I'm wondering what gas mileage I should be getting in my 3.2 A/T? On a road trip, my economy meter says I'm getting between 7-8 L/100km, when in fact I averaged around 9.5-10. I was averaging around 110kmh on 2 lane highway for about 90 minutes and 130kmh on 4 lane freeway for 30 minutes. It was 0 Celsius outside. Am I getting pretty normal gas mileage? What speeds would I have to do to get closer to 7.5-8 average?

 

Most likely your economy meter is more accurate than your calculations. Keep in mind, unless you average at least three full tanks (five would be better), of fuel together, your hand calculations for fuel economy will almost always be incorrect.

For my car I typically see about 6.75-7.00 liters per 100km when doing steady state cruising on the highway and more like 9.00 liters per 100km during my day-to-day driving. That said, my car has a manual transmission, so it will most likely be a bit better than what you can expect with an automatic transmission.

 

How fast are you going when getting 7L? If I run a full tank of fuel in I can go 550-600 which works out to 8.5-9L/100km. If I got 7L/100km like the computer shows I should be able to get about 700km on a full tank. To be fair, I always drive this thing spiritedly as it's a blast to drive. I'm not expecting really good numbers since it's cold out now. I also think there's some coils that need to be replaced as the car has thrown a misfire code once in a while at full throttle and doesn't idle perfectly smooth. I replaced the spark plugs with Denso platinums which helped a little bit. I'm also going to be running Lucas injector cleaner each tank for a while, and might try a seafoam treatment, hopefully it helps!

 

A few comments:

• My best fuel economy runs were done at about 120 kph, my single best tank of fuel yielded 855.69km on 56.34 liters, which works out to 6.58 liters per 100km; the OAT was about 4°C.
• Anecdotal reports consistently indicate our cars run best with the OEM NGK plugs; your Denso plugs might be the issue.
• As a general rule, coils either work or they don't, there really isn't a situation where they will work under normal circumstances and then not work at high RPMs.
• Lucas injector cleaner is an absolute waste of money.
• Seafoam is another absolute waste of money.

 

wow that's pretty good. I was thinking maybe my maf is dirty as well and could be hindering my performance. I noticed when changing plugs that cylinder 2 coil has been replaced, which makes me think there's probably at least 1 more coil that is on it's way out. The car has 360k on it

 

I do not believe our cars have a MAF sensor, instead they use a MAP, and quite honestly, I've never seen actual evidence a dirty MAP will cause such issues.

 

I would suspect upstream O2 sensors more than MAP, but still it would be hard to make a call. Using at least 91 octane? Not that it should matters anyways.

 

Well, I sure hope it's not a problem related to the spark plugs I just changed, but I'm sure the ngks might help a little. the car has a slightly rough idle that's consistent. Like I said, there was one time that I floored it to pass someone and the check engine light flashed when I hit red line, and eventually shut back off, I checked for engine codes and it had a cylinder 1&5 misfire. I used my blue driver to read o2 and map sensor data, they seem to be within spec. Maybe I ought to start with changing the coils on those cylinders

 

That's why you test for bad coils by swapping them around. I doubt they are the reason, but it won't hurt to swap them to other cylinders.

What plugs did you use? Hopefully not some cheap ones from ebay or autolite.

 

They are denso pk20tt platinum plugs. The plugs I pulled out were cheapo ACdelco copper plugs

 

What made you chose Denso over the OEM NGKs?

 

It's the only thing my local Canadian Tire stocked other than champion copper plugs, I also figured since it was still a platinum plug and gapped at .040" I didn't think it would be an issue, surely it would run better than what was in there. At the time I didn't know running OEM Ngk's was so crucial at the time. I'm not quite convinced my plugs are causing this. The car has 360,000 kms on it and I think a lot of stuff is original, including coils, I noticed cylinder 2 coil has been replaced at some time. I've watched videos by MODSFOREVER, when he replaced coils and cleaned throttle body it made a huge difference. I'm thinking I will try throttle body cleaning, and looking into carbon buildup in the egr system, and potentially replacing coils.

 

Yea, I also don't think plugs are your mileage problem. Just curious as Rockauto has NGK and I am also from Canada. On my 3.5, I have to drive conservatively at 100-110 km/h to get 7-7.5L/100km on my touring MXM4 tires. Winters get me 8.5-9L/100km on my very new and sticky Xi3.
what tires are you running?

 

I was running cs5's as my summers that are now bald, my winters are star fires, I figured it was mostly due cold weather, but I'd like to solve my rough idle. So far I've wound it down to coils, dirty throttle body/intake manifold, and possibly clogged egr passageways. These can all cause a rough idle and bad throttle response from what I've gathered. What I find weird is that rockauto stocks 2 different ngk plugs for these, but one is the oem that's 12$ per plug and one is like 4$. I'd love to get 7L/100 at 120!

 

Well tonight I did a Seafoam treatment by spraying the aerosol version on the throttle plate and following up with throttle body cleaner once the motor was hot soaking. There was quite a bit of carbon in the intake and the car definitely runs better and idles smoother. It's not perfect but a big improvement, I'll monitor my gas mileage for a bit and see if there's some improvement