tracked straighter with a head wind. And even while following behind a large tanker truck where the turbulent draft would ordinarily cause me to wiggle a little.
-AF 2 present, but at a different incline. This time it held 8th longer on an on ramp. Not a lot longer, but definitely held the gear farther and then shifted back into 8th sooner than normal. This one may have more to do with the weather though.
- AF 3 present
Day 5
Trip 1 - RULED OUT
20.5Mpg @ average 60mph with nasty cross wind and gusts. 29d ambient tempt
-Ruled out due to inclement weather impacting the road conditions, and not being able to safely maintain the test speed of 65mph.
-I continue to be impressed with the Bridgestone Dueler ATs. They handled the snowy roads wonderfully. Not entirely sure where all the hate on these tires comes from since ive had nothing but positive experience with them. Enough that I may end up getting another set of bronco take offs if I see them at a good price.
-AF 1 Present
-AF 2 did not occur
-AF 3 Present
~*~*~*~*~*~*~*~*~
The experiment is on hold for now. 31 Jan 2022 See post #57 (page 4) for details
~*~*~*~*~*~*~*~*~
time to get back to it boys!
My dealership had to rebuild my rear differential. It’s been nearly 1000 miles of me breaking it in and my gas mileage is finally starting to come back to what I would consider to be normal.
so I think it’s time to start the experiment back up again.
since getting the truck back, and the weather turning to nice, I have put my soft top on. I initially saw a decrease in my mpg, but I’m thinking that was more due to the new gears than anything.
So my intention is to do an entire new set of data and see if it kind of matches up with the soft top versus a hard top to the gains that we saw with the hardtop.
Day 1
Trip 1
23.6mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 2
Trip 1
24.7mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 3
Trip 1
24.7mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Trip 2
23.5mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 4
Trip 1
23.6mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 5
Trip 1
23.5mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
trip 2
24mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 6
trip 1
23.7mpg
-af 1 present
-af 2 present
-af 3 present
Day 7
trip 1
23.9mpg
-af 1 present
-af 2 present
-af 3 present
When I first got my JTmt I cut down the air dams on the bumper within the first week. Then did a mess of other things, much like many of you. After towing with the JT I decided to do a different experiment. I ultimately ended up back at a more OEM tire size to preserve my towing power / capability and my daily driver mpg. Here is a link to that little life lesson incase anyone want to thumb through it "Worth it to go back"
Now, in the same vein of MPG for daily driving, ive wondered if the air dams really did anything significant. And if they did, was it significant enough to keep the not so jeep look. Ive leaned more function over form with the JT..so it wouldn't be the end of the world if they had to stay on.
So the experiment.
Question:
+Do the plastic air dams on the OEM bumper increase MPG over the OEM bumper without them?
+ If there is any gain, at what point does it offset the cost to "go back" to the OEM bumper?
Research:
+A lot of opinion on the forum believe they help a little, in the area of 0.5-1mpg max
+jeep added them intentionally, which lends to them being useful since most all design features they did on the jt and jl are actually pretty impressive. (ex: the exhaust loop)
+after talking with an engineer, i was introduced to the drag formula. Now I dont know the coefficient of the JT, but what I glean from that formula is that it takes the entire frontal area of the JT into account for the drag of the vehicle. And the air dams make up a very small portion of that surface area. ( paraphrased from the engineer I spoke with). There is a school of though that the interior area of the wheel well creates a significant extra drag since it is open to the air movement now and sort of traps it. Certainly not something I can test, but interesting to think about. finding the drag coefficient of a vehicle
+MPG is largely determined by driving style, which is a variable I should be able to mostly isolate. interesting info on fuel economy
+I know what I already get for my regular travels.
-20.5 mpg average overall driving.
-22 mpg average for the test trip "track", given the same variables listed below
Hypothesis:
+The air dams will increase my MPG with daily driving
+The air dams will decrease drag, allowing me to get into 8th sooner
+The increase in MPG will not be significant enough to make the cost worth while for most of us
experiment:
I will drive to and from the same place during my regular work week.
-control variables
+fixed distance to and from work
+fixed time of day for travel
+consistent fuel source
+no further vehicle changes until end of experiment
-variables
+wind
-if wind is over 10mph, and/or primarily from the north or south the day will not be counted
-driving habits
-I will be using cruise control for the entire travel distance, barring unforeseen need to slow down
+inclement weather
-if the weather effects the road conditions, the day will not count
+acceleration
-while accelerating onto the highway there is a specific marker that ive chosen at the beginning / middle end of the on ramp. I will accelerate at a (relatively) fixed rate between these points until I hit highway speed (65mph) by the last marker.
+road type
- I will only record the data during the highway portion of my driving in order to keep this variable, and others, as controlled as possible.
Test.
Time to test and see!
Anecdotal findings worth note:
AF1- The Jt seems to need fewer steering inputs with normal driving. I use the right lane only for the entire trip, unless I need to pass a driver going slower than 65mph. In which case I merge and....slowly....pass them with out increasing my test speed.
AF2- The Jt seems to hold 8th longer on inclines during the trip.
AF3- The Jt shifts back into 8th sooner after automatically down shifting into 7th while in cruise at 65mph.
