Gear swaps for better fuel economy

["Jason and Maria Colavecchia" <jasoncolavecchia@domain.elided>]

After hearing all this discussion about swapping to higher ratios
(numerically LOWER...please anybody lets put some kind of STANDARD together
we can all accept when referring to differential gear ratios!!!), I have to
jump back in again to voice my opinion....

DEPENDING on the tire size you are running, as well as the type of power,
your final drive ratio when in direct drive is CRITICAL to obtaining
adequate performance.  Making the engine RPM slower is not necessarily
going to mean better fuel economy. Often, lowering RPM speed at highway
cruise speeds may cut your fuel needs somewhat, but as soon as you are in a
stop and go, inner city operation, you will throw all that extra fuel you
saved right out the tailpipe.  Conversely, you could gear for maximum power
and torque and gain INCREDIBLE performance around town/off road, but suffer
badly on highway.

A decision MUST be made concerning the expected operating conditions of the
vehicle. Here is my suggestion for an all-around type of truck which sees
some off road use but is a daily driver:

31" tires...direct-type transmission:  Try either 3.54 or 3.73 gearing. 
Either will give you good highway power and also good off-road and city
driving control.  I know alot of people run 3.07's with 31" tires, and this
is adequate.  My experience is that this ratio is NOT adequate with the
V-304 or the 6-4 cylinder engines with 31" or bigger tires.  You tend to
lug the engine too much, creating unnecessary heat buildup in the heads,
and increasing carbon deposits.  Running the engine at around its full
torque capacity tends to bring on increased wear and is unnecessarily hard
on driveline components, especially u-Joints and transmissions.

4.09 gearing is acceptable, but does not help your highway operation unless
your are planning to pull a heavy load behind the Scout. THEN the 4.09 will
actually improve fuel economy over "lower" ratios due to the fact more
multiplied torque is available to the wheels as well and the engine running
in a more horsepower-oriented section of it's bandwidth.  4.09 gearing is
not as important with the bigger-displacement motors, especially the 392.

Example number 2...

My travelall currently runs a 345 with a wide box T-18 manual (direct) and
3.54 gears.  this stock setup moves this gigantic crate of a machine quite
acceptably, loaded or unloaded.  I am planning to upgrade the truck's
pulling power by NOT changing ring and pinion, but by opting for the IH
T-36 "short 4th" transmission.  This would let me keep my deep first gear
(almost identical to the current setup) and provide me with a very close 4
upper speed ranges.  Under heavy loads or going uphill on steep grades, I
can drop to the Close 4th gear, allowing for more torque multiplication and
simultaneously giving me the required horsepower to make the grade without
losing speed.  The T36 was designed to work hand in hand with IH engines,
and dropping to 4th doesn't overspeed the engine (with 3.54 gears), even at
60 mph....Currently, I would have to slow down to 55 or less MPH to drop it
into 3rd on the T-18 to provide any advantage of torque multiplication
without overspeeding the engine.

Jason and Maria Colavecchia
1979 IH Scout II,  V304, modifications ongoing
1969 IH 1210D Travelall 3/4 ton, V345,  restoring slowly