Hello guys. Need your expertise and advice on a pretty serious issue.

I bought a 2010 Vios J-spec (MANUAL) with around 110K in mileage, back in Mar 2015. While I have been pretty happy with the FC and the overall performance of the car, I've noticed that my car revs up to 3000rpm at 110km/h. Some people claim that a normal Vios would hover around 2600-2800 rpm but not this high.

FYI, the car came with a set of sh!t-@ss Venomz 15" rims which look almost like deep dish wheels. The previous owner even paired them with Continental 195/50 R15 tires (Contisport 3 IIRC). I believe that this combo may have caused the car to suffer at the FC department. I get average 430-450km at 100% city driving (Penang), but can sometimes hit 580km per full tank when I'm on the expressway.

My question is: is this sort of FC normal? I am currently using Liqui Moly Mos2 10W-30 Semi-synth and always pump Petronas Primax 97.

Another more pressing issue: I've noticed these few days that my gear shifting is a bit more rough (as if it needs more effort to shift through the gates), and sometimes when I rev up till 2500rpm, the car doesn't seem to have much 'push' as before. What could have possibly caused this? I will be sending my car tomorrow to the SC to change the gear oil as I've heard that older oil may cause these symptoms. I pray to God it's not a slipped clutch.

Your advice is duly appreciated.