Possible MPG problem

shumthisway

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Joined
Sep 16, 2023
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17
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Jacksonville, Florida
Let me just start by saying jeeps get smiles per gallon not miles per gallon. I have no issue getting terrible mpg I just want to keep the 4.0 healthy. I have an 05 LJ 6 speed just under 80k miles on 35s and stock 3.73 gears. Currently I'm getting about 8 to the gallon and that seems abnormally low to me. I'm in florida so all my driving is on flat ground and I'd say my average speed is around 60. Is this normal? If not what are some things I can look into? Did a tuneup not too long ago and no change.

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I have an 05 LJ 6 speed just under 80k miles on 35s and stock 3.73 gears. Currently I'm getting about 8 to the gallon and that seems abnormally low to me. I'm in florida so all my driving is on flat ground and I'd say my average speed is around 60. Is this normal?

Just posting this for reference, not sure I have a solution for you though I would guess running top off will hurt your mpg. Not suggesting you not do that, just sayin'...

I have basically the same vehicle as you with a hard top, 115k on the clock, running 31's, no lift and whatever the stock gears are. On the highway, so long as I keep my speeds 60-65, I'd estimate I'm getting 16-18mpg. Over 65 and the fuel gauge tends to drop quite a bit quicker. 60 seems like the sweet spot for me.

Nice dog. :)
 
Definitely not normal. Did it always get lousy mileage? Did you change the speedo gear to the correct one? I got 17 mpg highway (60 mph) when I had 35s and 3.73 gears (5 speed). My brother averaged about the same with a similar setup (6 speed).
 
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I've averaged 12.52mpg over the seven years of owning my rig, so something is not right.
 
You're on 35's with the 6 speed, right? Most people here would recommend 5.13's to get your engine closer to the power band while driving vs your 3.73's. I suspect you're lugging that engine pretty hard. At 60mph (assuming that's actually 60 after you changed your tire size) you'd be at ~1800rpm, wheras on 5.13's you be around 2500rpm. The 4.0L is plenty happy running at higher RPM. I'd start there, which is obviously one of the most expensive places to start. That or put 31's on it and see what changes.
 
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You're on 35's with the 6 speed, right? Most people here would recommend 5.13's to get your engine closer to the power band while driving vs your 3.73's. I suspect you're lugging that engine pretty hard. At 60mph (assuming that's actually 60 after you changed your tire size) you'd be at ~1800rpm, wheras on 5.13's you be around 2500rpm. The 4.0L is plenty happy running at higher RPM. I'd start there, which is obviously one of the most expensive places to start. That or put 31's on it and see what changes.

This.

But since you have a manual, just drive it in a lower gear that keeps your rpm up between 2500 and 3k.
 
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Let me just start by saying jeeps get smiles per gallon not miles per gallon. I have no issue getting terrible mpg I just want to keep the 4.0 healthy. I have an 05 LJ 6 speed just under 80k miles on 35s and stock 3.73 gears. Currently I'm getting about 8 to the gallon and that seems abnormally low to me. I'm in florida so all my driving is on flat ground and I'd say my average speed is around 60. Is this normal? If not what are some things I can look into? Did a tuneup not too long ago and no change.

View attachment 514452

With your current setup, you're at 1800 RPM at 60 MPH in 6th gear. I'm also gonna assume because of that you're flooring it at 60 MPH. I'd run in 5th(2200), or maybe even 4th (2700 and also you're 1:1 ratio) to stay in the power band. Our Jeeps are shaped like bricks and are basically air resistance magnets. At highway speeds, you need more RPM. You have hog-ass tires and no gears to back it up. If you really want to amplify how big of a deal this can be, you're free to drive my 4 banger on 31's if you ever head up north. It sucks.

The real answer is you're lugging your engine. Your tires are much bigger than stock, and you don't have the gears swapped to account for them. A 29 inch tire, similar to stock, would put you around where your 5th gear is now. A 29" tire at 60 mph in 6th gear puts you at 2200 RPM. Your 35's essentially require you to kinda give up the 6th gear (so basically any overdrive gear) in most cases. I give up my 5th on 31's on 4.10's with a 4 banger. Ask the 42rle guys how much fun they have with overdrive and bigger tires. The all around issue is you're causing the engine to have to work much harder because it is out of it's range of efficiency. A third option from regearing or giving up overdrive is smaller tires. Look at my pfp to see why that is not a good option. Never skip leg day.

Play around on http://www.grimmjeeper.com/gears.html.
 
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Definitely not normal. Did it always get lousy mileage? Did you change the speedo gear to the correct one? I got 17 mpg highway (60 mph) when I had 35s and 3.73 gears (5 speed). My brother averaged about the same with a similar setup (6 speed).

I've only had it since September its always seemed a little extra thirsty but i didnt calculate my mpg until more recently.
 
I've only had it since September its always seemed a little extra thirsty but i didnt calculate my mpg until more recently.

Verify your speedo with your phone. A TJ naturally, despite its size, is gonna have half ton truck fuel consumption. So it'll always seem like a lot to me anyways, which makes it harder to find out if it's really bad or not. Either way, you're lugging your engine in 6th at 60 MPH anyways. Start using 5th or 4th. If you know the exact distance to a location from your house, you can use the trip odometer.
 
