I mentioned in the thread about calculating speed based on engine speed,
gear ratio, rear end ratio and rolling tire radius. Randy has also mentioned
the different rear end ratios and tire diameters.
I know there are formulas for this but I thought I would share a method I use
to determine max speed in different gears.
Some understanding of the vehicle is required.First to be determined is max
governed engine speed. The engine will likely have a plate listing max engine
speed. Older engines with
mechanical fuel injection can be tampered with (a driver could do this)so
that they can exceed the manufacturers specs. A diesel shop could confirm this.
Newer engines almost always have electronic fuel injection and require
equipment and a code to change the governed speed. Again a diesel shop could
confirm the settings.
You are pretty safe with the published ratios (for the particular make and model
of transmission - there will often be on a sticker somewhere on the truck)
for the particular gear that you are interested in. A gear cannot really be
changed, the whole transmission would have to be changed.
The rear end ratios can be changed and have to be confirmed.
The circumference of the tires can easily be calculated (and I think there is
a formula for calculating the rolling radius - rolling radius is the adjustment
for the tire size based on the weight of the vehicle and load - I don't think
it makes a hill of beans difference, but you might as well cover it if you can)
Static load radius for given tires is available from the manufacturer (and
maybe rolling load radius)but this is based on loading to max specs and may not
be what you have.
The method I use to calculate max speed in any given gear:
In heavy truck collisions the truck is often available (especially truck vs car,
the front axle may be ripped out but the drive train is intact). Sometimes the
vehicle cargo may still be on, if not try to duplicate the weight (this will
cover the rolling radius argument)
Place a mark on the ground and on a drive tire at the six o'clock position.
Place a chalk mark on the drive shaft and turn the drive shaft by hand to
eliminate any play/lash. Either have the vehicle driven slowly in low gear or
have a tow truck tow it in low gear.
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