You do not lose any mph since the Allison would be running in lock up.Lin -
Unfortunately, with the V-drive GMCs, you DO lose about 5 - 6 mph in top speed, which doesn't happen in the T-drive configurations. This is due to the bevel gearset ratio, which is lower in the automatic compared to the manual. Almost all of this can be overcome by going to 24.5 wheels and a tire that turns 470 revs/mile.George -
To echo others - if you think you want an automatic, then look for a coach so equipped, preferrably OEM. Don't waste time and resources on a manual to auto conversion.
Just like it makes no sense in today's market to buy a shell when you can get a fully converted rig at "fire sale" prices.
FWW & HTH. . .