Quote:
Originally Posted by Howard
Hanno,
I would have thought it perfectly reasonable to expect the heavier trucks to run at higher revs, making use of the available power in the upper rev range, versus the smaller that would be less unhappy chugging along at lower revs.
(Boy, this stuff is difficult to put into words, isn't it!?)
H
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Except that the resultant effect is the other way around to what you describe. If both the 30cwt and 60cwt have the same diff ratio (and I've dropped the "F", because both Ford and Chev have the same diff ratios in each weight class), then for a specific road speed the vehicle with the smaller 10.50-16 tyres will be running at higher engine revs to keep up with the vehicle with the larger 10.50-20 tyres.
While the Ford and Chev engines have different torque curves (which result in a different "Sweet spot" cruising speed), tyre differences of around 10% between the Medium and Large CMPs will see a mixed convoy experience the downshift issues Phil describes. If the 60cwt's rpms are approx 10% lower than the 30cwt (and carrying a heavier load), it will naturally need to shift down a gear sooner than the lighter truck. In fact, with the larger tyre size, it may actually be more efficient (for the convoy, not for fuel!) for the larger truck to be revving high in 3rd gear to keep up with the lighter truck chugging along in 4th.
Note that within each brand of CMP, all weight trucks have identical engines (and power and torque figures), and identical gearboxes (with the same ratios). The only variation for speed is diff ratio and tyre size