I have a 90s pickup, manual, no tach. I use it to plow snow on my steep, long driveway. It has "shift up" light, but it's always wrong. Without the windows open

I have a 90s pickup, manual, no tach. I use it to plow snow on my steep, long driveway. It has "shift up" light, but it's always wrong. Without the windows open I have no idea when to shift.