Cruise Control and MPG

[TBULL52]

I just did a 1400 mile round trip from Maryland to Charleston, SC and noticed that when using cruise control my MPGs went way down. I'm not one to pay close attention to what gear my automatic transmission is in but after seeing the MPGs go down while using cruise control I started to pay a little/alot more attention. What I notice is that when using cruise control my Jeep never went into 8th gear. I set it to 79 MPH and it would get up to speed easily and then stay in 7th gear and 3000 RPMs. When I turned the cruise control off and just maintained the 79 MPH the Jeep would settle in 8th gear and run at about 2200 RPMs. I can not think of a logical reason for this, has anybody else experienced this scenario. I am running 35's but I have used my Tazer JL to adjust my tire size. Thanks in advance for any input.

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[Shots]

The only thing I can think of is that 79 with 35's is right on the edge of 7th/8th gear. By manually controlling the throttle you may be making minor adjustments to keep the speed via throttle position, whereas the computer must be keeping the speed via gear selection and maintaining a given throttle setting. Or of course I could be completely wrong, I'm no expert in electronics, so I'm just guessing. Have you tried the same "experiment" at different speeds? For example trying it at 70-75 to see if cruise will settle in to 8th gear?

 

[TBULL52]

The only thing I can think of is that 79 with 35's is right on the edge of 7th/8th gear. By manually controlling the throttle you may be making minor adjustments to keep the speed via throttle position, whereas the computer must be keeping the speed via gear selection and maintaining a given throttle setting. Or of course I could be completely wrong, I'm no expert in electronics, so I'm just guessing. Have you tried the same "experiment" at different speeds? For example trying it at 70-75 to see if cruise will settle in to 8th gear?

Thanks for the input. I have not tried it a different speeds but I will on the way home from work today

 

[TA1ton]

@TBULL52; I agree with @Shots...it’s probably right on the edge between the gears. I’ve noticed that there is hardly any “slack” in the cruise on the Jeep. My old pickup would slow down by about 5 mph or so before it would downshift...the Jeep doesn’t do that and I can’t help but wonder how much better the MPG would be if it did. On some small hills it seems like it downshifts and then runs high RPMs for a while before it shifts back up when it could just lose a couple MPH and then gain them back on the downhill side. Another experiment you can try is to put it into manual mode and make it shift into 8th instead of hanging in 7th and see if that helps your MPG. I’ve done this and it helped on the smaller hills, but on larger hills it just didn’t have the torque and it wasn’t very advantageous. Let us know how it goes!

 

[Uhdinator]

did you re cal by selecting tire size specs or actual measurement? My tires are 32.7 based on mfr spec (could be based on max cold psi of 65). My tires are at 32-34 psi and actual measurement was approx 32". Re cal selectable choice used was 31.75" This resulted in speedometer reading 1 mph faster than speedo on smart phone app. Selecting 32" as measured resulted in 3 mph off. I believe the auto trans has adaptive learning too. So if your foot is heavy on the gas the trans gradually adjusts over time to shift at higher rpm's based on driving habits. See If you can find if there is a fuse for the trans module (TCM) or disconnect battery for a few minutes. TCM module resets. Then re cal using actual tire height and test with phone speedo app or gps to see is speedo is correct. Drive easy on the gas until you get to interstate speed at see what happens. You may be defeating yourself if you have a heavy foot, thus training the trans to shift at higher rpm's thus it wants to get to a higher rpm based on your habits before it shifts to 8th gear. Just a hunch.

My 2.0L runs at 1600-1700 at 70mph in 8th gear. I noticed it doesn't always go to 8th but changing a couple mph fixes that depending on winds and inclines. The torque assist electric motor likely helps to maintain power/torque at lower rpm's than the v6.

 

[RussJeep1]

I just did a 1400 mile round trip from Maryland to Charleston, SC and noticed that when using cruise control my MPGs went way down. I'm not one to pay close attention to what gear my automatic transmission is in but after seeing the MPGs go down while using cruise control I started to pay a little/alot more attention. What I notice is that when using cruise control my Jeep never went into 8th gear. I set it to 79 MPH and it would get up to speed easily and then stay in 7th gear and 3000 RPMs. When I turned the cruise control off and just maintained the 79 MPH the Jeep would settle in 8th gear and run at about 2200 RPMs. I can not think of a logical reason for this, has anybody else experienced this scenario. I am running 35's but I have used my Tazer JL to adjust my tire size. Thanks in advance for any input.

Extremely valid concern Jack @TBULL52 .

To the extent gas mileage factors into your concerns--and it may not, or at least not primarily--boy do I find myself taking a dive with said miles/gallon should I take my JL (a.k.a. my box on wheeks) North of 65....which you may know..

on a private closed track that is--only time I take my rig "North of 65" is Route 65

Seriously, say you're in non-cruise control, in eighth gear at 75 mph and then you enable the cruise control...what happens to the gear you're in if anything?

 

[WXman]

I just did a 1400 mile round trip from Maryland to Charleston, SC and noticed that when using cruise control my MPGs went way down. I'm not one to pay close attention to what gear my automatic transmission is in but after seeing the MPGs go down while using cruise control I started to pay a little/alot more attention. What I notice is that when using cruise control my Jeep never went into 8th gear. I set it to 79 MPH and it would get up to speed easily and then stay in 7th gear and 3000 RPMs. When I turned the cruise control off and just maintained the 79 MPH the Jeep would settle in 8th gear and run at about 2200 RPMs. I can not think of a logical reason for this, has anybody else experienced this scenario. I am running 35's but I have used my Tazer JL to adjust my tire size. Thanks in advance for any input.

That's normal. Cruise is programmed to maintain speed without much fluctuation. Feathering the pedal manually will allow fewer gear changes and better MPGs.

 

[Oilslick]

I noticed similar results, I drive with the cruise off a lot because I get irritated with the up and down 7-8-7 shifts.

 

[Torero]

I just did a 1400 mile round trip from Maryland to Charleston, SC and noticed that when using cruise control my MPGs went way down. I'm not one to pay close attention to what gear my automatic transmission is in but after seeing the MPGs go down while using cruise control I started to pay a little/alot more attention. What I notice is that when using cruise control my Jeep never went into 8th gear. I set it to 79 MPH and it would get up to speed easily and then stay in 7th gear and 3000 RPMs. When I turned the cruise control off and just maintained the 79 MPH the Jeep would settle in 8th gear and run at about 2200 RPMs. I can not think of a logical reason for this, has anybody else experienced this scenario. I am running 35's but I have used my Tazer JL to adjust my tire size. Thanks in advance for any input.

Interesting observation but somewhat incomplete. You say that on 8 gear the RPM go down, that’s a fact, but you don’t say you observed an increase in mpg. Did you?
Because lower RPM does not directly translate into better mpg. Mpg is a function of the throttle position and speed. If to maintain the same speed at the lower rpm the gas throttle has to be pushed further you will actually consume more gas.
There is a long hill that I climb everyday. Mine is a manual transmission. If I leave it on 6 gear it will climb the hill st 65 mph below 2000 rpm but I have to push the gas to the floor to maintain the 65. Mpg shows 7 mpg. If I shift to 4 gear, maintain 65 mph, the rpm climb to close to 3000 but I can keep the gas at about 1/4 and the mpg shows 10 mpg.
The other thing is that these cars above 75 mph consume a lot of gas regardless.

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