Double four times, and divide by ten as soon as possible (ie: the first time you see a zero on the end, drop it).
So 30 mph => 3 (drop the zero) => 6 => 12 => 24 => 48 km/h
The nice thing about this method is that you know that the kph is going to be more than mph, but less than double it, so you don't have to count the doublings very well- when you're in the right range, you're at the right number.
Example: 135 miles to destination is how many km? Okay, double to 270; drop the zero to 27; double to 54; double to 108;... we're still less than the mph, so we must need to double again to get 216 km. Now we're between 135 and 2*135, so we must be at the answer. 216 km. (actual answer is 217.3).
135, half is 67, a tenth is 13, add those two together to get 80, 135+80=215.