For the price of a tank of fuel there are millions of smart phones for sale. Obama even gives them away.
Many highways in Calyfornua no longer have mile markers.
Before smart phones, I too used mile markers and a stop watch, that cost me more than a used smart phone costs today. The smart phone has proven more convenient for me and accurate and I ain't required to find no highway to check my speedo or odometer.
Could help OP with his issue.
I recently went thru a speedo accuracy issue with wife's 55 T Bird in which I had swapped in an AOD 4 speed auto transmission. I not only had to sort out mph speedo, but too an odometer accuracy problem. Could have done it with mile markers and a stop watch,,,,but would have wasted a few additional hours doing so over using this app.
60 mph, how long to go 1 mile=
120 mph, how long to go 1 mile=
90 mph, how long to go 1 mile=