MPG, RPM and vacuum relationship - JeepForum.com
 
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post #1 of 11 Old 02-16-2017, 06:35 AM Thread Starter
kb9mfd
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MPG, RPM and vacuum relationship

I have been having a hard time finding a solid answer on this, and its probably one of those issues of everyone has a different opinion. I have been trying to find out why I have bad MPG and I am now at driving habits. I have a '95 ZJ 4.0 5 speed. Putting aside the issues with engine or emissions, I have a questions on the relationship of RPM and vacuum and fuel usage. I installed a vac gage and all standard vac tests do not show any issues. It idles fine at 17Hg. I biggest questions is this, is there a direct relationship with the vacuum and fuel usage? For example, If I go 35 on a stretch of road, in 3rd gear my rpm is 2000, vac is around 15. In 4th my rpm is around 1300 and my vac at around 10 and in 5th my rom is around 900 and vac around 8. Obviously the higher the gear the more I have to press the accelerator to maintain speed. All gears it's smooth, but in 3rd to me it sounds like I am racing the engine, BUT I have read the peak performance is at 2500ish rpm. Again at 65 4th is around 2500 rpm and the vac around 12 and 5th would be around 2000rpm with a vac around 8. So, in what scenario do I use the least amount of fuel or is it a wash? Conventional wisdom says higher rpm is more fuel, but I am finding out that may be wrong, so what is the truth on that?

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post #2 of 11 Old 02-16-2017, 07:31 AM
Boone96ZJ
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The answer is, it depends. Vacuum is related to fuel usage mainly at acceleration and cruising. RPM is related to mpg at all times. They are all intertwined. During acceleration, vacuum is lowest while RPMs climb because the valves are "open" at a rate that vacuum cannot keep up. During deceleration, vacuum is highest and RPM stay just above idle. At idle, vacuum should be 17-22 Hg unless you have a very high lift cam, then it is lower. RPMs at idle are...at idle lol. During cruising speed, vacuum is 65-85% of idle vacuum. Any lower and you are in the "power" vacuum stage. Modern vacuum gauges give you a number, but old school ones gave a color code and wording. Like so,
.

Think of this, when vacuum is low and rpms are low in 5th, the engine is actually working harder by lugging around unless you are going fast enough to keep momentum such as >65mph.

Another hidden factor is actually air temperature. Higher temps provide less power, colder temps provide more power.

Hell, the ZJ isn't exactly designed to cut through the air either. Its a big square brick, trying to push through the wind and we make it worse by lifting them and putting huge rotational weight with bigger tires.

Jeep wasn't exactly concerned with good gas mileage in the mid to late 90s either. Gas was $1.25/gal.

Be happy with what you are getting, the most fuel efficient vehicle in my fleet is a 2010 Tahoe lol. About 5 mpg from a big block 66 Mercury Comet, around 8mpg from an 83 K10, my 96 v8 ZJ gets about 13 and the Tahoe gets around 18.

Hope that helped some?

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96 5.2 242 3" 32x11.5
https://www.jeepforum.com/forum/f13/1996-grand-cherokee-laredo-5-2-a-2570290/
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post #3 of 11 Old 02-16-2017, 07:27 PM Thread Starter
kb9mfd
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Quote:
Originally Posted by Boone96ZJ View Post
... Modern vacuum gauges give you a number, but old school ones gave a color code and wording....

....Think of this, when vacuum is low and rpms are low in 5th, the engine is actually working harder by lugging around unless you are going fast enough to keep momentum such as >65mph. ....
Thanks for the reply. Actually the gage I bought (its a AutoGage) has the colors, so that is handy. Today I had to go 70 miles away so I paid attention. I went 70 in 5th, put cruise on and the vacuum was mostly between 0 and 5. Also sucked gas a at good clip. RPM was around 2200. (4" lift, 31" tires, 3.55 gears) After a while I down-shifted to 4th and went 65. RPM was around 2500 and vacuum around 10 ~ 15 (in the green on my gage). I think it also used less fuel but I am not sure. I did get up to 70, 2800 RPM and it stayed above 12 most of the time and unlike in 5th, I had less movement up and down hills, it actually would get into the yellow (de-acceleration on mine >23) going down hill. I am going to guess I should drive in 4th most of the time. I know I must have a performance issue somewhere because if I reset the computer I better mileage for a while. After I put the larger tires on I was getting around 16, now its more like 12. Still, would putting 3.73 gears in help? I am planning on keeping it at 4" lift and 31" tires so maybe butting the best gears for that application would be something to look into. I'm a bit cautious over 65, as the transmission change I did along with the lift has caused me to adjust my caster out of the recommended range so it drifts a bit at high speed. I am also trying to fix that but there is no easy or cheap fix. Thanks!
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post #4 of 11 Old 02-17-2017, 06:25 AM
Ace14
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Honestly the issue is that 3.55 and 31s is not ideal gearing. You are pulling between 0 and 5 in fifth because the jeep is working hard to keep it going at that speed. Personally I always like the gearing in my TJ when it was 3.73 and 31s did well all around. When I had 4.10s in my TJ on 31s it was even better but RPMs were a little high on the highway.

