Understanding Fuel Milage
 

In a quest to reduce my own fuel consumption and try to understand the numerous complaints about fuel milage, I logged about 3,5oo data points and sorted them in a table to try and find out what rpm range is the 6-Speed RX8's sweet spot for fuel milage. This is actual data, not a recollection of what your driving habits might have been over the last tank of gas...

Table 1..... MPG - Engine Load vs RPM

This is the most important of the tables I have attached as it shows fuel milage as a function of engine load and rpms (fuel milage is the white output cells). Looking at engine load is better then looking at throttle position or speed because the load calculation reflects not only the throttle position but also how much stress is being put on the engine (is the car going uphill/downhill/bogging in a low gear).

**Keep in mind the fuel milage calculation is only as accurate as the MAF/O2 sensor/speedometer so the actual numbers themselves may be skewed - HOWEVER the relative difference when comparing the numbers (is it higher or lower) should be fairly accurate.**

Bottom line - if you look over the results, you will see the engines best fuel milage comes between 3200 - 3400 rpms, with below 3200 rpm milage still being pretty decent and above 3400 rpms milage falling off very quickly. So for best milage you should try and stay in the 3000-34000 rpm band whenever possible. This also seems to correspond well to real world experience. As a side note, Mazda must also have concluded this as our car's [legal] cruising speed at 65mph in 6th gear is in this peak zone - yes the engineer's were doing their job when picking gear ratios.

ONe exception to the sweet spot is at slow speeds (say constant 25mph) the engine load itself is dramatically less in 5th gear then 2nd or 3rd gear, so you can still get good fuel milage at a less efficient rpm because the engine isn't working as hard.

Also note: I did not include much detail higher in the rev band because: a) it wasn't very good, and b) I have a very well tuned CZ unit above 6000 rpms and across all throttle positions and didn't want that to effect these results

Table 2.... MPG - Speed vs RPM

I included this just for a reality check and the fact its a little easier to relate to. The same results basically come out of it (3200-3400 sweet spot), but its less accurate since you can not tell if i'm going uphill or coasting or at WOT which will severly effect your fuel milage. I did try to filter out things above 40mpg to reduce the effects of coasting (120mpg can really skew things!) but those factors are still in there.

You did all this work to prove that going slow and not revving the engine means you get better gas milage?? I would have never thought of that!

What I wanna know is why my 7 ('84 gsl-se, 13b) got better mileage in top gear at 90 than it did at 65... :confused: :eek:

Similarly, my 8 is getting better mileage if I drive with gusto around town, taking off at about 1/3 or 1/2 throttle, shifting at 4 or 5 or even 6 k instead of driving like a corpse, taking off at very light throttle and shifting at 3..

Wankels have always confused me... I flog mine, and they get me better mileage..

:D

What about Carbon Buildup problems if always trying keep in the optimal RPM range to save gas?? Isn't this a problem for Rotaries??

's a problem for other engines too.. my miata didn't like putting about, and neither did my 7.

Drive 'em fairly briskly. They likey.

Old hi-performance cars with carburators were kinda brutal with carbon buildup.. ever hear the term 'Italian Tuneup?' It was what's done to fancy sports cars whose owners were too much of a wuss to drive her properly.... the horrors of carbon..

With today's motors, the problem's a bit less..

But if you're concerned for the internal cleanliness of your mill, just take 'er out to a freeway, and redline every shift a couple of times, with your foot fairly well into it.. she'll thank you.

All I have to Say
 

Here is the deal (my deal). My 8 is not my daily driver. I drive it on the weekend and then, I would call it "Spirited". When it has been driven, as I assume, as most of you would drive.....Normal 8 driving, a beep now and then......I'm in the 17mpg range. But when the skies are clear, the road is twisteee (check out the S.E. forum...Places in Atl for Vid's).... and we let the DOGS out! I have gotten no less than 20+MPG's. Now I ask, how is this possible? How I calculated this is simple...drive like a wuss "till I get there. Fill up and check mileage...around 17mpg. As the say in the forum, then, drive like you stole it, fill up again, always above 20mpg. Anybody want to put that in a graph....post it, and you may be on to something. Otherwise, by a Prius or a moped, or Shutup about mileage. I don't relly care.

It is what it is....so troll and flame....I won't be back to read it!!!!!!!!!!

a) this took me about 15 sec to do (you may notice its a 16x16 grid like someone may put together to tune an e-manage... :rolleyes: )

b) anyone that claims they get great milage at high rpms is full of ---- or severly delusional... this proves it as you can't get squat for milage at high rpm's even with being leaned out with tuning :o

c) what the hell is the point of driving around at 7k rpms in bumper to bumper traffic... oh right, thats "fun" :rolleyes:

I have to agree with the drive it hard theory. What really kills my mileage is slow creeping stop and go traffic, it stays under 4000 rpm but my mileage stinks. I once got 15 mpg doing only slow commutes. Normally I am around 18-20 mpg.

I'm starting to believe in the "higher is better" theory myself. If I drive through a tank of gas entirely within the city, barely ever going above 3500 rpms, I tend to get between 14-15 mpg. On the other hand, on tanks where I take repeated trips up to my favorite mountain twisties where I can take the rotary up where it belongs, I get 15-16 mpg. I despaired of ever breaking through 15 until I noticed that my 8 got more then 15 on those tanks where I got to wring the engine out a bit. It's a trend that has continued, interestingly enough.

I do still wonder about the ideal cruising range though...I've read comments so far that said it's better to be up around 3200-3400, while there have also been comments that said the ideal cruising range is still 2000 rpm. I've tried both and haven't noticed a major difference in MPG.

That's interesting. But sometimes the numbers almost seem random. For instance, why would the mpg go up over 10 mpg at 3400 rpm when going from a load of 35 to 40? Or drop 9 mpg at 3200 rpm when going from a load of 25 to a load of 30? Seems weird.