Gas is messing with my Budget!

OK, my budget, in case I haven't ever explained it on here, is a simple hand-written, self-created form.

I fold a piece of paper in half, and the top half is for the 1st through the 14th of the month, and the bottom for the 15th through the end of the month.  I get paid, clearly, on the 1st and 15th.

I start with my average paycheck, and then write the fixed expenses paid in that time period (ie, rent, gym, cell phone), then the amounts alloted to other expenses, like food.

Meanwhile, my car doesn't get filled up with gas all that much.  I work two miles from home.  The gym and the grocery store are both between home and work.   I fill up about every three weeks.

Initially, this worked out great.   I budgeted about $40 per pay period to gas, and some months it hit where I used the 2 fill-ups in one month, but most months I had $40 "free" money to cover over-spending or to "snowflake" into savings.

Then, gas prices started to climb.  It now costs me over $50 to fill up and all reports are that this will get worse, not better.

I really don't want to rework my budget and take away $20 from other things in order to budget for 2 $50 fill-ups a month. 

I really don't want to have to top off the tank in every two week period to use the money as I have it budgeted. . . even though that would mean I did stay under the $40 mark.

And I am really not looking forward to what happens when it costs me $60, $70, etc, to fill up.