The Mack M123 (G792) was a 10-ton 6x6 semi-tractor introduced in 1955; the Mack M125
was a heavy cargo truck version of the M123. The M123 was used to tow
tank transporter trailers while the M125 towed field artillery pieces.

Engine and driveline

All M123s and M125 built in the 1950s had a LeRoi T‑H844 engine, a 844 cu in (13.8 L)
overhead valve V8 gasoline engine developing 297 hp (221 kW) at 2600 rpm and 725 lbf⋅ft (983 N⋅m) of torque at 1700 rpm. M123A1s built in the 1960s had a Cummins V8‑300, a 785 cu in (12.9 L) naturally aspirated V8 diesel engine
developing 300 hp (220 kW) at 3000 rpm and 580 lbf⋅ft (786 N⋅m) torque
at 2100 rpm. This engine was also retrofitted into M123s to make
M123E2s. No M125s were converted to diesel power.^{[1][url=en.wikipedia.org/wiki/M123_and_M125_10-ton_6x6_trucks#cite_note-FOOTNOTEDoyle2003283–288,_290-3][3][/url][5][6]}


The drivetrain was a repeat of that in the Mack NO, with a Mack TRDXT72 5 speed transmission
with the transfer case mounted solidly to the rear of the transmission.
This transmission was direct in 5th. The transfer case had a 2.50:1 low
range, engaged the front axle, and had a power take-off to operate the
winch(es).^{[url=en.wikipedia.org/wiki/M123_and_M125_10-ton_6x6_trucks#cite_note-FOOTNOTEDoyle2003283–288,_290-3][3][/url][7][8][9]}


The front axle was an unusual triple reduction type which did not need universal joints
on the outer ends and allowed a tighter turning radius. The two rear
axles were a double reduction type. Final drive ratios were 9.02:1 for
gas powered trucks and 10.11:1 for diesels. The driveline for every M123
and M125 was built by Mack, regardless of who manufactured the rest of
the truck.