In that exercise, you are using the transport stock turnover methods so the fuel economy values you specify apply to new vehicles in the year when they are first purchased. You can also specify a degradation profile to describe how the initial fuel economy values decline over time as each new cohort of vehicles gradually ages. You can use the same approach to specify how emission factors degrade over time.

This approach is useful for modeling fuel economy since it closely reflects both how fuel economy standards tend to be set by regulators (i.e. regulated values are for new vehicles not for the stock average) and it also describes how actual vehicle performance degrades over time as vehicles get older. Using this method, the average fuel economy across the total vehicle stock (all vehicles on the road in any given year) will be an OUTPUT from your model - taking into account what vintages of vehicles are on the road in any given year.

If you simply want to specify the stock average fuel economy as an INPUT to your model, I would suggest NOT using the stock turnover method. Instead, just use one of the simpler activity level/energy intensity methods.

I hope this is useful!

Charlie