The small gauges I used are 2 1/16. You see a lot more of the black bezel around the gauge using that size but they fit in the hole very nice and tight. The 2 5/8 must be mounted behind the instrument cluster like I did with the speedo and tach. Here are some pictures. It might give you some ideas. Keep in mind I did mine for about $500. Mustang depot has this one
http://www.mustangdepot.com/OnLineCa...NW67-EPKit.jpg for 759 and the black finish one for 659. You can save some money doing it yourself but it ain't easy. I didn't use the aftermaket gauge supports either. Those are $35. I just used the stock back piece and modified it to work. I think you can see in the pics
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