Using reserve light to check MPG

[funola]

How accurate would you say the reserve light is? It's a 21.1 gal tank and the reserve light is suppose to come on with 2.9 gal left. It's a fixed position float switch on the bottom of the sender and my guess is that as long as it is on level ground it should be accurate and repeatable.

I just came back from a long trip where I drove 500 miles on VO before the reserve light came on. That works out to 27.47 mpg! I used a GPS to keep track and the average speed was 50 mph. Some of it was stop and go, most of the back roads taken was 55 to 65 mph and 30 to 45 mph.

Reason I have to check MPG this way is I use the main tank for VO and have a small 5 gallon tank for diesel and have no accurate way to measure the VO I put in (non metered pump).