Hi everyone,
Regarding axle ratios, I personally feel there is nothing to lose in going
to as low a ratio as is possible. Certainly anything up to a 3.90 or so
would not be too much, camper special pickup trucks routinely use 4.11 and
4.56 ratios after all.
GM recommended a ratio of 3.42 in the early 1970's on most full size car
lines with 454-455 engines in the trailer towing packages. This was
considered adequate to tow a 7000 lb travel trailer with, say, a Caprice or
LeSabre. 1973 up Three Quarter and One Ton pickups with Camper Special
Equipment had a mandatory 3.73 ratio as a minimum requirement.
For best economy for travelling under a heavy load it is desirable to gear a
vehicle for operation at or up to ~400 rpm below the net torque peak at the
usual road speed. This assumes the engine makes adequate torque to carry the
load at that rpm. I generally would shoot for a 60 mph cruising speed, and
appropriate gearing for that. Driving a large vehicle over 60 starts to
really use a lot of power and fuel, and setting them up for say 75 mph
cruise is kind of pointless, theres not much scope for good economy at that
speed anyhow.
Aerodynamic drag is the largest issue from a cruising perspective. According
to a chart I have in a GMC Dealer Data Book heres the Air Resistance Demand
Horsepower figures between 30 and 70 MPH for a relatively square nosed
vehicle of 60 sq/ft frontal area.
30 7.8
35 12.3
40 18.4
45 26.2
47 29.9
49 33.8
50 36.0
52 40.4
54 45.3
56 50.5
58 56.1
60 62.1
65 79.0
70 98.7
As you can see, horsepower just for wind drag is very significant. Driving
56 mph takes about half the output to overcome the wind that 70 mph does.
Rolling Drag losses go up proportional to road speed/weight, and not
exponentially as wind drag. Twice as fast or twice the weight = twice the
drag for weight and rolling resistance for the most part. Takes about 20HP
to overcome rolling resistance of a GMC at 60-65 MPH. At 30 MPH it would be
more like 10HP.
Another thing to consider is there is approximately 145-160 net HP availible
in the first place from which you can deduct the drag;-)
I would definitely think the factory installed 3.07 is too tall for the
application. The GMC would be 'driveable' right down to a 2.41 in all
likelihood, but with disasterous economy and performance implications. One
real gas eater is operating an engine below 7" Hg or thereabouts vacuum
readings- your power enrichment system is activated and mixture strengths
are increased quite drastically and this pulls fuel economy way down. If a
higher numerically ratio set of gears permits you to cruise and handle minor
grades and headwinds without a susbstantial loss of manifold vacuum, you
will get better economy. Liberal use of second gear can help also. Keep
vacuum over 8" as much as possible.
Just for reference, the tires on a GMC rolls about 675 revolutions per mile.
Multiplying 675 times the axle ratio gives the engine RPM at 60 MPH. 675 x
3.07 = 2072.25 rpm @ 60 MPH.
Dividing 2072/60 = 34.5 rpm per MPH. At 100 MPH the GMC would only do 3450
rpm at this rate- thats a pretty tall ratio! Of course there is slight slip
at the torque converter, so the actual figure is slightly higher as
indicated on the tachometer.
The torque peak of the 455 is about 2800 RPM as I recall, this means a
range between 2400-2800 RPM would be the ideal for cruise economy. 2800/675
= 4.14, which would be about the lowest ratio you'd want, and 2400/675 =
3.55 which would likely be the ideal, at 60 mph average cruise speed in any
event. A 3.42 is not far off that, and with slight converter slippage would
be pretty darn close to ideal.
I've seen the Caspro Power Drive Chain system and the aforementioned box for
switch pitch control made by the Toro List member and they are both
excellent units and I will be using both myself. The switch pitch converter
is desirable if you have an opportunity to obtain one, not vital, but a
definite refinement.
Anyhow, basically to sum, the benefits of a high numerically ratio axle are
substantial for the typical owner, and although even a 2.73/3.07 etc. will
work and drive acceptibly with adequate performance, the penalties for
economy and performance would probably substantially pay the costs of an
upgraded axle ratio in a short time. Go for the 3.21 3.42 3.54 3.73 if you
can get them!
Hope this helps!
