There seems to be a bit of confusion about Blu-ray's read speeds in
comparison to DVD's read speeds specially given the Xbox 360's 12x DVD
rating. I am going to attempt to clarify the differences.

The first
difference that should be noted in a DVD vs Blu-ray discussion is that
DVD's are Constant Angular Velocity (CAV) and Blu-ray is Constant
Linear Velocity (CLV).

CAV: The drives transfer rate is
variable. Information on the inside tracks of the disc is read at
approximately half the speed as the information on the outside of the
disc.

CLV: The drives transfer speed is linear or
constant regardless of whether information is on the inside tracks of
the disc or the outside.

This is important to note because each of DVD's X ratings have a minimum and a maximum read speed and the advertised read speed is taken from the maximum, consequently a DVD drives maximum read speed is almost never realized. On the other hand Blu-ray's only has one constant read speed:

A comparison of Blu-ray's constant read speed to DVD's variable read speed at each X rating.
Another
important note about CAV drives (e.g. DVD) is that because information
on a disc is written from the inside to the outside the X rating is
never realized unless the disc is entirely full.


Example:

The maximum read speed of a disc that is only half full on a
12x drive is only 12MBps compared to the drives maximum of 16.5MBps.

The comparison


Mb = megabits
MB =megabytes

Blu-ray 1x: 36Mbps / 4.5MBps
12x DVD: 66 - 132Mbps / 8.2 - 16.5MBps

Blu-ray 2x: 72Mbp / 8MBps
12x DVD: 66 - 132Mbps / 8.2 - 16.5MBps

Blu-ray 3x: 108Mbps / 13.5MBps
12x DVD: 66 - 132Mbps / 8.2 - 16.5MBps

Blu-ray 4x: 144MBps / 18MBps
12x DVD: 66 - 132Mbps / 8.2 - 16.5MBps

A comparison of Blu-ray's read speed to a 12X DVD's maximum and minimum baseline.

At 2x Blu-ray can read as fast as 12x DVD's minimum read speed. At just
3x Blu-ray is comparable to DVD at 12x; through the first half of the
disc 3x Blu-ray is faster, through the second half of the disc 12x DVD
is faster.
And at only 4x Blu-ray manages to best a 12x DVD's maximum read speed
by 9%.