Vigo wrote:
85mph, with a sweep that looked like it went to about 140.
I dont think there was any kind of legal enforcement that the speedo couldnt go past 85. I think it was a voluntary arrangement for the OEMs. I know the 86 and 87 GLHS's just had a sticker applique stuck on the speedo that extended the MPH from 85 to 135. Incidentially, based on this, all us old k-car goofs know that when we get the speedo all the way back around to 5 mph, we're doing 135. 
it was a DOT regulation that came about in the late 70's some time.. the thought was that the only reason people sped was because they had speedometers that went well past 100mph. the slower speedos would cause people to slow down to save gas and save lives. how can you argue with that logic?? it wasn't a very big deal to most people since there weren't too many cars made from '75-'85 or so that could get up past 85mph in a reasonable period of time.
personally, i like 120mph speedos- that way, 60mph will be with the needle pointing straight up which makes it easier to maintain my regular 60mph cruising speed..
on the topic of the SVO Mustang-
i read somewhere on teh internets that the SVO was the first car sold in the USA with plastic headlights. Ford wanted to go with the plastic headlights in the '84 Mustang, but the DOT said no.. they got a special DOT exemption for the SVO since it was a low volume limited edition car. that regulation was dropped after the '86 model year, and Ford quickly redesigned the Mustang to get the plastic headlights. i think that was also when the 85mph speedo regulation was dropped, too.