When a vespene geyser in for instance on the top of command center you just need 3 SCVs for efficient mining which gives you 296 gas per minute (fastest).
Not efficient gas mining:
- 3 SCV's mining vespene geyser placed on the bottom of command center. Result: 240 gas per minute.
- 3 probes mining vespene geyser placed on the bottom of nexus. Result: 272 gas per minute.
- 3 drones mining vespene geyser placed on the bottom of hatchery. Result: 264 gas per minute.
In that case you need 4 SCVs/probes/drones mining gas to get 296 gas per minute. I haven't tested it on other maps but it should be the same. What do you guys think about it? Did you know about it?
* img shows about which expansions i am talking about.