Re: [Amc-list] T-5 OD gear

[farna@xxxxxxx]

It sort of points to camming differences, but not as much as you think. I run a 258 "econo-power" cam from NAPA that's designed for low and mid range power in my 4.6L. I don't have the specs handy, unfortunately. My car would pull 3.08 rear gears with the 0.70 OD with no porblems, but overall economy fell by 2 mpg compared to 3.54 gears I'm running now. If the engine isn't near it's torque peak highway mileage will suffer. Logic states that the 3.08 gears should have been great with a torquey engine, but cruising at 65 mph only ran it at 1800 rpm. So I agree with you that my cam obviously "likes" to run at 2000 rpm or more, which would be 73 mph w/3.08 gears. Not to fast, but running at a slightly lower speed (65-70) resulted in less rpm than the engine wanted, but not so little that a downshift to direct drive was warranted. 

I still remember my dad's 83 or so Ford F-150 w/300 six and the OD four speed (fourth was OD). It was the same way -- wound up in third at 60 or so, but not enough rpm to economically pull fourth (OD) on anything but level ground. If there weren't any hills the high OD and rear axle might work, but with varying conditions it's false logic. That won't keep manufacturers from offering it anyway! The Eagle 4x4 was the same way -- only about 1 mpg difference between AWD or selectable 2/4WD, but consumers *believed* it made enough difference that they readily paid the $300-$400 more for a selectable transfer case. The same with the high rear axle ratios and OD -- the engine works to much at lower rpm (though it won't "lug" or labor to much) to get the expected economy on anything but a long level, steady speed run. There just aren't many roads like that, except out west and some mid US areas. 

---------------
Date: Tue, 22 May 2007 11:22:10 -0400 (EDT)
From: adh@xxxxxxxxxxxxxxxxx (Sandwich Maker)


" From: farna@xxxxxxx
" 
" The 0.62 OD gear is probably going to be a bit tall with a 3.55
" axle. I have a 0.70 OD (early AW4) and cruise at 76 mph @ 2400
" rpm. With a 0.62 OD my car would be running 85.6 mph at the same
" rpm, 71.3 @ 2000 rpm. You need to cruise above 1800, preferably
" around 2000, with the 4.0L for best efficiency. 2500+ rpm will
" cause the computer to enrichen the mixture enough that you'll
" notice the mileage difference -- found that out on my 93 Cherokee
" years ago. My Renix engine acts the same. That's why I geared and
" adjusted tire size to cruise around 75 @ 2400 rpm. I generally
" cruise at 70 (2200 rpm). I'm running 205/70R15 tires, yours may
" be smaller. A 205/70R14, for example, would run 82 mph @ 2400 rpm
" w/3.55 gears/0.62 OD, 69 mph @ 2000. That might be better. Just
" means you'll only use fifth on the interstate (or similar
" highways).

this points up camming differences.  the original mustang pulled that
0.62 5th with iirc a 2.73 axle, but that was the factory setup from
the start, and the engine must obviously have been set up for it.

cherokees with the 4.0 and auto are geared 3.55.  the smallest tires
they run are 215/75r15s, with 225s and 235s more common.  4th isn't
usable at all below about 50 and not of much use until you get over
60.  65 is about 2200-2300 and that's comfortable.

cherokees with the 4.0 and stick are geared 3.07, and that's widely
considered to be tall gearing for them.

iirc the stock 4.0 cam has something like 270* duration.  if you swap
it for something oriented for lower-rpm operation - like a 2bbl 258
cam with 257*, or a '60s-'70s 232 cam with 244* - you can efficiently
pull all sorts of tall gears.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
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