@alipeer... tyre size does make a difference, but u are right... i'v also observed a 5km/hr difference between altis and the new civic... when the altis is at 120, civic shows 125-126...
So yes, if the tyres sizes are stock, u'll be getting this difference... one more thing, i guess the corolla meter is correct, coz the motorway radars also (always) got my speed on the civic with a -5 difference...