Weird MPG Question

[HOrnbrod]

EDIT:  Topic Moved

 

I have a quick one for y'all. I've been down to FL a few times with the stroker and always averaged at least 21 MPG, normally closer to 23. It has a 4.10 rear and the bigger 96-99 injectors (49psi @ 23.2lb/hr) and I run it around 65-70 MPH turning 2900-3100 RPM. The speedo is correctly calibrated and checks spot-on. Yhe problem is around town I only make 11-13 MPG tops no matter how I drive. Anyone know why there is such a difference?

[Ωhm]

Under both of your driving examples, is the vehicle in closed loop mode?

 

Only the ECU knows if your in open or closed loop.

[HOrnbrod]

Yes, I have a A/F meter that monitors the ECU and displays the existing A/F ratio. It switches out of closed loop search mode at about a minute after starting. I can see the A/F ratio going from search then locking in at ~13 when the ECU switches to closed loop. And it stays locked in closed loop as long as your underway.

 

Around town I've tried driving like a fool with burnouts and the opposite extreme driving around no more than 40MPH with slow acceleration, etc. and the mileage is never over 13 MPG. Runs and idles fine at all times, driving slow or fast too.  

 

At least 90% of my driving is "city" driving, and I'd like to improve the 11-12 MPG average if possible. Any ideas, things to check, or just live with it?

[Tex06]

Damn, you sure on the 21mpg? I don't get that with my nearly stock 4.0 and AX-15. Have you tried tracking mileage in town while in Florida? I don't think it would account for a nearly 10 mpg difference, but I know Alabama gas gives me worse mileage than anywhere else. Mississippi is the best...I assume the blend is different.

Also, is your auto transmission stock? If there was any issue with it, it could account for some loss of mpg.

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[HOrnbrod]

Thanks for the reply Tex. I've been buying my gas at a local station that supposedly sells ethanol-free gas for the last couple of years, so I don't think it's bad fuel causing this. And my AW4 is completely stock and operates as it should; RPMs are correct for the speeds on the tach, and the O/D works fine. I just have never seen such a difference in mileage between city and highway MPG, and it's puzzling. Maybe it's the 62mm bored out throttle body and intake I have causing the difference, but I haven't seen this mentioned anywhere else.  

[jeeppapa]

Damn, you sure on the 21mpg? I don't get that with my nearly stock 4.0 and AX-15. Have you tried tracking mileage in town while in Florida? I don't think it would account for a nearly 10 mpg difference, but I know Alabama gas gives me worse mileage than anywhere else. Mississippi is the best...I assume the blend is different.

Also, is your auto transmission stock? If there was any issue with it, it could account for some loss of mpg.

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I actually got 21-22 on my 2001 Cherokee between CA and MO. I run a KN cone with 1” riser and flow master cat back. Averaged about 60-70 mph. I use fuel system cleaner pretty regularly. Following a moving truck. If I go above 70 it makes a huge difference in MPG. Though it’s hard to keep my foot out of it


88 Pioneer 4x4 auto LB D30/D44
88 Eliminator 4x2 5sp SB D35

[Tex06]

Following a semi can make a huge difference but is dangerous and illegal in most (if not all) states. I averaged 29 mpg in my old 05 Canyon doing this for a tank between Ga and Texas instead of 20-21. My 92 MJ gets about 15-16 mpg around town and 17ish mpg on the hwy doing 65-70mph.

 

I use my GPS to track my mileage since my odometer is off. Still don't know how such a big difference in hornbrod's mileage happens though.... unless he tailgated like this to "hypermile" on the trip to Florida.

 

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[Tex06]

Thanks for the reply Tex. I've been buying my gas at a local station that supposedly sells ethanol-free gas for the last couple of years, so I don't think it's bad fuel causing this. And my AW4 is completely stock and operates as it should; RPMs are correct for the speeds on the tach, and the O/D works fine. I just have never seen such a difference in mileage between city and highway MPG, and it's puzzling. Maybe it's the 62mm bored out throttle body and intake I have causing the difference, but I haven't seen this mentioned anywhere else.  

