Why do short tanks give high MPG?
 

I've seen it mentioned here a couple of time and have seen it on my car on my last tank.

Why does prematurely filling your tank give you a high MPG result?

Just looking for some enlightenment on the matter :)

Thanks,

if your car has a decent evap system (like the old honda insight) you get elevated MPG figures for a while after filling due to the vapor in the carbon filter. If you don't drive far your tank stays near full pushing on the evap.

I am "uncertain" if the effect matters much as you are just burning gasoline that evaporated in your tank but if it improved how the vapors burnt by being so much rich, I have no idea.

A short fill means any variation in pump cutoff makes a dramatic difference in the calculated mpg. It may be high or low, though.

They should average out over time though ? Although with a 90-day average that is maybe less of a factor.

Actually the 90 day thing is a bit daft - I only get 3 tanks in (normal) 90 days, others maybe 5-10.

It does average out over time. But a short tank doesn't get that averaging.