Disputing gas saving tips

[Ecomike]

Interesting. Given the same construction, the torque output of individual engines becomes a function of crankshaft speed (assuming same head/cam/breathing/...)

I get best mileage cruising at 2800-2900rpm crankshaft speed with 31s and 3.07. I still want to change the gearing (it really doesn't sit well with the transmission, it's a horrible complement to the tyres as well...) and that should end up changing the relationship between cruise speeds and crankshaft RPM - I'll have to find my notes.

However, if you were to do a longer-term test to find your "sweet spot" for crankshaft speed at cruise, I'd like to see your notes when you were done.

I did some limited tests already using the AW4 like a manual tranny running it at different shift point rpms on stop and go driving. At 1,400 rpm and 3,000 rpm, with 235R75 15" tires and 3.54 rearend the mileage dropped to about 10 mpg. Actually it was worst upshifting at 3,000 rpm, and not much better upshifting at 1400 rpm. I find it interesting that now that I have the TCU wires all working and a new properly working, and well wired (good grounds) TPS, with all the other sensors pretty much dead-on per the FSM, that at 10% to 50% of WOT the TCU upshifts at 2000 to 2200 rpm, and the next gear kicks in at 1800 to 2000 rpm. A very tight rpm range from first to fourth gear while the TC is still unlocked and while running in D. If I push the throttle harder to say 75% of WOT the rpms shift a good bit higher, but gas mileage suffers.

I suspect your rpm sweet spot is so high due to the gearing and tires. From what I have read here you really should be running something in 4.x gear range with those tires.

What kind of mileage are you getting anyway???

 

[RichP]

Keep in mind a full tank of gas weighs about 125lbs. So I chose to put in only a 1/4 of a tank at a time. That's at least almost a 94lb difference which helps a little. Plus, since you fill up so often, you realize how much you spend in gas which makes ya not want to drive lol. Works for me anyways Oh, also, running full synthetic everything helps a lot too.

Don't try that with a 97+ jeep if you have alot of very steep hills to travel on. My daughter 'ran out' of gas on one really steep SOB in Philly and there was still 4 gallons left in the tank. It was one of those hills that causes the seat belts to lock, too bad she was going up and not down.

 

[RichP]

Interesting. Given the same construction, the torque output of individual engines becomes a function of crankshaft speed (assuming same head/cam/breathing/...)

I get best mileage cruising at 2800-2900rpm crankshaft speed with 31s and 3.07. I still want to change the gearing (it really doesn't sit well with the transmission, it's a horrible complement to the tyres as well...) and that should end up changing the relationship between cruise speeds and crankshaft RPM - I'll have to find my notes.

However, if you were to do a longer-term test to find your "sweet spot" for crankshaft speed at cruise, I'd like to see your notes when you were done.

My sweet spot with LT235 BFG/AT KO's at 70mph is 4th gear at 2400rpm with 3:07's and my 5 speed. With the 2.5L 5 speed, 4:10's and 30" BFG's in the TJ it's 3,000 in 4th at 65, 2500 in 5th with no hills and a good tail wind. All bets are off when the top comes down. With the XJ all bets are off if the windows are open in the interstate.

 

[POSJ]

I don't remember exactly but on a lifted XJ you are screwed. 6" w/ 33's and 4.56 gears gets about 200-300 miles a tank from what I found at about 60-65 mph.

I havent driven my XJ in awhile but I can chime in on my Dodge. I work swingshift 1500-2300 so I get to go as slow as I can tolerate on the way home and last night I found my best speed limit was about 52 mph with a slight headwind. I usually set the cruise at 58 mph since it isn't too slow and I would get about 17-21mpg highway but with it set at 52mph I got about 21-25mpg. I only average about 16mpg per tank since my truck weighs twice what an XJ does though. BTW 2001 Dodge 5.9L CTD Auto SRW, if anybody was wondering about the type...

 

[5-90]

What kind of mileage are you getting anyway???