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With your current setup, you're at 1800 RPM at 60 MPH in 6th gear. I'm also gonna assume because of that you're flooring it at 60 MPH. I'd run in 5th(2200), or maybe even 4th (2700 and also you're 1:1 ratio) to stay in the power band. Our Jeeps are shaped like bricks and are basically air resistance magnets. At highway speeds, you need more RPM. You have hog-ass tires and no gears to back it up. If you really want to amplify how big of a deal this can be, you're free to drive my 4 banger on 31's if you ever head up north. It sucks.

The real answer is you're lugging your engine. Your tires are much bigger than stock, and you don't have the gears swapped to account for them. A 29 inch tire, similar to stock, would put you around where your 5th gear is now. A 29" tire at 60 mph in 6th gear puts you at 2200 RPM. Your 35's essentially require you to kinda give up the 6th gear (so basically any overdrive gear) in most cases. I give up my 5th on 31's on 4.10's with a 4 banger. Ask the 42rle guys how much fun they have with overdrive and bigger tires. The all around issue is you're causing the engine to have to work much harder because it is out of it's range of efficiency. A third option from regearing or giving up overdrive is smaller tires. Look at my pfp to see why that is not a good option. Never skip leg day.

Play around on http://www.grimmjeeper.com/gears.html.

Honestly that makes a lot of sense. Looks
With your current setup, you're at 1800 RPM at 60 MPH in 6th gear. I'm also gonna assume because of that you're flooring it at 60 MPH. I'd run in 5th(2200), or maybe even 4th (2700 and also you're 1:1 ratio) to stay in the power band. Our Jeeps are shaped like bricks and are basically air resistance magnets. At highway speeds, you need more RPM. You have hog-ass tires and no gears to back it up. If you really want to amplify how big of a deal this can be, you're free to drive my 4 banger on 31's if you ever head up north. It sucks.

The real answer is you're lugging your engine. Your tires are much bigger than stock, and you don't have the gears swapped to account for them. A 29 inch tire, similar to stock, would put you around where your 5th gear is now. A 29" tire at 60 mph in 6th gear puts you at 2200 RPM. Your 35's essentially require you to kinda give up the 6th gear (so basically any overdrive gear) in most cases. I give up my 5th on 31's on 4.10's with a 4 banger. Ask the 42rle guys how much fun they have with overdrive and bigger tires. The all around issue is you're causing the engine to have to work much harder because it is out of it's range of efficiency. A third option from regearing or giving up overdrive is smaller tires. Look at my pfp to see why that is not a good option. Never skip leg day.

Play around on http://www.grimmjeeper.com/gears.html.

Honestly that makes a lot of sense. Really good write up. I guess I have a 5 speed until I regear 😂
 
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Honestly that makes a lot of sense. Looks


Honestly that makes a lot of sense. Really good write up. I guess I have a 5 speed until I regear 😂

Correct. That's how I see it. I gaslight myself into thinking I'm in a squarebody with a SM465. I don't touch 5th (pretty much your 6th) until I'm at around 60 MPH.

To top that, if your acceleration from a stop doesn't bug you too much, you don't have to do anything except give up 6th. Running at 60 MPH at 2200/2700 RPM on a well maintained motor will cause no problems. One guy on here rode a 2.5 at 3k on the highway everyday and burned no oil when he totaled it at 212k. But, if you really gotta floor it just to get around, I'd look at gears or 33's.
 
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Let me just start by saying jeeps get smiles per gallon not miles per gallon. I have no issue getting terrible mpg I just want to keep the 4.0 healthy. I have an 05 LJ 6 speed just under 80k miles on 35s and stock 3.73 gears. Currently I'm getting about 8 to the gallon and that seems abnormally low to me. I'm in florida so all my driving is on flat ground and I'd say my average speed is around 60. Is this normal? If not what are some things I can look into? Did a tuneup not too long ago and no change.

View attachment 514452

Nice jeep!
I'm on 35s and for my last fill up I measured 8.5mpg.
But that was almost mostly spent in the mountains, in 4-low, crawling through wet, heavy snow. I'd say you have a problem.

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I’d say you have a problem too… and not just the gears.

My ‘05 TJ is on 33’s with the factory 3.07s and 6speed… I do 16-18 mpg at 65+ mph on any road we have here, and drop to 5th only when traffic slows me down when going up hill. No, I do not drive with pedal on the floor… actually not too much deeper than when it had 31’s or the stock tires. Yes, we know what was done to and how this Jeep has driven since we bought it new with 8 miles on the odometer. Yes, I know it needs gears, but it has not been as big a priority as a daily driver it does well.
 
I have an 05 LJR 6spd 32s and 4:10. I get 16mpg on the hwy if I keep it below 70. Headwinds drop that to 12mpg. I do run it like I stole it. I try to keep it just under 3,000 rpm. RPMs determine my shifts.
 
Correct. That's how I see it. I gaslight myself into thinking I'm in a squarebody with a SM465. I don't touch 5th (pretty much your 6th) until I'm at around 60 MPH.

To top that, if your acceleration from a stop doesn't bug you too much, you don't have to do anything except give up 6th. Running at 60 MPH at 2200/2700 RPM on a well maintained motor will cause no problems. One guy on here rode a 2.5 at 3k on the highway everyday and burned no oil when he totaled it at 212k. But, if you really gotta floor it just to get around, I'd look at gears or 33's.

Yeah I plan on getting a regear in the near future. What is it 5.13s is the max I can go with the dana 30 up front right?