Horn, if you always use that station at home maybe try a different one for a few tanks before your next trip (like Racetrac) and try to stick to that during the trip as well. That's gonna give you the closest thing to the same fuel for both types of driving, but I don't believe this is the cause of the difference just be interesting to look at.

With the increased power of the stroker motor, should the shifting of the transmission have been adjusted? For example, perhaps your engine is revving much higher than necessary or shifting too soon. A slipping transmission or one that "hunts" for the right gear constantly can kill your gas mileage. A good example would be the 6cyl autos of the 60s and 70s.

Conversely, once you got up to speed with the auto, it wouldn't be shifting anymore and your mileage would improve drastically.

Just a hypothesis, consult an expert before acting on this.

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[Wounded_Fighter]

3 hours ago, HOrnbrod said:

EDIT:  Topic Moved

 

I have a quick one for y'all. I've been down to FL a few times with the stroker and always averaged at least 21 MPG, normally closer to 23. It has a 4.10 rear and the bigger 96-99 injectors (49psi @ 23.2lb/hr) and I run it around 65-70 MPH turning 2900-3100 RPM. The speedo is correctly calibrated and checks spot-on. Yhe problem is around town I only make 11-13 MPG tops no matter how I drive. Anyone know why there is such a difference?

 

Higher rpm and trans gear that you're in. Lower gear/rpm is usually lower mpg.

Same was true on my ram 3500 5.9 tdi

[HOrnbrod]

9 minutes ago, Wounded_Fighter said:

 

Higher rpm and trans gear that you're in. Lower gear/rpm is usually lower mpg.

Same was true on my ram 3500 5.9 tdi

 

True, but a 10MPG difference? I don't think that's normal. My engine runs great, and obviously is very efficient at highway cruising, but literally shows maximum suckage at stop and go. I'm willing to try some mods to bump up the in-town mileage even if I have to sacrifice some highway MPG since over 90% of my driving is running around town. Just what to try though - that is the question....0

 

One theory I have is maybe the improved highway mileage is caused by my intake. I have it sucking air in from behind the grille, and on the highway at speed I could be getting a slight "ram" effect pushing more air to the intake manifold. Just a theory though and don't know what to do about it to help around town...

[Tex06]

Any excuse to show off the cleanest engine bay on the site, huh?

I believe the ram air effect would be negligible, watch your AF ratio to tell for sure.

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[HOrnbrod]

6 minutes ago, Tex06 said:

Any excuse to show off the cleanest engine bay on the site, huh?

 

Negative Tex. There are plenty on rigs on here that make my engine bay look like a garbage pit. Alexia and Gjeep come to mind. And yeah, you're probably right about the negligible ram effect...

[DirtyComanche]

I think your problems are at least partly transmission related.  The AW4 is not very efficient outside of lockup, and I'm guessing you're seeing very little time in lockup during your around town driving.

 

[HOrnbrod]

Very true Dirty. But a ~10 MPG difference? My other Jeeps are about maybe a 4-5 MPH difference. But they don't have AW4's either. Guess I'll just live with it then. Was hoping someone might have a silver bullet solution.

[Crash]

As weird as it may seem, the mpg difference is not unusual per-se. On the higher end of the spectrum.

When I was dd ing my MJ.. 88 renix, 4.0, 5 spd, 3.73, 32s, bored tb, 5.0 injectors, 3ish inch lift. I was averaging 18.5. On road trips as much as 28 consistently.

When I had a shorter commute it was closer to 15.

City driving was closer to 13 give or take.

 

 

Addon:

Depending on the stroker setup, that may also be a factor in the efficiency, cam, tune, etc.

[HOrnbrod]

22 minutes ago, Crash said:

Addon:

Depending on the stroker setup, that may also be a factor in the efficiency, cam, tune, etc.

 

This probably has more to do with the crappy city mileage than anything else.

[Tex06]

As weird as it may seem, the mpg difference is not unusual per-se. On the higher end of the spectrum.

When I was dd ing my MJ.. 88 renix, 4.0, 5 spd, 3.73, 32s, bored tb, 5.0 injectors, 3ish inch lift. I was averaging 18.5. On road trips as much as 28 consistently.

