Figured out for anyone interested. I'm going to buy some and plastic off amazon cut it into a 4 1/2 circle. Next I'll cut the hole for the new gauge, I'll then plastic weld it into the back of the cluster. For the sensor I'll use this after market sensor that uses 4 magnets attached to the drive shaft. Total cost will be around $210.

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84 300sd 2.47 dif 360k
26 mpg needs lots of love
given to me in pitty

89 civic hatchback auto 140k
33mpgs soon to be low to mid 40's
been in the family since mile 1

85 300d with unknown knock. driven over 30k with knock and still going. sold for $800