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Double Overdrive??


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Eagle is right, get to low of rpm and it turns into a dog.

My 5 speed XJ with 4.10's got 21 mpg with 265/75/16 tires running at 80 MPH from CT to Tennesee.

It was spinning around 2500 or more the whole way.

 

IMHO 2500 RPM is the sweet spot for these engines, and 2000 to 3000 seems to be more or less an optimal operating range.

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If you're looking for fuel economy out of a 4.0 equipped MJ, just park it and get a little Jap car for commuting, groceries etc.

 

I agree.

Last tank on my mj,4.0/auto/3.55 gear/265-75-16/corrected speedo (according to radar signs), all short work commute miles, was 12.56mpg...sigh.

My LT1/fi/700r4/3.55s/corrected speedo, XJ does the same commute at 14.5-15, best on a 1500 mile trip has been juuust under 22.

PS I bought a LJC just for that reason...I hate it.

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Back in the forties when gas was cheap , .15cents a gal, A guy walks into a Cadillac dealership looking for a new car. After the demo ride and all the sales talk the guy asks the salesman, what kind of mileage did it get. The salesman looks at him and repiles, If you have to ask then you can't afford it.

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I saw that after I entered my post. My bad.

 

 

 

I think you're making a serious mistake. I think your gas mileage will be worse, not better.

 

Let me explain: Completely aside from the fact that the most efficient engine speed is the speed at which it produces maximum torque, you should also take into account the heritage of this engine design. The injection dates to 1987, but the basic engine is an AMC engine that was first introduced in 1964. I grew up in a Hudson ==> AMC family, and we owned a number of different cars with earlier versions of this engine. Ours were all 3-speed manuals, and they all came with small-ish tires and 3.08 gears. The overall drive ratio worked out to 24 MPH per 1000 RPM, which conveniently came to 2500 RPM for a 60 MPH cruise. That's what the engine was designed to run at.

 

But we didn't cruise at 60 MPH. My brother and I were gearheads. We raced ... any kind of racing you could think of. And, being young and immortal, we drove perhaps "a bit" in excess of posted speed limits. This is New England, however, so prolonged cruise at 90 MPH wasn't viable, but 75 was pretty customary. Another neat round number from that final drive ratio: 3000 RPM gave us exactly 72 MPH. And cruising at 3000 RPM or a bit more still gave us decent performance and very respectable fuel economy ... although my Rambler American wouldn't get 28 MPG at 75 MPH, it was still in the mid-20s.

 

I had the American while I was in the Army and stationed in Maryland, and I got it in my head that it was important to be able to break 100 MPH in a box on wheels. I tweaked the tuning, installed a straight-through glasspack muffler, and converted to an open air clear ... and very late one night on I-95 somewhere a bit north of Edgewood Arsenal, MD, I pushed the box-on-wheels up to a calculated 104 miles per hour. I was a happy camper.

 

Winter arrived. I needed snow tires, and in 1967 the pay for a PFC-E3 wasn't anything to get excited about. My brother at the time was driving a bigger Rambler with a V-8, and he had just switched over to Michelan radials. So he gave me his bias-ply snow tires from the previous winter. I don't remember the sizes, because they were in one of the old tire sizing schemes, but the bottom line is they were a couple or three sizes larger than what came standard on my American, and probably about two inches larger in diameter. And with those snow tires on and no other changes, my top speed DROPPED from 104 MPH to 80 MPH. The gas mileage took a dive, too, but I don't recall the exact figure. The speed, though, is permanently etched in my brain because I remember being out there on an empty stretch of I-95 looking at the speedometer reading 74 with my foot pressed all the way to the floor, thinking "WTF?"

 

Bottom line, the power curve crossed the drag curve. The engineers knew what they were doing when they chose the gear ratio and tire size, and they had optimized the drive train. 104 MPH in stock configuration was 4333 RPM, which was right about the horsepower peak of the engine. What a surprise -- once you reach max power, you can't go faster! I was shocked, I tell you ... shocked!

 

My best guess is that the larger snow tires were about 2 inches taller. Assuming that the originals were approximately 25" tires, which I think is about correct, that's about an 8 percent difference. Doesn't sound like a lot. But with the stock tires, 80 MPH would have been 3,333 RPM. Cut that by 8 percent for the bigger tires, and that's 3,067. That's where the drag curve crossed the power curve.

 

And you want to cruise at 90 MPH at 2000 RPM?

 

NOT ... GONNA ... HAPPEN. Not in this world.

 

Well stated, and I agree completely.

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Without digging deeply into physics yet (we could, but lets hold off for now), consider this.

Today I can make 90 mph, and I can accelerate beyond 90 mph, on a flat road. Therefore,

I have more than enough power while turning just over 2900 rpm. Can I make 90 mph

with reduced RPM? The answer has to be "yes". The question then becomes, at what RPM

does the answer become "no". Since I don't have an affordable dyno in my neighborhood,

I've got no reference to actual available power other than this generic chart:

which shows the nearly linear HP increase and the famously flat torque curve. I'm

likely a bit better than stock, but not too much. If the engine's "sweet spot" is 2500 RPM

(and I agree that's a good area), and if I can reduce my RPM ~20% or so to get into that

spot, I may, or I may not, still generate enough HP to reach 90. A charge up the Pd/Cd

calc hill would tell for certain (with an accurate plot), but I still contend that I could operate

at my desired speed range, at lower RPM's, and not have the fuel rate plummet. It may

not be 90 at 2000, it may be 88 at 2300, but that's ok. Now if you ask whether it's worth

doing, that's a much easier question: "Not in this world". Merry Christmas!

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