Estimation of craft (UFO) speed

Most of the time the craft (UFOs) that are captured are only seen on one image in a series of ten because they are moving so fast. But occassionally we capture a craft that is present in two consecutive images. On the morning of the 14th January 2013 I captured a craft in two consecutive images (see DSC08735 and DSC08736 below) that was flying at high speed over Darlington, Western Australia. The craft was identified as one of the oval, transparent types that have four nozzle-like structures underneath it as previously captured (see DSC08309 below for comparison).
I have captured images of this craft before and it appears to be a small craft with what looks like two occupants. Based on this information I could make an estimate of the speed of the craft if I made some assumptions based on an estimate of the size the craft.

The craft is flying from right to left. The time between the two images is 0.46 seconds when the camera is set to continuous shutter mode.
Based on other images of this craft showing what looks like two occupants (see DSC09899 below), I am assuming the craft is about the size of a car of about 5 m long (of course I could be way out, but it is just a starting point until I get more data). On my computer screen the craft in a high resolution version of DSC08735 is 3 mm long and the distance travelled on the screen to the position in DSC08736 is 260 mm. Scaling up, this means a 5 m long craft would have travelled 433 m in 0.46 seconds. This is equivalent to 941.3 m/s or 3390 km/h or 2106 miles/h. The craft appears to be moving away from us as the craft in DSC08736 is smaller in length than in DSC08735. This means that the craft would be travelling faster than 3390 km/h (based on my assumption of a 5 m long craft.) If the craft is larger than 5 m then the speed of the craft would be greater. For example, if the craft was 10 m long the craft speed would have been at least 1884 m/s or 6783 km/h or 4215 miles/h. Of course it is also possible that this type of craft comes in different sizes.

DSC09899-craftSemi-transparent craft with nozzle-like structures underneath it. Presumably this image captured the craft from the front. It looks like there are two occupants in the craft.