ORIGINAL: SkrapIron
Bone stock, with RTR components, you can expect 32-40mph from the Hyper 7 buggies.
You can of course find a smaller spur gear and a larger clutch bell to go faster, but that may well overtax the engine.
The other option is to buy a kit, and get a big block .26 or .28, even a .30 engine, if all you want is speed.
Buggies are heavy. Very heavy. It takes alot of power to get them going, and even more to keep them there. 50mph is about max for a single speed transmission 1/8 scale. I've seen it done with a .30 engine and 35% nitro. But the engine lasted only 4 tanks before blowing the connecting rod.
Those big displacement engines are made for monster trucks. These buggies are very light... their power to weight ratio is insane and they don't need anywhere near the torque that those bigger engines put out. The racing .21 engine will reach higher max RPMs then those .26+ engines can reach giving you a higher max speed. A friend of mine got his buggy to hit about 72mph (it was a police radar gun made for full size cars so it can't be to accurate for this... minimum speed was deffinatly 60mph) with an RB Concept WS7II running lean on 30% nitro and geared a bit higher. You can gear a bigger engine higher because of the torque but again with the weight of these buggies and how freely the drivetran works you can gear a .21 just about as high as a .26+.
Stock most RTR buggies will reach somewhere around 40mph and you can get them to just about 50mph if you gear them higher. You just can't overgear it otherwise you will lose top speed and either way your acceleration is going to be slow.
Edit: The mayhem is the only RTR 1/8 buggy (that I know of) to have a proven recorded speed of 50mph stock.