mysteriously worsening fuel economy solved

[dlb]

my wife's civic VX normally gets about 18-19 km/L (42-45 mpg) in the summer but over the last month it has been steadily decreasing to about 15 km/L (35 mpg). for the first few tanks, i thought 'no big deal, one or two poor tanks here and there is normal due to driving habits, conditions, etc. i'll wait and watch the trend.'

but the next tanks kept getting gradually worse. i asked her if she had noticed anything weird while driving it, she said no. convinced something was up, i jacked up each corner and checked for brake drag. all seemed fine. then i checked the air filter. it was clean but i banged out the dust anyway. then i checked the plugs. they looked fine. i thought 'i'm not sure how old these spark plug wires are. maybe i should replace them. but faulty wires would probably cause some rough running so that's probably not it.' i had to leave for work so i told jenn i wasn't sure what was up and would give it more though.

when i got to work, i checked my 'common causes of poor fuel economy' list that is in the FAQ here on the forum. right at the top of the list: low tire pressure. i don't know why i hadn't thought about them. the tires didn't look low but i realized i hadn't checked their pressures in a long time.

checked the tires tonight and one was down to 32 psi, two to 25 psi, and one to 20 psi. mystery solved! i love an easy fix, even if it's an obvious one that i should have guessed right at the start.

[Petros]

I have also wondered how much difference one brand of tires vs. another might change fuel economy. when we had several weeks of freezing icy weather I finally switch to a set of studded snow tires, economy normally about 28-29 mpg in my mixed driving, dropped to about 19-20 mpg. ouch, those are costly way go just so I can drive the last 1/4 mile of road up our hill.

[dlb]

i have checked mileage when we switch from summer to winter tires (no studs, mind you) and haven't noticed any difference.