Im NOT a scientist and...im sure....i probably botched some of the basic writeup. So feel free to nitpick me friends.
*+*+*My setup for those wondering*+*+*
i currently have the iron rock off road 2in spacer lift with…
Core 4x4 track bar
Fox 2.0 steering stabilizer
MPP front and rear sway bar ends
MPP front LCAs
Front geometry correction brackets
Rear Drive shaft drop block(can’t remember brand)
Rear bump stop extensions (axle side)
Sumo springs bump stops all around
Rear track bar bracket
Air lift bags
255/75/17 Bridgestone dueler a/t (bronco take offs)
Jeep JT 17in steelies (for now)
Data:
Day 1
Trip 1
24.5mpg average for the trip
-no notable increase in how fast the JT shifts into 8th
-interesting finding. While traveling up a bridge I remained in 8th longer than usual. Routinely the JT will shift into 7th before a specific sign. Today I stayed in 8th for an additional 3/10th of a mile. Seems small but points to a possible decrease in wind resistance. Interesting enough that Ill keep an eye on when the JT shifts on most inclines now and maybe add that as a data point.
-anecdotal observation...the jeep seems to track straighter with fewer steering inputs on straight line, right lane, driving.
Trip 2
24.1mpg
-had winds that varied in direction, nothing sustained or enough to rule out the trip.
-the jt shifted down to 7th at about the same point as usual on an off ramp. But it did seem to shift back to 8th very quickly compared to w/o the air dams.
-repeat of the same anecdotal finding from trip 1. Noticeably fewer small steering inputs needed with regular driving.
Day 2
Trip 1
22.9mpg
-notably colder this morning. Day 1 AM temp was 29d, today it was 21d. Not sure if that is a contributing factor or not. but there are no other changes Im aware of that would change the trip 1 average so much from day 1. Time will tell.
-anecdotal finding relating to shifting on an incline (now going to call this AF2) was not repeated today. The JT down shifted into 7th IAO the same place it typically would.
-anecdotal finding relating steering inputs (now going to call this AF1) seemed consistent with day 1. Fewer small steering inputs were needed during normal driving conditions.
Trip 2
23.1 MPG
-af 1 was present
-af 2 didn’t happen
-af 3 was present
- minor wind and 26d outside, not enough wind to rule out the day. The temperature has me wondering if its a contributing factor or not. Ex: maybe the lower air temp @ the intake changes the fuel use requirements in a negative way. If I remember (Cadbury egg brain thanks to the army) ill use the jscan app the see what the intake temp is.
Day 3
Trip 1
23.6mpg
Mild wind, 29d ambiant temp.
-af 1 present
-af 2 present, traveled an additional 0.3 miles before shift
-af 3 present
Day 4
Trip 1
23.6Mpg
-30d, no wind
-af 1 present
-af 2 didn’t occur
-af 3 present
Trip 2 - RULED OUT
22.6mpg
-Trip ruled out due to 10+ mph sustained head wind combined with inclement weather.
-Interesting to me that, despite the weather factor, I still had a MPG average in the same range as one of the days in the data pool.
-AF 1 present. In addition to fewer steering inputs, I noticed the JT tracked straighter with a head wind. And even while following behind a large tanker truck where the turbulent draft would ordinarily cause me to wiggle a little.
-AF 2 present, but at a different incline. This time it held 8th longer on an on ramp. Not a lot longer, but definitely held the gear farther and then shifted back into 8th sooner than normal. This one may have more to do with the weather though.
- AF 3 present
Day 5
Trip 1 - RULED OUT
20.5Mpg @ average 60mph with nasty cross wind and gusts. 29d ambient tempt
-Ruled out due to inclement weather impacting the road conditions, and not being able to safely maintain the test speed of 65mph.
-I continue to be impressed with the Bridgestone Dueler ATs. They handled the snowy roads wonderfully. Not entirely sure where all the hate on these tires comes from since ive had nothing but positive experience with them. Enough that I may end up getting another set of bronco take offs if I see them at a good price.
-AF 1 Present
-AF 2 did not occur
-AF 3 Present
~*~*~*~*~*~*~*~*~
The experiment is on hold for now. 31 Jan 2022 See post #57 (page 4) for details
~*~*~*~*~*~*~*~*~
time to get back to it boys!
My dealership had to rebuild my rear differential. It’s been nearly 1000 miles of me breaking it in and my gas mileage is finally starting to come back to what I would consider to be normal.
so I think it’s time to start the experiment back up again.
since getting the truck back, and the weather turning to nice, I have put my soft top on. I initially saw a decrease in my mpg, but I’m thinking that was more due to the new gears than anything.
So my intention is to do an entire new set of data and see if it kind of matches up with the soft top versus a hard top to the gains that we saw with the hardtop.
Day 1
Trip 1
23.6mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 2
Trip 1
24.7mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 3
Trip 1
24.7mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Trip 2
23.5mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 4
Trip 1
23.6mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 5
Trip 1
23.5mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
trip 2
24mpg
-af 1 present
-af 2 didn’t occur
-af 3 present
Day 6
trip 1
23.7mpg
-af 1 present
-af 2 present
-af 3 present
Day 7
trip 1
23.9mpg
-af 1 present
-af 2 present
-af 3 present
Sponsored
Last edited:
Great writeup 
