THE PRODUCTIVITY INDEX
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This is where the canny operator will start to get sceptical: any calculation of "productivity" requires assumptions that are bound to be unsuitable for some applications. For some operators the net payload is most important; for others average speeds are everything; but for most hauliers, fuel consumption is the most significant factor. Nevertheless, we have grown used to using a productivity factor that is simply the product of the fuel consumption (in mpg), the average speed (mph) and the payload (tonnes).
We have assumed a combined tractor and trailer weight (unladen) of 15 tonnes, giving a notional payload of 26,000kg at 41t GCW and 29,000kg at the new maximum weight This represents an increase of 11.5% in payload—but, of course, if your vehicle's tare weight is more than 15 tonnes, there will be a larger percentage increase in payload.
Using the results from Volvo's preliminary tests (above)—with the 460hp FH12 operating in still conditions— we get a Productivity Index of 8,180 at 41 tonnes, compared with 8,501 at 44 tonnes. This difference of 3.9% at the higher weight represents a bigger improvement than you will get from most operational changes.
The improvement is more marked when the going is easier: on the dual-carriageway section of the route (with an average fuel consumption of about 9mpg) the average speed was hardly affected by the extra weight, so the Productivity Index was 9.5% better at U tonnes. However, on the much tougher mixed section (a combination of Aand B-roads) the fuel consumption went up to about 6mpg. Here, both fuel economy and average speed took a hit at the higher weight, so the Productivity Index went up by just 1.7%--hardly enough to cover the extra costs of maintenance and VED at U tonnes.