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AW4 with 4:10's too big for a daily driver?


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Run the 215s until they wear out, then move up to 235/75s. The 235s are almost exactly the same diameter as 30x9.50s. With 235s behind an AW4 you'll be turning about 2200 RPM at 60 MPH and 2580 RPM at 70. That puts your cruising RPM right in the "fat" part of the torque curve, and that's where you want to be for maximum efficiency.

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You are going 180 degrees in the wrong direction for peak highway MPG. Some of the gear sites and 4wd catalogs have a chart listing tire size vs. ratio with it broken down into three sections: Highway MPG vs. all-around performance vs. off-road only. http://www.4wd.com's printed catalog lists 4.10's with a 28" tall tire as returning poor fuel economy but increased towing power. For that tire size they list 3.55's as the best all-around performance and 3.07's for best highway fuel economy. The cheapest way to go for 3.07's would be to find a set of generally cheap axles from a 5-spd XJ/MJ and throw them under your truck. For 4.10's you would have to go to a 34" tall tire to get in the "good highway MPG" range. Remember that peak MPG can be seriously different for around town vs. highway driving. Great highway gears may make your engine work too hard to move the vehicle in heavy light-to-light type driving and driveability suffers. The factory axle gears with a factory sized tire generally offers the best compromise on power vs. MPG and your rig should have the preferred 3.55's in it. Most people who change tire size generally change the gear ratio to duplicate the factory combo's numbers in order to maintain that balance unless it is a towing or true off-road vehicle. The cam can make a difference if it radically moves your power band. Use a chart like

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I'm planning on upgrading to 4:10s on my DD manche with 215/75 R 15 tires (27.7" diameter) with a Renix 4.0,is this too steep of a ratio? if not can I go bigger? This rig will see loooong highway trips any input on driveability and mileage is welcome,thank you.

 

The Renix gets best fuel economy at lower rpms. With the stock 3.55 gears and 235/75R15 tires (a bit bigger than yours) I got 24mpg as long as I stayed at 60mph. Going 70 I only got 19.with 4.10 gears the engine rpms are about the same going 60 as going 70 with the stock gearing.; you would love the performance but not the mileage. For optimum gas mileage at highway speeds (70mph) you would probably want 3.07 gears out of a 6 cylinder/manual transmission Comanche or Cherokee, but performance would suck.

 

All around I would stick with the stock gears.

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You are going 180 degrees in the wrong direction for peak highway MPG. Some of the gear sites and 4wd catalogs have a chart listing tire size vs. ratio with it broken down into three sections: Highway MPG vs. all-around performance vs. off-road only. http://www.4wd.com's printed catalog lists 4.10's with a 28" tall tire as returning poor fuel economy but increased towing power. For that tire size they list 3.55's as the best all-around performance and 3.07's for best highway fuel economy.

Forget what those charts say -- they're wrong.

 

I bought my '88 Cherokee new. 4.0L 5-speed, so it came with (and still has) 3.07 gears. Original tire size was 205/75-15s, but I've been on 225s or 235s (depends on what set I have handy) for many years. I'm in Connecticut, so we don't have long stretches of 65 to 70 MPH highway. Typical highway cruise is about 60. And at that speed I get better gas mileage staying in 4th than when I use 5th.

 

Years ago, I determined that I'd be better off running 3.73s. But then I got the '88 MJ, with 31" tires and 3.07 gears, so the new gears went into the MJ and the XJ stayed stock.

 

You gents are forgetting two things: First, both the XJ and the MJ are "boxes on wheels." Aerodynamic drag increases exponentially with speed. The fact you get worse mileage at 70 than at 60 is not due to engine RPM, it's due to increased drag and there's not a damn thing you can do about that other than drive slower. I had the same thing with the 1999 Grand Cherokee I owned. That was with the newly-designed 4.7L V8. On a trip from CT to wstern PA (about 500 miles) I logged about 19 MPG on the way out, cruising at 70. On the way back, I had plenty of time so I set the cruise for 60 and I got 22 MPG.

 

The other thing you forget is that the base engine on which the 4.0L is built was originally designed by AMC back in 1963, first appearing in the 1964 Rambler. Overdrive was extremely rare back then, and a standard transmission was a 3-speed. AMC was noted for building economy cars. Standard gearing was 3.08, with small tires. The typical gearing resulted in 2500 RPM at 60 MPH. That meant that a 72 MPH cruise speed ran the engine at 3000 RPM. Toady, you lads consider those engine speeds as excessive, but that's what the engines were designed for, and they would last 300,000 miles as long as you changed the oil. My old Rambler American used to routinely deliver 28 MPG set up like that. Of course, it was lighter in weight than an MJ, and we didn't have gasoline diluted with ethanol. But that's the engine speed range that's going to deliver the best economy.

 

3.07 in today's XJs and MJs was not selected because it's best for any real world purpose. They used that ratio because it generated good results on the unrealistic Federal mileage tests. Seriously. With stock tires in 5th gear and running 3.07 axles, 70 MPH is 1960 RPM. That's several hundred RPM below the engine's torque peak -- and several THOUSAND RPM below the torque peak for the HO engine. You can't deliver optimum economy running well under the torque peak.

