I think you're slipping the clutch way too much. This will not only cause premature wear, but also lower mileage considerably.

 

Manifold vacuum pressure is usually measured in the intake manifold between the butterfly valve & the cylinder head. In laymans terms it's the the amount of resistance in the intake system to the suction caused by the action of the pistons drawing air (and vaporised fuel) in through the intake valves.

So when you're cruising on a flat road at say 40MPH in 5th gear at a little over 1000RPM you barely have your foot on the gas, and so the butterfly valve is barely open thus restricting air entering the intake manifold; manifold vacuum pressure is quite high. So you want to accellerate to 60MPH. Now if you suddely stomp on the gas at 40MPH in 5th gear the manifold backpressure will virtually dissappear as you completely open up the butterfly valve to allow the engine to suck in as much air (and also fuel) as it can. However in such a high a gear at such low engine speeds (particularly with the 3.8L V6) not much will happen (in an auto, the gearbox kicks down immediately to select a lower gear and allow the motor to rev to produce more power) and you will pretty much remain at the same speed and RPMs (or accellerate very slowly) while the motor sucks in a ton more fuel; this is usually refered to as "lugging" the motor.

If you were to shift down a few gears to say 3rd or even 2nd the motor will instantly rev up to the powerband (the RPM range where the motor starts to really develop lots more power & torque relative to the amount of fuel it's using) and thus you can accellerate quickly by only adding a little extra throttle and fuel, working your way up the gears and keeping within the powerband until you've reached 60, then shifting into 6th and cruising once again at low revs and at high manifold pressure.

Many cars manufactured in the late 70's & early 80's during the second oil shock when people became obsessed with gas prices (and all manufacturers drastically downsized their vehicles so the could install smaller motors that used less fuel) had manifold vacuum or "economy" guages on the instrument panel. These usually had an an arc from red at one side (low pressure), yellow in the middle and blue or green at the other side (high pressure), and the aim was to keep the needle towards the blue or green end at most times if possible to save fuel. When I was a poor student and didn't have money for gas I could achieve incredible mileage in my car (almost double the usual) by being incredibly gentle on the gas peddle & shifting gears frequently to keep that damn needle in the green.

Sorry for the long winded explanation; manifold vacuum pressure can be a little hard to explain I've just found out!

As for the clutch, due to reports that the JK's one wears quickly, I usually try to engage it fairly quickly at low RPMs (without stalling obviously) and avoid riding it too much in order to reduce wear, but that's just me.

 

I mostly shift in the upper 2k range..... I'm pretty sure the manual says not to use 6th gear unless you're going over 55. I get alright mileage, although can't believe anyone is getting 400 miles out of a tank. I don't really pay much attention though. I didn't buy the thing for the mileage.

running 33s on 4.10s

 

I'm with you! My 6th gear is worthless........

 

I got 433 miles a few weeks ago driving with 5 people and all our luggage through hilly terrain with the AC on. I still had about 2 gallons left in the tank when I filled up too. Though I do have the unlimited so I have an extra gallon or two compared to the 2-door...

 

That is really impressive.........I might be getting 225 MPT

 

Thank You for the Awesome Explanation About Manifold Pressure about the clutch I don't really completely release at 14 that's only if I need to accelerate fast and I'm at an incline going into the freeway. So would you not recommend using sixth gear at all if I'm only going 40 mph with 3.21 gearing and 33's? I'm also love to keep my vehicles tip top shape no matter how old. So you would say try to release the clutch completely as fast as you can without stalling or bucking jerking jumping with as little gas as possible to perserve the clutch.

 

Apt name with an attitude like that! That is some funny stuff though.

 

I did some experimenting today.....

I have a vacuum gauge installed and am fine-tuning my shifting patterns based on manifold pressure (I've done this in all my vehicles). I found that I'm getting better mileage cruising at 2500 RPM than upshifting to 2000 RPM. For steady speed city cruising, 2500-2800 seems to be the efficiency sweet spot.

As for clutch slippage, my clutch is disengaged in first by about 4 mph. Granted, I have 4.88s (and 35s) and the gearing makes a huge difference.

 

Yeah, what kills clutches more quickly than anything else is heat build-up. Sometimes when you're starting from standstill on a steep hill & really give the motor some revs & ride the clutch for a good 10 or 20 seconds or so to get going and you get that rancid smell in the cabin; that means the clutch has been cooked & you've possibly glazed the friction surfaces (not good). Just riding it a bit more from start on a flat surface will tend to wear down the friction material a little more quickly in the long run but won't tend to quickly kill a clutch.

What's cool about the JK is that is has an anti-stall feature on the fly-by-wire throttle that means you can virtually idle up steep hills & it won't stall (as long as you keep your foot OFF the gas); the motor doesn't increase in revs at all, but the computer adjusts the mixture, timing, and throttle opening to greatly increase torque at idle thus avoiding stall. So this means you can actually engage the clutch at idle RPMs and the Jeep will move smoothly forward without bucking or stalling (again, as long as you don't touch the gas peddle, which will cancel the programming & you will stall).

I often use this feature in heavy stop/go traffic, often never touching the gas peddle for miles. The problem with our tall 3.21 gearing and 33" tires is that there's far more strain on the clutch than Rubi drivers with their 4.10s experience (many don't even use first gear at all on flat ground because they find it too low & start off in 2nd!) so every little thing to reduce clutch wear helps I find.

And with 6th, I usually never use it until at least 50-55MPH unless the road is completely flat, though yes it is possible to cruise at 40MPH in 6th at like 800RPM but there's not much throttle response down there.