Never Use Too Many Figures
If you express a distance as 3020 miles, to within plus or minus about 20 miles, it would be expressed as: 3.02 x 103 miles (nothing new here)
If you change the units to inches, your calculator would display 191,347,200 inches. But now the uncertainty (20 miles) is 1,267,200 inches. We would report 1.91 x 108 inches. The uncertainty expresses itself in the last digit (1).