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I agree that the charts are pretty generic. I think they work better with an auto tranny than a stick, as well. The difference between the 1st gear ratio's between the two types is pretty extreme in many cases. But I still maintain that the factory gear ratio's are a good compromise for power and efficiency and ton's of people who re-gear agree that going back to a near factory balance is preferable.

 

Isn't there a difference between peak horse power or peak torque and maximum efficiency? The idea is that you need peak power ( of either type) to reach a given speed but less to maintain it. If peak horsepower and peak torque translated to maximum efficiency it doesn't seem like the peaks would be some 1,300 rpm's apart? Just curious. Even if the assertions about spinning the inline sixes near the redline is true, I doubt anyone recommends keeping the RPM's at the 4,600 RPM mark (which is peak horsepower for the chart for a '92 4.0 that I located online) for a 500 mile highway trip for peak mileage.

 

BTW: by switching from a 27.1" tall tire (205/75/15) to a 28.8" tall tire (235/75/15) you changed your 5th gear RPM's at 65 MPH from 1954 to 1834. 4th gear for the same speeds is 2474 & 2322, still very far below peak TQ or HP figures but hardly several thousand (around 1,100 (TQ) to 2,300 (HP)) below. Again, peak does not necessarily = most efficient, either. Under modern testing perimeters a 196" Rambler I6 would probably produce less than 100 hp peak NET ( they only made 138 GROSS) versus 170+ NET for the Renix 4.0, which could really help pull the vehicle at a lower rpm as the 4.0 probably makes considerably more TQ & HP at 1900 RPM than the 196 did even at peak. The low tech carb'ed 196 may be the distant basis for the 4.0, but the injection and ecm alone make them completely different animals. The "one setting fits all drags & rpm's" un-adaptability of a carb, especially an ancient 1 or 2 barrel, versus even the Renix FI system's on-the-fly adjust-ability is a huge difference maker. Move onto the OBD1 4.0 H.O. and your are really talking apples and oranges.

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Ignoring drivetrain drag, rolling resistance and air resistance, peak efficiency is at the same rpm as the torque peak. Add in the other things and it ends up being slightly below the torque peak. Once you pass the torque peak gas mileage will take a nose dive.

 

the torque peak on the HO engines being so high explains why my 96 Cherokee bone stock got 22 mpg (averaged over an 800 mile trip) running 70, but 21 mpg (again averaged over the same 800 mile trip in the opposite direction) running 60, even though the 60 mph run went down 1000 feet elevation and the 70mph trip went the other direction, ending 1000 feet higher than the start.

 

All I know is that my 4 cylinder, running 235/75R15 tires used to get 19 mpg at 70mph. Now after the addition of an overdrive gear but still running the same 3.55 gears it gets 24mpg at 70mph. Mileage at 60mph seems to have gone from 27 to 28, but with a decimal added are close enough as to be identical (27.4, 27.7) due to rounding errors and never being able to fill the tank to the exact same level.

 

I was originally going to regear it to 4.10 after the addition of 5th gear until I realized 4.10 in 5th gear on an ax5 is the exact same final drive ratio as 3.55 in 4th gear. It would have put me back at 19mpg....

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Even if the assertions about spinning the inline sixes near the redline is true, I doubt anyone recommends keeping the RPM's at the 4,600 RPM mark (which is peak horsepower for the chart for a '92 4.0 that I located online) for a 500 mile highway trip for peak mileage.

Who "asserted" spinning it near redline? I wrote that maximum efficiency is when you cruise at the torque peak ... nobody said anything about the horsepower peak. For the Renix 4.0L, the torque peak was at 2400 RPM and the horsepower peak was at 4750 RPM. Highway cruise at around 2400 to 2600 RPM is, as I wrote, right in the "fat" part of the torque curve for the engine.

 

Again, peak does not necessarily = most efficient, either.

Basically, yes it does. In fact, the working definition of torque is "horsepower per revolution," so the torque peak IS, by definition, the RPM at which the engine itself is operating at maximum efficiency. As has been noted, exponential increases in various drag factors as a function of vehicle speed enter into it and cannot be ignored, but the simple fact is that a highway cruise that's almost 1000 RPM below the torque peak (like the way the 5-speeds came from the factory) cannot deliver optimum fuel economy in the real world. As proven by the fact that my '88 Cherokee gets as good mileage in 4th gear on the highway as it does in 5th.

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99 xj...built 4.0. cam's sweet spot is 1900-ish.

 

still get my best mileage @ 1500rpm, with 235's, and 3.07 gears. 3.55's, same setup, got less mpg.

 

average tank (mixed driving, under 60mph) gives me 20.8 mpg. highway, I stay below 70. anything over that and I'm getting under 20mpg.

 

 

and 1st gear still is pretty short. wish I had another gear to run it up to.

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