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85 I believe. If I remember correctly, the 100mph speedo was dropped mid-77. Well, scheduled to be dropped anyhow...like many things that Ford changed, they used the old ones until they ran out, so I've heard of at least 1 late '77 model with a 100.
But by the time your 78 was manufactured, I'd bet it was the 85.
85 MPH for 1978.
You'll need to know if the original cluster had oil & amp gauges, or warning lights for oil & amp. You can't install one where the other was originally, at least not easily. Gauge clusters use a plug roughly 2 1/8" long, warning light cluster plugs are slightly shorter.
100 & 85 MPH clusters can sort of be swapped back & forth, but there are usually some differences that cause problems. Brake, hi beam & seat belt lights in different places, some have a low fuel economy light as well. The wiring is different for each type of cluster.