Don't be too surprised if the wing makes zero difference to the fuel economy.
If your serious about fuel economy you need to get the basics right.
Check your tyre pressure all round.
Make sure the car has had a wheel alignment.
Make sure the air conditioning has been serviced.
Are the wheel bearing ok?
Any brake calipers getting stuck?
That's a starting point.
Then go nuts under the bonnet.
RA snorkel.
K&N air cleaner.
Wrap the radiator hoses in exhaust wrap.
Wrap the intake in gold heat reflection tape.
The exhaust wrap and gold reflection tape will be less then $100 to do the lot.
The biggest thing you can do is get the ECU tuned by
Merlin.Tuning@gmail.comThe ECU tune from the factory is absolutely not helping you at all.
Use some decent fuel.
BP98 is magic kit.
I have been down this road and tried 98 fuel from just about everyone you can think of who sells 98.
BP is by far the best power and performance and fuel economy you can get as a 98 fuel.
Once those area are Covered , drop on some extractors , wrap them before hand , 2.25 inch system , 200 cell high flow cat.
You will absolutely reap the rewards.
Top it off with a 4G69 TB and I can assure you the power and fuel economy will be a lot better than your getting now
You have said before you get about 600kays to a tank
I can get about 1000 Kay's to a tank on BP98 on the highway.
It works. I know it does.
You don't need to do everything at once.
Do it piece by piece.
Also , any extra weight you don't need in the car , remove it eg a boot full of spare parts you bought last week
Keep the car service regular as well.
Believe it or not the tread pattern on your tyre will also effect your fuel economy as well.
Is your suspension ok? Is there a crook strut?
There is so many things that can and will effect your fuel economy.
Cover the basics and you can't go too far wrong