Formulas to relate velocity?

Hi,

I'm not sure if this is the right place to post this, but it is part of a calculus problem I am working on, so hopefully it's okay.

I know how to do the problem (I'm working with related rates), but I have to find velocity, and all I have are two graphs, one of gallons/hour, and the other of fuel efficiency as a function of fuel - (miles/gallon)/gallon. My problem is finding a way to relate these equations, or finding another equation so that I can relate them, and solve this problem!

Thank you!