I have a 87 5.0 notch with aod.
I just got a 140mph cluster to replace the 85mph.
Is this a direct replacement or do I need to replace a speedo gear or something?
This has been covered dozens of times, but basically it depends on what cluster you have. If it is from a 89 Mustang, or it is the FMS replacement speedo, (with a flat bottom on the cluster) then you will not have to do anything at all, just swap it over. If it is from a 90-93 car (with an arch cut out on the bottom, above the steering shaft) it is not a direct fit, the wiring is all different. If that's the case, you have two choices. Either rewire the plugs to match the newer cluster, or swap out the speedo part and install it in the older cluster. But that again is not a direct swap since Ford changed the gauge face clear plastic backings and how they mount in the clusters!
Either way, you will not have to do anything with the speedo gear unless you alter the gears in the rearend of the car. All the speedos are calibrated the same, for the same number of cable turns per MPH, so it should read the same as the one you are replacing.
Where in Houston are you? We have a lot of local guys on www.westsidemustangs.com It is a great place for local help and info and such.