anyone know about engine vacuum?

1991 4.0l, AW4, 4.5"lift, 33x12.5" mfg m/t's, regeared to 4.56.
Getting really poor mileage(like 13mpg) and i have tried almost everything, i've done all maintenance and tune-up stuff as well as changed my driving habits.
I hooked up a vacuum guage to the line that goes between the manifold and MAP sensor. i ran it into the truck and obsereved it while driving. I have been told numerous times that higher vacuum means better mileage. here are the results:

3rd gear - 2250rpm/15in.-Hg
3rd gear - 2500rpm/15in.-Hg
3rd gear - 2750rpm/14in.-Hg

OD(4rth)- 1500rpm/14in.-Hg
OD(4rth)- 1750rpm/12in.-Hg
OD(4rth)- 2000rpm/12in.-Hg
OD(4rth)- 2250rpm/10in.-Hg
OD(4rth)- 2500rpm/9in.-Hg
OD(4rth)- 2750rpm/7in.-Hg

I really don't understand what this all means but maybe you do?
the only thing that I am assuming is that 4.56 gears are a bit undergeared with33" tires at highway speeds and that maybe if i was using a bit bigger tires(35") i would be running less rpm at highway speeds in OD and get better mileage? although i can't seem to understand why there was more vacuum in 3rd gear(that would lead me to believe that i would get better mileage in 3rd gear running 2750rpm on the highway(getting 14in.-Hg) than being in OD running 2250rpm on the highway(getting 10in.-Hg). but maybe i have no idea? let me know.

You have higher vacuum in 3rd gear because the throttle is open more in 4th gear to keep the vehicle up to speed.

The most important thing is vacuum at idle. Is it a steady figure? It should be around 18 at idle and go up to about 20 at 1000-1500rpm.

You said you did a tune-up, does that include the O2 sensor? When they go bad you'll get bad mileage.

I wouldn't expect good mileage with a lift a bigger tires anyway tho, more of the jeep is in the wind and with bigger tires you're lugging around more weight.

yeah, i get about 18in.-Hg steady at idle. and the tuneup i did included spark plugs, wires, cap, rotor, air filter, oil change, oil filter, fuel filter, fuel pump, removing and cleaning fule injectors, oxygen sensor, new 2.25" exhuast, and muffler(but the guy cut out the cat without me asking him to). i can't think of much else that would help with the mileage. but i've heard of more than a couple guys running 4.56 gears and bigger than 33" tires and they say they are getting between 15 and 19 mpg. i just want to figure out why i can't do that, my jeep is very well maintained and i don't drive it hard, there should be no reason why it can't get better mileage?

i just ordered a k&n cone filter and will be making my own intake, hopefully that will help with the mileage as well.

Drive with the vacuum gauge on the motor. Keep it above about 12in/Hg and you'll get better gas mileage. I used to run one in my cavalier. 89 4cyl 5speed and could squeeze 32mpg out of it driving to work and back using a vacuum gauge to drive it. I also used the acetone trick, 2oz/10gal. Works well, I ran it 3 tanks of acetone/gas to one take straight gas, and could get 36mpg outta that old car with the acetone and gauge.

so, can anyone confirm that bigger tires will probably give a bit better mileage with the info i have given here?

Forget it! Unless you regear to compensate for the added weight you are going to suffer bad mpg! 4.56's are ideal for 33" tires, 4.88's are ideal for 35" tires. The regearing keeps the engine in it's optimum torque range. I understand your logic but it is counter productive because your engine has to work harder to spin the larger tires.

yeah, i just remember that my 88' xj was getting about 15mpg on the highway stock, and then i lifted it and put 31's on with stock gears and i lost a bit of power but was running lower rpm on the highway and was getting like 17mpg. so, anyways, with the vacuum numbers i posted above, does this mean that since iam running more vaccum at highway speeds in 3rd(even at higher rpm) i will get better mileage than running in Od with lower rpm(and lower vacuum)?

mitsumotors said:

yeah, i just remember that my 88' xj was getting about 15mpg on the highway stock, and then i lifted it and put 31's on with stock gears and i lost a bit of power but was running lower rpm on the highway and was getting like 17mpg. so, anyways, with the vacuum numbers i posted above, does this mean that since iam running more vaccum at highway speeds in 3rd(even at higher rpm) i will get better mileage than running in Od with lower rpm(and lower vacuum)?

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The way I see it is:, You need to drive in the highest gear possible then try to get as high a vacumn as possible. There will be a trade off between high vacumn and useless high RPM's.
I got higher vacumn and really bad MPG's driving in 3rg gear verses 4th gear. Not so bad on the flat highway but going up hills that needed 3rd registered 4+MPG less than 4th on the flat. Driving on a low gear as opposed to a higher gear will nearly always reduce your MPG.
Use your vacumn guage while driving and find the speed/RPM that will yield the highest vacumn in as high as gear possible.

Vacuum is created in the manifold when the cylinders suck in air through the valves. When the throttle is open, more air flows, resulting in less vacuum pressure in the manifold (outside air is filling the vacuum). That's why vacuum drops when you are giving it more throttle.

A healthy engine will produce 15-22 hg inches at idle, so at 17 you are in the zone.

FWIW my '91 stock beater puts out 17 hg inches too, and it gets 15 mpg city 20 mpg highway.

mitsumotors said:

i just ordered a k&n cone filter and will be making my own intake, hopefully that will help with the mileage as well.

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I didn't have any luck doing that. It was noisier and got 2mpg less on the hwy.

You have about the right gear, but 33X12.50s are big tires with alot of cross section. Thay take hp to start moving and HP to keep moving. If you have MTs they are harder to keep moving than ATs because it's like you have a bunch of squares for tires instead of a circle.

I see you have an automatic. With 4.56's and 33's you should have no issues with using "D" all the time. You just need to remind yourself that agressive driving has it's price.

You say 2250rpm at hwy speed? What is that hwy speed?
By the vacuum readings, it seems under geared, but it may be something else, like a clogged CAT. If it's a CAT you have been abusing off-road, it could be broke up inside causing a perfrormance drop and you needing to crack the throttle more.

yeah, but i have a feeling that since i opened up my exhuast so much but haven't done anything to the intake it is making bad mileage, but if both the exhuast and intake are open to compliment each other a bit more it should be better..... i hope.

mitsumotors said:

yeah, but i have a feeling that since i opened up my exhuast so much but haven't done anything to the intake it is making bad mileage, but if both the exhuast and intake are open to compliment each other a bit more it should be better..... i hope.

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I think you created your own problem.The 4.0 isnt a breather and needs back pressure.Ive havent ever even done a tune-up(0-37K) and I still avg 17mpg!