~9% improvement in one tank! Just not sure why...

[Mifunego]

In the last 3 months of earnest efforts at improving my FE through driving habits only (and extra-inflating the tires) I managed to improve my average by about 12%. That got me a ~32% over EPA - now "hypermiler" status I made one mechanical change since then. At the end of the tank before last, I swapped out one of the three serial cats on my exhaust with one of the high-flow race cats. That next tankful was just slightly higher the same mpg as I have been getting, averaging 26.17. But then after that tank, BAM! the big jump an additional 9% (over average) to a whopping 28.55 mpg! My commute has been the same, and I even had a dreadful 30 mile, bumper-to-bumper, stop and go jam thrown in to bring the FE down even more. Yet, I showed a more than standard variation this tank.

Could it be the switch to summer gas already? Even if it was, would the FE improve that much? Based on the averages i was getting in my leadfoot days, I saw no substantial difference between summer and winter gas mpg. If this is a result of the new cat, I'm still glad, but just a bit surprised that it skipped a tank to show its benefits. Of course, it's only one tank, so who knows what will happen next time I fill up. But I'll be optimistic anyway.

[hummingbird]

Weather can be a differentiator. if you see long termers' fuel logs, you can clearly see a sinusoidal pattern that dips in winter, and gets better in summer. IC engines just luuv warm temperatures.

9% gain as an isolated occurrence can be because anything. And it can disappear as mysteriously as it appeared. If you can average the same for the next three tanks, then go looking for causes.

Looking at your log, you seem to get very good FE for a couple of tanks, then fall back to your usual 25-26 pattern. What is different for those couple of tanks? Long trips? Change in gas? pumped up tires? Did you think about analyzing that seriously?

[colonelk]

could it be the engine management finally tuned into the new cat? They take a little while to learn dont they? Or I could be talking rubbish....

[wagonman76]

Pump may have kicked off prematurely. I'd see what the next fill looks like, it should average out.

[Mifunego]

Hummingbird: Yes, I've heard of the seasonal temp change as a reason for improved FE, I've just never experienced it; even in this car. And while it is warmer in Chicago today, I think the average temp is still pretty close over the last couple tanks. But like you said, that 9% bump could be a fluke - it's just one tank. I sure hope not! Those few 28-29mpg trips are long, traffic-free, 99% expressway jaunts. I've yet to be able to stick to 60 or slower in a 70mph zone.

Colonelk - I've heard the same thing. I reset the ECU after I installed the race cat. There may have been some learning going on for a while.

wagonman76 - that was my first thought: there was a simple miscalculation. But my factory mpg display seems to indicate otherwise. It shows a similar improvement in average mpg over that tankful (although it is consistently optimistic by about 2.5 mpg all the time). And I sure hope it doesn't average out! However, I just replaced my winter wheel/tires with the summers. The switch in tires should be beneficial, but the rims went from 17 to 18 which we all know is usually bad. So who knows what will happen.