Brent
Regarding axle ratios, I personally feel there is nothing to lose in going
to as low a ratio as is possible. Certainly anything up to a 3.90 or so
would not be too much, camper special pickup trucks routinely use 4.11 and
4.56 ratios after all.
GM recommended a ratio of 3.42 in the early 1970's on most full size car
lines with 454-455 engines in the trailer towing packages. This was
considered adequate to tow a 7000 lb travel trailer with, say, a Caprice or
LeSabre. 1973 up Three Quarter and One Ton pickups with Camper Special
Equipment had a mandatory 3.73 ratio as a minimum requirement.
For best economy for travelling under a heavy load it is desirable to gear a
vehicle for operation at or up to ~400 rpm below the net torque peak at the
usual road speed. This assumes the engine makes adequate torque to carry the
load at that rpm. I generally would shoot for a 60 mph cruising speed, and
appropriate gearing for that. Driving a large vehicle over 60 starts to
really use a lot of power and fuel, and setting them up for say 75 mph
cruise is kind of pointless, theres not much scope for good economy at that
speed anyhow.
Aerodynamic drag is the largest issue from a cruising perspective. According
to a chart I have in a GMC Dealer Data Book heres the Air Resistance Demand
Horsepower figures between 30 and 70 MPH for a relatively square nosed
vehicle of 60 sq/ft frontal area.
30 7.8
35 12.3
40 18.4
45 26.2
47 29.9
49 33.8
50 36.0
52 40.4
54 45.3
56 50.5
58 56.1
60 62.1
65 79.0
70 98.7
As you can see, horsepower just for wind drag is very significant. Driving
56 mph takes about half the output to overcome the wind that 70 mph does.
Rolling Drag losses go up proportional to road speed/weight, and not
exponentially as wind drag. Twice as fast or twice the weight = twice the
drag for weight and rolling resistance for the most part. Takes about 20HP
to overcome rolling resistance of a GMC at 60-65 MPH. At 30 MPH it would be
more like 10HP.
Another thing to consider is there is approximately 145-160 net HP availible
in the first place from which you can deduct the drag;-)
I would definitely think the factory installed 3.07 is too tall for the
application. The GMC would be 'driveable' right down to a 2.41 in all
likelihood, but with disasterous economy and performance implications. One
real gas eater is operating an engine below 7" Hg or thereabouts vacuum
readings- your power enrichment system is activated and mixture strengths
are increased quite drastically and this pulls fuel economy way down. If a
higher numerically ratio set of gears permits you to cruise and handle minor
grades and headwinds without a susbstantial loss of manifold vacuum, you
will get better economy. Liberal use of second gear can help also. Keep
vacuum over 8" as much as possible.
Just for reference, the tires on a GMC rolls about 675 revolutions per mile.
Multiplying 675 times the axle ratio gives the engine RPM at 60 MPH. 675 x
3.07 = 2072.25 rpm @ 60 MPH.
Dividing 2072/60 = 34.5 rpm per MPH. At 100 MPH the GMC would only do 3450
rpm at this rate- thats a pretty tall ratio! Of course there is slight slip
at the torque converter, so the actual figure is slightly higher as
indicated on the tachometer.
The torque peak of the 455 is about 2800 RPM as I recall, this means a
range between 2400-2800 RPM would be the ideal for cruise economy. 2800/675
= 4.14, which would be about the lowest ratio you'd want, and 2400/675 =
3.55 which would likely be the ideal, at 60 mph average cruise speed in any
event. A 3.42 is not far off that, and with slight converter slippage would
be pretty darn close to ideal.
I've seen the Caspro Power Drive Chain system and the aforementioned box for
switch pitch control made by the Toro List member and they are both
excellent units and I will be using both myself. The switch pitch converter
is desirable if you have an opportunity to obtain one, not vital, but a
definite refinement.
Anyhow, basically to sum, the benefits of a high numerically ratio axle are
substantial for the typical owner, and although even a 2.73/3.07 etc. will
work and drive acceptibly with adequate performance, the penalties for
economy and performance would probably substantially pay the costs of an
upgraded axle ratio in a short time. Go for the 3.21 3.42 3.54 3.73 if you
can get them!
Hope this helps!
Brent
glad you enjoyed it!
