Re: [AMC-list] Jeep mods/mileage

[Frank Swygert <farna@xxxxxxx>]

Jesse, did you read the whole message? What I said was:
"It will cruise at 75-80, but gas mileage is cut nearly in half doing. Did it once, checked mileage -- 10 mpg. It normally gets around 17-18 in all around driving."
As long as I stay at or under that 65 mph threshold where wind resistance starts to really take a toll the truck gets 17-18 mpg. Only when pushing it and "standing in the four barrel" does gas mileage plummet. I have a vac secondary carb, but I've got it rigged so that the secondaries open at about 2/3 throttle whether there's enoug h vacuum to pull them open or not. The engine has over 100K on it and I don't think it produces enough vacuum sometimes to pull the secondaries open. I may have had them open on that high speed run more than needed.

I've done the OD gear ratio calculations. I was going to run the thing with 3.54 gears and the four speed -- no OD. Top cruising speed would be limited to 60-65. With the five speed it will be much better, but I don't expect to cruise at 75 with it. 65-70 will be about it except for a possible rare occasion such as that 75-80 run. 65 will put it at 1895 rpm, which will be low in the power band but should be okay with a light or no load. Fifth gear will only be used for high speed cruising, as 60 would only run the engine at 1750 rpm, too low for my comfort. Of course I might just go down to a 30" tire -- then 60 would be back up to a more comfortable 1808 rpm. Still a bit low, but tolerable. Another inch drop to 29" tires only brings it up to 1870 rpm. I don't need really big tires, 28" would be the lowest I'd go, but I don't want it to be too high in rpm in the lower four gears either. When towing or hauling much I'll never use fifth gear anyway. I'm sort of stuck with the 3
.54 gears though, don't want anything higher -- the 2.73s have GOT to go!! 3.08 is what the Cherokee/Commanche uses, and I always thought they were a bit high when used with larger than stock tires. I forget what tire size my Commanche had, only that they were larger diameter than stock. It could have used 3.31 gears easily!


-----------
Date: Wed, 2 Jun 2010 15:48:20 -0700 (PDT)
From: Jesse<j2sax@xxxxxxxxx>
To:amc-list@xxxxxxxxxxxx
Subject: Re: [AMC-list] AMC-list Digest, Vol 15, Issue 5
Message-ID:<897090.57332.qm@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
Content-Type: text/plain; charset="iso-8859-1"

While I agree that these bricks aren't made to slice and dice through the air, 10 MPG is abysmal!? My 360 4 spd J20 did better than that as did all my Auto 360's at that same speed....

3.54 gears (with .76 OD) at 75 MPH with 31 " tires
RPM= 75x2.69x336/31= 2187 RPM?... sounds pretty good for a 258.? Now let's look at the resuls with a 3..08 rear.

RPM= 75x2.34x336/31= 1902 RPM getting pretty low in the power range.? Let's look at the adjusted 2.73 with OD

RPM= 75x2.07x336/31=? 1682 RPM YIKES.. won't have much power there!?



--
Frank Swygert
Publisher, "American Motors Cars"
Magazine (AMC)
For all AMC enthusiasts
http://www.amc-mag.com
(free download available!)


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