Just finished swapping in a '90 dash into my car. The way I have the elec speedo set up the vss goes to a dakota digital box then straight to the speedo, no comp. Using one of those things the police put up to let you see your speed I tried it at a few different speeds. At 20mph it was dead on. But when my speedo read 30 it was like 26 mph and when my speedo read about 35 I was only doing 30. I know those signs are not accurate but I think they would be consistent. So I guess is there a fix? One of the other outputs on the box? I don't understand why it would just not be say 10 mph off at all speeds. Or is it typically a percentage or something? I am using app #2 on the DD box and output #1 I believe.

Thanks,