Speedometers showing different speeds.
 

Hello everyone, I just got a 1998 Seville SLS Northstar, it is my first Cadillac and I wish I started driving Cadillac a long time ago. She is showing her age and has had several things done to it before I purchased it. But a pretty low mileage (71k).
I believe a panel cluster and a ton of switches, steering wheel, mirror assembly, buttons and minor parts donated from another. The main issue I am having are:
the speedometers show 2 different speeds. The digital number readout is about 5mph higher than the classic needle version at around 45mph, the difference is slightly higher at 60 is it possible to have one in km and the other in mph? that is about how it appears.
It has a slight misfire, from my testing its due to a fouling plug wire, however that wire seems to be seized in the hole, i haven't dug any further till i have the time money and parts to see how bad. Because of that I have concerns for when its time to swap the plugs out. Since it is cast aluminum I am petrified about removing them, any suggestions there?

Speedometer is a little strange if cluster is out of an STS or programmed for a STS could be the cause. The STS/ETC have a 3.73 final drive and the rest 3.11. You would need to go to dealer to have it checked. The spark plugs many times come right out. If they have been changed and anti seize wasn't applied to threads maybe not. Plug wires can be tough to get loose sometimes twisting helps. Also check for rust at coil and wire ends, it will cause misfiring and stalling. Rust requires new coil and wire(s).

thank you for that, i am pretty sure that is what the guy said, he got the parts from an sts, is there anything i can do to make the speed correct? Also, any guess which one is the most accurate? I wish i could take it to a dealer, but I live on very limited means and have to do all my own repairs. Barely keeping the lights on here.

He did the coils, and maybe the wires but there is 1 that dont match the rest, it is sunk in the hole the others protrude out from the top. that one that is different dosent wiggle at all like the others (just a slight give showing they would come out if i tried) using this image is a comparison, not actual parts of course, but the 4th one from the right resembles the one i am worried about, the 3rd are more like the rest of the boots/wires. http://www.magnecor.com/magnecor1/images/boots.jpg

If you have a GPS or know someone with one, you could compare GPS speed reading to speedometer. My guess would be digital since it would be sent from pcm to cluster. If you have a smart phone there is a GPS speedometer app. Module programming will require trip to dealer.