With the setup I've got (88, stock height, 31s, and 3.07's cruising 62-65mph in fourth,) I average about 17-18mpg or so. It drops off drastically when I use fifth on the freeway under 80mph or so (down around 14mpg.)

 

[Dustn525]

My tip is to dish out $1500 for a 45mpg Honda CRX or $800 for a Metro, and just drive your Jeep when you really want to. If you do alot of driving, the little cars will pay for themselves in no time.

 

[gone postal]

I don't buy the "shut off at red light" theory. I'm sure the savings is negliable and at the cost of your starter and if you hit enough red lights, your alternator (considering your battery would have to draw cranking amps every light). I haven't had to replace either on my '97, but on my brother's '00, the alt. was $170-ish and starter was $150-ish I think - it's been awhile. Either way, $300 for round #'s - that's 90 gallons of gas. That's a lot of gas to save from red lights.

Actually reminds me of a Mythbusters I saw recently. They were testing the same theory, except w/ light bulbs (evidentally the same rumor of leaving a light on is better than shutting on/off everytime you enter/exit room). The break even point for most lights (normal, compact flourescent, LED, etc) was .25 second. The ONLY one that took longer was a regular tube flourescent - think office building, classroom, etc - that took 23 seconds. So, if you're one of those types, turn your damn lights off!! Maybe you can save some electricity and afford more gas...

Some other 'gas theories' I've heard are buy in the morning - the gas is denser and you get more for your $$ and going back to the aforementioned vapors - pump slowly. If you pump fast, more turns to vapor and leaves the tank. I don't know how much this all adds up to, but those are the theories.

 

[RichP]

Actually fluorescent light bulbs in buildings serve two purposes, one they are used to control heating and cooling in over nite and weekends. It can be spooky when the master controls turn entire floors on and off during the nite based on thermostats.

 

[falcon556]

Frequent starts accelerate engine wear.
I think using common sense will go a long way, if it feels like it will take a while turn it off, if not keep it running.
What is a "while"?, YMMV

 

[Rick Anderson]

Escaping Fuel Vapors while fueling, I'm pretty sure that is a Pollution Control Thing. I.E. it is negligible for your actual fuel being in the tank, BUT, raw gasoline vapor is a major contributor to ground level ozone, SMOG.

There already is the flip cap and opening barely larger than the fuel nozzle (granted, that is too keep people from putting leaded gasoline into unleaded only vehicles, which leaded fuel is long gone) but NOT much vapor is going to escape, even without it, how many ounces do you loose to evaporating to vapor, the five minutes the gas cap is open and mostly blocked by the fuel nozzle?

That is why you see those vapor recovery boots on gas pumps, it is NOT because the gas station is trying to recover the vapors from your fueling to pump back into their tanks, it probably goes to an activated charcoal filter, to keep it from escaping into the air and contributing to smog.

Ethanol has roughly 1/2 the energy density that gasoline does, meaning you have to burn twice as much of it to get the same amount of power. The chemistry works out, it consumes the same amount of oxygen to burn twice as much ethanol, so it works fine with your O2 sensor. Its just that your engine computer will recognize a lean condition with ethanol, extra O2 in the exhaust, then adjust by richening the mixture until there is the proper amount of O2 in the exhaust. I.E. injecting twice as much ethanol.

SO, if gasohol is 10% ethanol, and you have too burn twice as much to get the same mixture as gasoline, that means you should lose about 5% in mileage.

And I agree with the fiasco about alternative fuel ethanol has become. This is what happens when government gets involved instead of relying on market forces. Instead of market forces driving and developing the best alternatives, when they are viable, you get bad alternatives being pushed too early by politics.

 

[sjx40250]

My experience with ethanol is much more than 5% or 10%. More like 15-20%. That is for all three jeeps, 2 4.0l and one 4.7. Now that spring is coming I am starting to see small improvements.
With the loss in MPG, I end up buying more of that Arab oil in the form of gas! That is hardly an energy independent move! Too political (ignorant) and non-sensical for this old fart.