Obviously its not worth spending the money to regear for a 31inch tire so you may just have to deal with it. Most zj guys will tell you that 3.55 is fine for 31s which is true for the auto. With the stick shift I think they are slightly to steep.

That being said its a lifted jeep and its only going to get 12-16 MPG anyway lol. My TJ with 4.10s and 33s does about the same. Now my stock niner does 11-12 haha.

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1998 5.9 ZJ -DD/ work in progress
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post #5 of 11 Old 02-17-2017, 08:21 AM
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I have a 1996 4.0 Auto 3.55 ZJ with the 242 and 31's on steel rims and have been consistently getting just over 20 mpg on the highway (15ish in the city). I have never checked the vacuum and don't have a gauge, so I can't comment on that, but I did change my speedo gear when I got the tires. Not sure if you did that it may make a difference.

I may need to adjust my TV cable because with cruise control on when in overdrive (60-75mph) if I encounter a incline the 42re downshifts to 3rd and revs like crazy. So what I have been doing in the interim is hitting cancel on the steering wheel when I see it start to drop out of OD and being easy on the pedal try to not slow down too much and keep it from shifting out of OD, then when I'm at the top of the incline press resume accel. I think that has helped me stay above 20mpg.

Every time I get in the jeep I turn the OD off and it makes a huge difference around the city, better shift points and peppier all around, not that this applies to you. I keep it off when I get on the highway until I'm at about 60. I'm also hoping this helps me get some more life out of this 42re(I also have a 30,000 gcvw B&M super cooler stuffed behind the grill).

-Tim
(2) '96 4.0s
#1-'10-'18
#2- HP30/Ford 8.8 - 4.56, OME HDs, 5100's, IRO Adj Arms/Track Bars
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post #6 of 11 Old 02-17-2017, 08:23 AM
Boone96ZJ
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0-5 Hg in gear sounds like a vacuum leak

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https://www.jeepforum.com/forum/f13/1996-grand-cherokee-laredo-5-2-a-2570290/
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post #7 of 11 Old 02-20-2017, 07:16 AM Thread Starter
kb9mfd
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Quote:
Originally Posted by Ace14 View Post
Honestly the issue is that 3.55 and 31s is not ideal gearing. You are pulling between 0 and 5 in fifth because the jeep is working hard to keep it going at that speed.
I was thinking about the re-gearing as I saw a table somewhere with the chart of the gearing / transmission / tire size and 3.73 was more ideal. I do not have a vacuum leak as the vacuum is fine doing a normal vacuum test, I know it is low because of the accelerator being pressed. But, the guy I got my donor jeep (a '93) had 230k+ on it, where I got the AX15 and 31" tires from said he was getting around 20mpg, I kind of expected to get the same as the only difference is I have a 242 and he had a 231 (lets not get into that debate), about the same lift (he had 3.5" and I have 4") and I have less miles (160k). (I did change the speedo gear, checked it with a gps) and I am using the '93 computer. But, I had crummy mileage with the auto so there must be something else. I just changed my O2 to a NGK one last fall so that is not it. When I reset the computer I get great mileage for a half tank or so. Well this weekend I swapped out the throttle body and MAP sensor with the '93 (after cleaning the IAC) and reset the computer. It ran much better this morning and the vacuum was higher than what I am use to seeing, so I guess I will see. The only reason re-gearing is something I am looking at is because I need to re-do my front end. I need to put a different drive shaft in to correct my caster (pinion is too low) so I have to remove the yoke, but I think the pinion bearing is going out as its quite noisy (makes a whrrr whrrr sound while driving) so I was figuring on re-building it and if I had it all apart that would be the ideal time to re-gear. (but then of course I would have to do the rear), I was also thinking about lockers but I do not know if I will have the money.
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post #8 of 11 Old 02-20-2017, 09:06 AM
StPaul59
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There's a guy on the xj forum that claims to get 25 mpg hwy with 30in tires.

The main things he did-

205 thermostat

Regeared to 3.73

Kolak ignition upgrade

Ill find the link...
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post #9 of 11 Old 02-20-2017, 09:07 AM
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post #10 of 11 Old 02-20-2017, 09:42 AM Thread Starter
kb9mfd
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Completely awesome thread, thanks!
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post #11 of 11 Old 02-20-2017, 12:05 PM
StPaul59
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Originally Posted by kb9mfd View Post
Completely awesome thread, thanks!
Yep! I'm running the 205 t stat, granted I have a different motor, but am happy with the results so far. It should be fine in the midwest summers as well. It helps that I have an all aluminum radiator ..
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