I want to swap my 80 mph sppedo to one that has a higher reading can i just swap the speedo or do i need to swap the entire cluster or even more

 

Easiest to swap the whole cluster. Take out the 10 or so Torx screws that hold the bezel on; then 2 nuts at the bottom corners of the cluster; slide it out far enough to reach behind and disconnect the speedo cable. Installation is the reverse of removal. No changes to anything else are required. Accuracy is the same.

People have the 140 mph clusters for sale in the classifieds on this site all the time. They usually go for about $75-150 US depending on condition.

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"So many Mustangs, so little time..."

 

I would like to offer my advice...
swap just the speedo...
reason---the rest of the gauges might not work properly if the wiring/plastic sheet in back doesn't line up properly and then you will have difficulties such as lights always on or never on... turnsignals which choose to ignore your request for their participation while driving...other little gremlins.
just my 2 cents.

 

Both ways are about the same. You still have to take the whole cluster out to put in a new speedo. If you can get just the speedo it is only 3 more screws to take off and it easy. I agree about the plastic backing. This why you should start the car and test everything out before you lock everything down again. I was able to get a whole cluster at a frea market for $20 and everything worked and now I have a speedo that reads 145.