My is Suzuku RGV 120, can go up to 145-150km/h (without modification). But its too danger to ride that fast, now my max speed is 80-90km/h for any bike,

Sorry to say that, why do we need to know our true speed via calibrated radar gun courtesy only? Is this the most accurate way? It might be. However, we do not need to measure that way to evidence us as if we use our brain, we can figure out how accurate the speedometer is. First, what does km/h means? Sorry if I am wrong, km/h means total kilometer distance that a vehicle can reach in an hour. Try to have a ride to a place for a given distance, say 110-120 km, and have our bike or car speed try to maintain at 110km/h, and if we can reach the destination in an hour time, or 65-70 minutes, then we can roughly know that the accuracy of speedometer of bike and a car is near or equivalent. If both type of vehicle can reach for almost same duration, then the speedometer for the vehicles is tally. I have tried my bike against fren's proton wira at 110-120km/h, and I found that we are at the same speed. I am not sure, but I guess that the speedometer measurement between car and bike should be identical, unless there are some problems with the speedometer.

Err, I think is still on the mark that you mean.. you have to know, I say try to maintain the speed at 110km/h, but as we know, its impossible that we can always maintain the speed as we might slow down on the road. 65-70 minutes are meant for that. Also, I mentioned about 110-120 km/h, not exactly 110 km/h, so there must be some tolerable time allowed like adding 5-10 more minutes. And if the bike reach the above distance at 61m59s, and the car reach at 61m50s, it is approximately the same. Anyhow, its quite interesting to test against this question as I have been bothered for quite long time ago, nothing but just curious, aren't you?