rough estimate (just numbers):

$10/hr now = $400/work wk

$12/hr new job = $480

10miles to job now (20miles round trip) = 100 miles/wk

u fill up every 4wks so to speak, so every month, right now is $40/month on gas, speaking just on work

40miles to new job (80 miles round trip) = 400 miles/wk

u fill up every wk so to speak, so every month, in the future is $160/month on gas, speaking just on work

so here is your equation

$1600/month - $40/month = $1560 left over

$1920/month - $160/month = $1760 left over

as of that equation you have $200 to play with w/ your future job..plug in your numbers accordingly and make your decision that way...