Binger -
I think you have found a comfortable sweet-spot for your Mirage. It's especially nice that the road's speed limit makes your speed look "normal". 20 MPH is probably the drop-dead-bottom-practically-idling 5th gear speed for my car too.
I wanted to find some gear ratios for your tranny, but all I could google were these URLs :
1991 Mitsubishi Mirage
Mitsubishi Mirage pictures 1991 models
CarloSW2