Re: [AMC-list] AMC-list Digest, Vol 15, Issue 5

[Jesse <j2sax@xxxxxxxxx>]

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.  I would take a look at your 390's secondary circuit to see if your opening up while cruising and make some adjustments or even play with your secondary jetting.  It sounds like you may even need to lower your gears.  Check your plugs immediately after a long highway cruise and see if they are looking "fuelish" (let the engine cool alittle first!).   Often times this condition can be hidden in vehicles that run clean around town.  Also check RPM at speed with below formula to see if you're losing power to high end clutch slippage.  Can be VERY subtle! 

Here's the formual to figure top RPM at a given speed.  If you have an OD, first we figure your gear ratio when run with the .76 (aprox) 5th gear of the trans.  So, 2.73x.76= 2.07 - 3.08x.76 =   2.34, 3.54x.76=02.69, 3.73x.76= 2.83, 4.11x.76=3.12.  

Take your choice of ratio, plug in your tire size and see what RPM your run at at a given MPH

If we work the formula: rpm = mph x gear ratio x 336 / tire diameter
 
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!  

Many times with smaller engines and taller tires they are thought to be working too hard, but the main problem is that the engine is not operating at a good RPM range for it's power band which causes the "lugging" uphill and such.  If your RPM seems way off from what you expect after working teh formula, you might want to make sure your clutch isn't slipping due to adjstment/wear.  

Newer Wranglers with 4 cyl 5/6 speeds have a 4.11 rearend ratio, so you can see that even a 3.31 is pretty tall for any power on teh highway, especially when you hit any kind of altitudes!  

If you are working these calculations with an Auto trans. you have to allow for some slippage if you do not have a lock up trq cnvrtr/  


>>>>>>>>>>>>>The tires aren't too tall, though I use a size or two smaller to reduce diameter and help a bit. All FSJs are 4x4, Jeep didn't make another 2WD until the Commanche came out in 86 or 87, everything prior to then was 4x4 back to 63. The old Willys wagon could be had in 2x4, and maybe the Willys pickup, possibly after 63. The 4x4 stuff is mainly more weight when in 2x4 mode. So throw a couple hundred pounds in the back of the wagon and go! The good thing about the 4x4 is you have low range, but it's a 4:1 ratio! Top speed is about 50 mph, and the six sounds like it might want to come apart if pushed any further then (I've never gone over 45 in low range). If only the t-case was synchronized so you could go from low to high on the move! Even the Commanche with five speed only had 3.08 gears, not really low enough, but 1st gear is a bit lower in those, 5th only useful for interstate cruising at 60+. Lower and you have to shift out of OD for all
 but minor hills. I have a Holley 390<<<<<<<<<<<<<<<<<
/4V carb on mine with a Clifford intake. 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.
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