Need speedometer help

[flat6s]

Guys,
Sent my speedo in for new odo gear. Comes back, its registering half the speed, give or take. Put my original old speedo in, works just fine
Car is 1980 911 SC. speedo shop says they set it to 1984 impulse specs and it runs just fine when their software is set to 1980. Speedo shop says their is no difference in spec between 1980 and 1984. My mechanic says there is a difference.
Speedo shop says they do not know what to do with it.
Can anyone tell me, will me speedo for a 1980 work just fine set to 1984 impulse ?

Thanks for the help
Joe

[Drisump]

I don't know, but when you say "speedo shop" the board would usually recommend North Hollywood Speedometeter....did you send it somewhere else? I have sent one speedo there for an odo gear and it came back fine. On another car, I did the gear myself and it turned out fine as well....fact is to perform this task, no adjustment is necessary. I would be concerned that anyone other than NHS, would have a proper data base for our old cars. Cheers

[sky1jord]

Drisump is right. Send your question directly to NHS