Basically, my new punto when i bought it, it came with an exhaust system and an k&n filter. Now both are tatty really and doesnt look as nice as what i would do. However, i noticed that my mpg lately has been really rubbish, basically, i put in a full tank yesterday, and ive only done 30 miles and its already quarter gone, whats all that about, its actually past quarter.

My mechanic said it could be the air filter and best to put original one back on. I was thinking of getting the gsr but will this actually benefit in terms of mpg?

Also im getting a new back box, will that help for my exhaust as well?

K&N are rubbish, replace it with a GSR or even the factory air box and your MPG will improve

Since getting a new backbox im going through almost double the amount of petrol! Im not driving it any differently either. So id look at changing the filter before putting a new backbox on.

try cleaning the airfilter, a dirty filter will cause poor mpg

Put the standard one back on. K&N are notorious for making your car have a drinking habbit!, this is even more pronounced on the 1.2 8v engines. As I said put the original back on with a new clean filter.

Then start looking at your exhaust if it hasn't improved much.