The overhead does not lie and it can be pretty accurate, however it is only as accurate as you want it to be. I you never reset the readout it will onlybe as accurate a from your last reset. If you reset it tank by tank or when you make serious changes in the typeof driving you make you will be having a more accurate reading. So far off the past 2 tanks of gas I have in my truck with mostly highway miles I am averaging about 19mpg in both the overhead and mathmatically. Now if I change to city driving this will be factoring in the highway driving I have done so it's not gong o be as accurate if I did not reset the overhead.

 

yeah i am getting the same reading however my tires are a bit bigger then the stock ones so my speedo is off too