To add to what MS has said above, the engine
and the transmission can have an enormous impact on the
acceleration and speed behaviour of a bus.
Basically, if you go from a 5-speed ZF transmission to a 6-speed ZF
transmission without changing the diff ratio or engine, you will
gain top-end speed. The same occurs if you change the diff ratio to
a high ratio.
The 6-speed ZF transmission has thus enabled operators to use
smaller engines (downsizing) to produce the same result. Instead of
12-litre engines, 9-litre engines can be used with a 6-speed
transmission and lower diff ratio to provide similar acceleration
and top-end speed characteristics of previous buses. Also note that
engine development has meant that new smaller engines actually have
much higher power and torque output of similar engines from, say,
10 years ago.
If 60km/h is the absolute maximum speed allowed in Singapore, a
5-speed ZF transmission would be the best choice, matched with a
very low diff ratio that allows cruising at 60km/h well within
"green zone" engine speeds for the lowest fuel consumption
figures.
Regards,
Dave[/b]
