So, it looks like you halve the time remaining when switching to each cone mode, and multiply back when switching to line mode. The problem is it goes like:
4-2-1-2-1-0.5-1-0.5-0.25...

Now, you could multiply by 4, but that still seems like it's a risk for trouble. Wouldn't it be a better in terms of implementation for each fuel item to add a fixed value of "fuel amount" and have each lamp mode subtract a different amount each tick or each second?