Works fine on 3GS
This app works just fine. Contrary to what other reviewers have said you can have multiple vehicles. And the only way to figure out mpg is to input your miles (duh) and the gallons you are adding so of course you have to know how far youve driven. You can use a trip odometer ("tripometer") or the odometer reading and it will compute the miles driven for you. Just set it up with your starting information and everytime you fillup you enter how many miles and how many gallons. The mpg will compute and log it. Just like Ive been doing on paper but this is easier. The only complaint I have is that its a little hard to find all the directions. Apparently you get directions depending on what page you are on when you request help so its not all in one place. If you follow the directions though itll work fine. Good app for 99 cents.
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Gas Log - MPG Fuel Calculator