 

[BillBraski]

Until the US government invest in real alternative (Ethonal is not the answer yet, becuase its raising the price as we speak) like Hydrogen its gonna be expensive. Anyway, when I'm not driving my xj's, I drive a 1987 Suzuki Samurai that gets 29 MPG.

 

[PurpleCherokee]

I think that "real" alternative is to continue improving fuel economy, and start drilling and refining more oil! It's about moderation, not abolishing oil products altogether. What'd be amazing (which I've heard that there's actually progress being made towards) would be if we can come up with a fully synthetic "gasoline". I believe I heard about turkey manure being converted into fuel somewhere in MO.

 

[BillBraski]

Heres something I find strange. This weekend I towed a 88 porsche 911 (2800 lb. track car) on a 1000 lb. trailer 250 miles with my fairly built XJ (2 inch lift, very strong 4.0 with ported head and port matched intake, D44 rear that is a fresh salvage yard find and rebuilt) using E10 gas. I drove 120 miles to pick up the car with the trailer and got a estimated 10 MPG, it took nearly 3/4 a tank. On the way back with the car, I only used a little over 1/4 tank! I used cruise control in 3rd gear, 45 @ 3000 RPM both ways because of all the hills. I think the weight of the car helped push. I am still shocked.

 

[BillBraski]

I think that "real" alternative is to continue improving fuel economy, and start drilling and refining more oil! It's about moderation, not abolishing oil products altogether. What'd be amazing (which I've heard that there's actually progress being made towards) would be if we can come up with a fully synthetic "gasoline". I believe I heard about turkey manure being converted into fuel somewhere in MO.

Thats a bigger pipe dream than Hydrogen vehicles, we are truly addicted to cheap energy. Its a fact. If oil only lasts another 30 years, were screwed if theres no alternative.
Here in Wisconsin its 3.34 a gallon, and I drive only when I need to, it still costs me 70$ a week in fuel. People think that they can waste all the enery they want and it doesnt effect all of us. Thats partly why gas is so expensive.

Hydrogen can be a viable energy, it just takes lots of energy to make it so it can be used like this. I am all for Nuclear power to supply the power to produce hydrogen. Too bad all the loser environmentalists gotta bash it with 30 year old studys. Its cheap, clean and not as dangerous as billions of tons of coal being burned and wrecking the environment.

 

[PurpleCherokee]

Wait wait wait... so you didn't have the car on the way up?? So you totalled 240 miles to a tank, half of which was pulling a 3800lb load?

 

[BillBraski]

Wait wait wait... so you didn't have the car on the way up?? So you totalled 240 miles to a tank, half of which was pulling a 3800lb load?

Yeah, I just had the trailer on the way to pick it up. I don't get it, i have gotten better millage hauling loads before, but not like this.When I topped it off and I was shocked! I think its becuase of the weight pushing forword when I had the car coming back because of all the hills i was going down. I got way better mileage on the way back +3800 lbs.!!! But at 3000 RPM!

 

[PurpleCherokee]

That is kinda nuts... 240 miles to a tank is good for NO trailer OR car lol...

 

[RichP]

Personally I don't see the prices coming down, the oil industry had gotten a taste of the apple and will now squeeze it for all it's worth. It would not surprise me if they were quietly acquiring as many of the alternative energy stocks as they can manage so that when someone does make a breakthrough they are still in the catbird seat... Or maybe further down the food chain as in farms and crop land so they have a piece of the source just like they do with oil.

 

[falcon556]

Until the US government invest in real alternative (Ethonal is not the answer yet, becuase its raising the price as we speak) like Hydrogen its gonna be expensive. Anyway, when I'm not driving my xj's, I drive a 1987 Suzuki Samurai that gets 29 MPG.

Who needs the alternative when we have the real thing?
There is no shortage of oil, just government restrictions to the supplies.
It reminds me of the diamond industry, one large monopoly restricting the supply and controlling the prices.