Thank you again, guys for your responses!

The manual says that it has a 2-speed transmission.

I have a Savage 25 made by HPI that has a 2-speed transmission, too. When pulling the trigger, the engine sound goes rapidly up, and then, with the shfting up, the engine sound goes down.

But I don't hear this from my Nitro Blizzard. When the pulling the trriger of the transmitter, the engine sounds goes up to high pitch and the vehicle runs a kind of fast. That's all. There is no engine sound change that signifies the shifiting up.

So I adjusted the screw in the gear according to the manual for earlier shifting up. (I tried twice, so I did total one turn anti-clockwise.) But still it is same.

I checked the sound of other Nitro Blizzard in the videos on You Tube. But I dont' hear gear shifiting from the other Nitro Blizzard in those videos.

Any ideas about this issue?

Your advices will be greatly appreciated. Thank you!