Re: [A/S] ? Aftermarket Overdrives \ Underdrives ?

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

More gear ratios are often good. But the American Eagle and other such
over/under drives have to run a really long time to pay. I get 16 mpg
unloaded with my small Triton V-8 F-150, 15.5 with Caravel at 50 mph,
almost 18 with the air conditioning off and unloaded on good fuel. In
100,000 miles, I'll use 6,250 gallons of gas (at 16 mpg). If an
overdrive (and this truck already had too much overdrive with an OD in
the transmission and 3.08 axle, engine running under 1600 rpm at 55 mph,
lugging range) saved fuel, it would have to save 1,667 gallons to pay
for a $2500 installation cost. That's a 27% improvement required. $3500
installation typical of some auxiliary transmissions would be even
harder to justify in merely 100,000 miles.

I needed more pull, so I changed the rear axle gears from 3.08 to 4.10.
Pulls fine, didn't change the gas mileage unloaded at all. Cost $480,
plus about $35 for the new speedometer sensor gear.

Most diesel engines have a narrower preferred running speed range than
my little V-8 that likes to turn fast (HP rated at 4600 rpm) so more
gears may prove to be more beneficial.

Gerald J.