The W22 & W24 have always utilized an "auto park" emergency braking system on the front of the driveshaft. Some used an enclosed drum brake system and my W24 used an enclosed disk brake system.
Beginning with the 2006 model year all Allison transmissions have 6 forward speeds. The 6th gear was just another overdrive. The first 5 gear ratios were the same in both Allison transmissions.
I own a 2005 W24 with 5 speed Allison. When the 6 speed came out in 2006 I wish I would have waited. But now I don't think it would make any difference.
For the pre-2006 5 speed Allison.
1st 2nd 3rd 4th 5th
3.10 1.81 1.41 1.00 0.71
For the post-2005 6 speed Allison.
1st 2nd 3rd 4th 5th 6th
3.10 1.81 1.41 1.00 0.71 0.61
For me the BIG change from 2005 to 2006 was the 600w inverter. Before 2006 the inverter was 300w. If I had that 600w inverter I could run my TV, satellite receiver and my high power laptop. With 300w I can not run the high power laptop with the TV and sat receiver.
The front end styling also changed from 05 to 06.
The grade brake was introduced in 2005 and is a great feature, IMHO. I never turn mine off.
-Tom, W3TLN