JB86: and i heard opposite cases too, that even while headers & tune should reduce exhaust restriction, add power (and thus lessen need to floor it more for same performance), run on leaner AFR, some had worsened economy in daily driving. I'm guessing, that because of fuel economy mostly dependent on "drivers' foot", and one that has done performance mods, may subconciously drive more aggressive, floor more, drive at lower revs, do some small tire chirping slides here & there, coast less, install also grippier (but of worse rolling resistance) tires instead of stock eco primacies, to feel acceleration/hear engine noise from added mods more. I myself probably simply wouldn't care for fuel economy change in both cases, be it unmodded or modded car, as this is chosen/bought with prioritizing driver's enjoyment, not practicality (or economy), and most economical driving is not exactly most enjoyable, so i just fill full tank whenever it's empty, not minding how much car uses up, and filling with 98, not cheaper 95, even while car capable of using later (at cost of slight power drop).
I’ve measured the difference when using 95 vs 99 fuel and while obviously not in lab conditions, economy is noticeably better on 99.
Though that’s more likely due to E10 vs E5 fuel. All of my vehicles, both cars and both bikes dropped in economy since E10 became the norm. Not quite so certain about it’s eco credentials, I bet it helps with the petrol company profits though…
One thing that appears not to make much of a difference… tyre size.
My car has 9.5J 18” wheels on with 255 tyres. So a lot wider than the stock 7J wheels. I’m running Kumho PS71’s.
My long distance motorway cruising MPG is the same ~40mpg that my Dad gets in his completely standard 86 on Eagle F5’s.
My highest ever avg MPG was 44.5mpg over 180 miles on a particularly boring drive which matches the highest value my Dad’s ever achieved, even on the original primacy tyres.
Kind of surprising, I expected the wider wheels to have a noticeable impact on fuel economy but doesn’t seem to be the case.
Normally on mixed driving trips, 35mpg is the norm for me and around town or on more exuberant drives, 30mpg is what I expect to see and as you say, I didn’t buy this car for economy, so it’s not driven gently.
I’ve been surprised though! I sold my hybrid Golf for this car, which could produce a similar 200bhp.
It usually got around 45-50mpg on motorway trips over 100 miles. So the fact the GT86, a 10 year old car, with a NA 2.0 performance engine, manual gearbox and no hybrid system isn’t a million miles off VW’s very efficient 1.4TSI blue motion engine, with a DSG and electric motor to help, is really very impressive!!
Probably helps the 86 is a good 350kg lighter!
Of course on short trips, the Golf managed well over 100mpg but I’ve been impressed by the efficiency of the 2.0 Boxer.
Car is currently in for its valve spring recall. So let’s see if tearing the engine down has any impact…