Personally I love my Speedmaster for a whole range of reasons.

The look, the feel, the manouverability,

the sound, the quality, the look, ( did I say that twice? )

the retro image, the way it makes me feel and the smile

it puts on senior citizen's faces at service stations. Okay it lacks power but we can improve that.
Now the Thunderbird has the power but it lacks in all the other areas
and i cannot fix that.

Just my opinion but I would take a Speedmaster, America, Bonneville or Thruxton anyday over a Tbird.
