Arop:
I had the same question about the levels as well until I decided to test what a one level at a time change would do to reliability, acceleration and fuel cost. I also used low_grade's loco spreadsheet to find the possible range of quality levels for each one. I test several different train engines, including electric engines, to see what an incremental one level change would do. I even tested it going backwards by reducing one level at a time to see if the effect was constant.
Here's what I found: changing reliability, acceleration &/or fuel costs +/- 'X' level changes its quality by +/- 'X' level.
For Fuel Economy, there are 10 possible quality levels:
Atrocious
Extremely Poor
Very Poor
Poor
Below Average
Average
Above Average
Good
Very Good
Outstanding.
<> Changing fuel cost by subtracting one level, which improves fuel economy, moves the fuel economy condition up to the next quality level. For example, if the condition is Very Poor, subtracting one level moves fuel condition up to Poor. Subtracting two levels moves fuel economy up two quality levels, up to Below Average, Subtracting 3 levels moves it up to Average and so on.
For acceleration, there are also 10 possible quality levels:
Very Poor
Poor
Below Average
Average
Above Average
Fast (different than fuel econ & reliability)
Very Fast
Ultra Fast
Virtually Instant
Instant
<> Changing acceleration by adding one level, improves acceleration by one qualifier level: from Very Poor to Poor for example. Adding 2 levels improves acceleration by two qualifier levels: Very Poor to Below Average. And so on.
For reliability, there are only 9 possible quality levels:
Very Poor
Poor
Below Average
Average
Above Average
Good
Very Good
Outstanding
Near Perfect
<> Same story...changing reliability by adding one level,improves reliability by one qualifier level, adding 2 levels improves it two qualifier levels and so on.
This means, that changing fuel economy, acceleration or reliability by ONE level is about the same as a 10% change, TWO levels are 20% change and such.
You can REDUCE reliability, acceleration and reliability using the same one-to-one relationship.