Thanks gents!
It does seem from the numbers on the Chevy specs page (
http://www.mapleleafup.org/vehicles/.../chevspec.html) that the vehicles could all be significantly overloaded from their 'class'. If I take the MGR (in lbs) and subtract the CW (in lbs) given in that table, and then use 112lb per cwt, I get the following payloads in cwt:
1. C8 - MGR 7250, CW 5750, payload 13.4 cwt
2. C15 - MGR 11200*, CW 4805, payload 57 cwt**
3. C15A - MGR 8500, CW 5485, payload 26.9 cwt
4. C30 - MGR 11200, CW 5920, payload 47.1 cwt
5. C60S - MGR 15700, CW 6503, payload 82.1 cwt
6. C60L - MGR 15700, CW 6658, payload 80.7 cwt
* if you add the axle weights in the table for this vehicle, you get a total of 7570 lb which seems far more likely than 11200. This looks like it might be a typo as the C30 has an MGR of 11200 lb.
** if you use say 7600 lb as an MGR for this vehicle, assuming the CW in the table is correct (it seems reasonable), then the payload would be about 18.6 cwt which would be a bit more credible than the figure above.
I note that apart from the C8, the table lists the CW as "chassis and cab" rather than "cassis, cab and body". Could this also account for some of the 'extra payload', since the body would have to be fitted in order to carry a payload?
Regards
Steve