When I bought my Volvo V50 1.6L diesel in 2005, it had the best FE of any Volvo : 5L/100km combined, or 47.0 mpg (US).
The roofrails were left off - a factory option - as I wouldn't use them anyway, and they'd only add drag. (Does that qualify as a "mod" ?
)
The car was also fitted with the free but unpopular optional diesel particle filter - a rarity in those days in Belgium.
The V50 came with fuel saving Michelin Energy E3 tyres which I disliked.
In its first year, the car got 3 sets of tyres, going from 195/65/15 to 205/50/17 in the process.
Fuel consumption was well above the advertised numbers :
5.97 L/100km or 39.4 mpg (min 5,42 Ø 5,97 max 6,92 )
The second year was a little bit better in terms of FE :
5.83 L/100km or 40.4 mpg (min 5,05 Ø 5,83 max 6,60 )
Late in its 3rd year - mid 2008, just as the economic crisis was about to hit and fuel prices were soaring - I made a trip to the UK and discovered the positive effects of driving 10 km/h slower.
5.76 L/100km or 40.8 mpg (min 5,39 Ø 5,76 max 6,26 )
During the car's 4th year, I finally saw the light : fitted Michelin Energy Saver tyres on 16" rims and really started changing my driving style
5.43 L/100km or 43.3 mpg ( min 4,89 Ø 5,43 max 6,13 )
Still above its rated FE, but going in the right direction.
In the 5th and current year, FE has gone down a bit due to the unusually long harsh winter we've seen in Europe, and a leaking injector.
5.51 L/100km or 42.7 mpg (min 4,94 Ø 5,51 max 6,35 )
Going from 39.4 to say 43 mpg is just over 9% improvement - but it's still 10% above the rated fuel consumption of 47 mpg.
I wouldn't call it a success story yet, but it's a start
