RPM Reading / Vehicle Speed
Minimizing RPM and Maximizing Speed is the GOAL
Here are some examples
divide rpm /1000 , 1000=>1, 2000=>2 3000=>3
divide speed by above figure
eg
30 /2 = 15 xpg (@2krpm)
60 /3 = 20 xpg
140 /4 = 35 xpg (@4k rpm)
Now with experience try to figure out xpg to mpg ratio by comparing it with actual mpg, and you can just readout your mpg without requiring any instrumentation