My car Geo prism not running over 65 mph, what is your opinion ?

check transmission fluid

The possibility exists that the exhaust or exhaust catalyst is plugged or restricted. Are the any check engine lights lit?.

A mechanic's shop could check it. In the old days (before oxygen sensors) we would drop the exhaust down at the exhaust manifold and drive it to see if it helped, but with an oxygen sensor I would no try that. Something else that could cause this is a slipped timing belt or a clogged fuel filter. Has the fuel filter recently been replaced?

Check the engine's vacuum with a vacuum gauge. Run the engine at 2 ,000 to 3,000 rpm. If vacuum drops and is unusually low maybe 12" or less the exhaust is restricted or plugged.