So I have had my civic for awhile now, and would say average mpg on a tank has been 30-32, occassionally a bit more or less if I do a lot more city or a lot more highway, but basically around 30-32.

After considering selling my car and getting an Integra I decided to wait a bit longer since school is starting up again soon and I'm not taking my car with me this year. I decided it would be nice preventative maintenance to have the timing belt/waterpump done and the 120K mile service (sparkplugs, transmission fluid, and a few other things). I got 31mpg out of my tank right before the service, and I just finished my first full tank after the service and got...40mpg!

Does it make sense that a simple tune-up can increase fuel efficiency that much or could there be some other factors? I mean I know sparkplugs can get fouled and reduce efficiency, but that much? Either way not complaining haha, just curious, not too worried bout 40mpg

 

I replaced my spark plugs, fuel filter, cleaned air filter, and replaced both O2 sensors all in the last few months and I can say it made a big difference.

The other day I got 360 miles on 9.5 gallons(very close to 38 mpg) before filling up in my 00 Ex. Usually I'd fill up around 280-300 on 8.5-9 gallons(<33 mpg on average). I have only been over 300 before filling up just a handful of times, but it's been easy ever since I replaced all that stuff.

 

Yes, it can make an enormous difference. At one point, I was only getting ~22 MPG. I did a basic tune up and my gas mileage immediately went up to ~27 MPG average on the next tank of gas, with the same driving style and driving environment. My last tank of gas averaged 29.5 MPG, after recently replacing my fuel filter, cap/rotor, and PCV valve.