I can understand and agree with the reasons why the site doesn't list everything in the order of best fuel economy, and I encourage you to continue ordering the data using the current 'default'
It is a different matter if I (as a user) elect to sort the data in the fashion I desire. So, why not provide it as an option that I can choose to exercise (or not)?
As an alternative, make the "browse vehicles" grid downloadable as an Excel or CSV file (so I can sort it myself)
Unless you make the data so it can be manipulated/analyzed, I fear the utility of your site will dwindle to zero (there will be little reason to invest my time in providing data)