I've got Martin's 'original' cone filter on my car after I made a deal with him at Haynes last year. The only issue is I don't have a dishwasher to wash it in, especially a brand new one. ;)
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I fitted that to my p1 V70 T5, and it might just have been a coincidence, but the maf sensor went immediately after fitting it. I now use JR panel and cones, and plan to stick with them. As soon as I notice my mpg is dropping, I clean them, which is more often than 10,000 miles.
Just squirt it with some flash, wait 5 minutes and blast it from the inside with the shower mate.
With the mileage James does, I'd say the filter needs a clean.
Don't forget with the cold weather mpg drops anyway, as the warm up cycle is longer, and heater going full blast and heated rear window/seats takes power as well.
If I drive like a Granny I can squeeze 30 - 31 average mpg out of it at a push and that's resetting the trip computer for each trip. I usually average 27 - 28 mpg, the terrain around here is very 'up and down' though.
In the summer I could get 33 -35 mpg.
i say if your watching your mpg you shouldnt be driving a t5. our second car is a 307 hdi so we dont need to worry about it.
I did a before and after RR session and my old Simota air filter made 5 BHP.