Efficiency is defined as; Work out / Work in. For an engine the work out, is in measured in terms of HP and work in, is measured in terms of fuel usage. The Nebraska Tests are an excellent source to compare different tractors. The best efficiency is typically near rated power. You should be able to find examples of small tractors and of large tractors which are very efficient since efficiency (HP -HR per Gallon of Fuel) is more about design than size.
Fuel useage is different than efficiency in that the amount of work perfomed is not accounted for. At idle an engine is using fuel while doing zero work. Therefore, all engines have an efficiency of zero at idle. However, fuel usage is at a minimum or at least very small at idle.
In answer to your question I would propose:
- The best efficiency is typically achieved using a tractor sized such that it is operating near its rated HP to perform the task.
- The lowest fuel useage is typically achieved using a smaller tractor for the small jobs. Again the engine will be closer to it's rated HP. In addition, there is less tractor weight to pull across the field.
Certainly there will be exceptions however this is how I would place my bets.
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