98 escort zx2 shifting?

I have a 98 ford escort zx2. 4 cylinder. As soon as I go past 40, the automatic transmission doesn't shift down. Maybe it's supposed to be like that. Anyway, when I'm on the highway and the rpms are way too high, I switch to over drive and keep the engine working nominally, or so it seems. Do you think anything is wrong with always switching to overdrive when going a certain speed? Thanks in advance