When I had a shorter commute it was closer to 15.

City driving was closer to 13 give or take.

 

 

Addon:

Depending on the stroker setup, that may also be a factor in the efficiency, cam, tune, etc.

My 92 has the HO motor, upgraded intake (took a hit on mpg with that), injectors, 3.5"lift, 3.07 gears, and 31" AT tires; in other words, very similar. I would love to see those numbers....I always wonder what causes the difference between machines that seem so similar.

 

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[yxmj]

What was the average air temp outside on the trip as compared to your day to day driving?

[HOrnbrod]

1 hour ago, Tex06 said:

My 92 has the HO motor, upgraded intake (took a hit on mpg with that), injectors, 3.5"lift, 3.07 gears, and 31" AT tires; in other words, very similar. I would love to see those numbers....I always wonder what causes the difference between machines that seem so similar.

 

Not quite similar. These are the dyno numbers shortly after the stroker went in and the upgraded intake. Not bragging, just posting the readouts. RED is HP, BLU is torque. I'm definitely not an expert interpreting them, but Lee Hurley @ Hesco seemed happy with them. It actually has good street manners, but seems to run much better at a higher RPM, which is probably the major reason why the mileage is so much better on the freeway.

 

 

1 hour ago, yxmj said:

What was the average air temp outside on the trip as compared to your day to day driving?

 

Mileage was based over several trips in temps ranging from 50* to 90*. All about the same MPG. Good thinking about the temps though..

[Ωhm]

Considering the ECU uses a 'Speed Density' system for fuel/timing lookup values and A/F ratio doesn't seem to be the issue, then perhaps your losing engine timing (advance) during your city drive cycles.

[Crash]

My 92 has the HO motor, upgraded intake (took a hit on mpg with that), injectors, 3.5"lift, 3.07 gears, and 31" AT tires; in other words, very similar. I would love to see those numbers....I always wonder what causes the difference between machines that seem so similar.  
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Swap axles with a min of 3.55 gears, your mpg and drivability will improve.

[HOrnbrod]

Just now, Crash said:

53 minutes ago, Tex06 said:

My 92 has the HO motor, upgraded intake (took a hit on mpg with that), injectors, 3.5"lift, 3.07 gears, and 31" AT tires; in other words, very similar. I would love to see those numbers....I always wonder what causes the difference between machines that seem so similar.

Swap axles with a min of 3.55 gears, your mpg and drivability will improve.

 

With the 31s the 4.10 ratios are very close to an MJ with stock size tires and 3.55 gears. Which is what I had originally with the old D35. But after snapping two right axles, I threw the bigger tires on with the 4.10 D44. No more busted axles.

[Tex06]

Swap axles with a min of 3.55 gears, your mpg and drivability will improve.

I've got a set of 4.10 for my Dana 44 and a Detroit locker. Now just waiting to find a deal on a carrier for my D30 up front.  

I would think with the higher gear ratio I would lose mpg, not gain them. It's not like the motor lugs about currently during daily driving.

 

 

Hornbrod: Earlier I was referring to my truck being similar to Crash's. Both 4.0 (renix vs HO), very similar setups apart from the gearing. Yet he's seen 28mpg when I've only seen 19.5 once.... going downhill. I know you're a whole different animal with the stroker and auto plus gearing differences.

 

 

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[HOrnbrod]

48 minutes ago, Ωhm said:

Considering the ECU uses a 'Speed Density' system for fuel/timing lookup values and A/F ratio doesn't seem to be the issue, then perhaps your losing engine timing (advance) during your city drive cycles.

 

That's a thought. Hmmmm, how can you monitor ignition advance in the cab? A knock monitor? Can the Renix REM do that on a Renix engine?

[HOrnbrod]

13 minutes ago, Tex06 said:

Hornbrod: Earlier I was referring to my truck being similar to Crash's. Both 4.0 (renix vs HO), very similar setups apart from the gearing. Yet he's seen 28mpg when I've only seen 19.5 once.... going downhill. I know you're a whole different animal with the stroker and auto plus gearing differences.

 

Gotcha - my bad.