Forgive the silly question, but why do you need a macro?

If you want today's date, enter CTRL ;
The other entries are just entries.

To calculate the Miles per gallon (using Miles in E1 and Gallons in C1)
enter =E1/C1

I've been tracking my mileage and have also included the name of the gas
station.  I'm seeing that I get better mileage from some stations than
others (although haven't done a full statistical analysis of it).   Who
knows if it's because of the fuel or if the tank is more full at some than
at others.   My MPG also appears be more stable if I fill the tank with
approximately the same amount of gas each time.   I've had very low MPG when
I fill the tank and it's still half full.

I know you said you're looking for a Macro but I'm a fan of formulas and
would like to offer the following

Column E=Miles/Gal
version 1  =((D2-D1)/B2)
version 2  =(G2/B2)

Column F= Cost/Gal
=((C2/B2))

Column G=Distance Traveled
=D2-D1

I know it's probably too simple minded.  But I like things that just work
and this works.

Hope this helps

jacob@roeckerfam.com