My Gen2 campro engine also same.. consume petrol so much when RPM over 3k. 

You are wrong. It will not change. Your speedo comes from your gearbox. It is fixed. Changing the size of the wheel will not change the readings on your speedo meter. It does not matter whether you change to 20inch. If the rpm is 3000rpm at 110km/h on the respective tacho and speedo meter, it will remain at 3000rpm at 110km/h even if you change it to 20inch (even though you are travelling at 130km/h).

You have to remember that there are always 2 speeds.

1. KM/H on speedo meter (this cannot change as it is a fixed reading from the gearbox)
2. KM/H actual speed (this will change depending on the OD of your wheel)

What we are talking here is the reading on the speedo and tacho meter. Actual speed aka real speed aka speed captured by speed camera will change but not the reading on the speedo and tacho.

haha..dont have it but asking around how to change gear box?   

your main interest is in the fuel consumption rite?
the lower the RPM, the better the fuel consumption. only true when you're cruising and maintaining the same pressure on the accelerator.

but lets say (for Auto) you're driving at the lowest RPM with the lightest pressure on the accelerator, the gearbox might not change to the final gear, thus you're wasting petrol because slower when compare to final gear speed at the same RPM.

and for Manual, because of the assumption that lower RPM gives better FC, you might change gear too early and thus need to have higher pressure on the accelerator to build more momentum, which lead to wasting petrol.
basically to get lowest FC, is to get the optimum pressure on the accelerator vs the RPM speed.
but this is my theory lar.
correct me if i'm wrong.

erm...my civic ej6 b16a manual 4k rpm at 110kmh..but last time auto reach 3k at 110kmh....so manual sux?????

my car RPM metre spoilt d.... lol..... i wonder how much it cost to replace

anyway, Iswara about 3.2k RPM @110KM/H

Paid attention to the rpm meter over the weekend and it confirmed my previous recollection that a E90 325i (2.5L) revs around 2.0-2.2k at 110km/h (6th gear). 

A Freelander TD4 (2.0L diesel) is another low-revving vehicle. Usually drives around city <2k rpm, and should probably rev around 2k at 110km/h too I believe. Will check it out next time I get a chance to drive it. I wonder if diesel-engined cars are low-revving in general? Anyone know?