Drive axle question

[Pu241]

Got a surprise too!
Filled up in STL for the trip to Indystiva and on the way back I thought to myself "I've come a long way on this tank."
Made the round trip on a single tank 504.6 miles and put 9.522 gals in it, adjusted for the 155's I'm running via the GPS and I get 50.4 MPG.
Not bad for having a Check Engine Light on (throwing a 15 and 17 code), broken down pipe bolt and flange to the front of the cat, as well as a suspected stuck/clogged IAC valve.
Now I did have a full upper grill block!
I try to keep at 60 MPH, but with the construction on the way in and finding a truck crusing at 57 MPH on my way back, my average was near 55 MPH for the trip.
But hell, I'll take 50 MPH every day.
Just begs the question exactly how much does the O2 sensor affect MPG, not to mention that the IAC must only function at lower RPM.

BTW, I want to thank Gauge and his wife for having us.
Not only was he the host but the cook as well.
On top of chief mechanic at his own meet!
Thanks again Dennis and as always it was too short!

[TominMO]

Just filled up after driving the last 60 miles to Indystiva, then back home. Mostly drove 65-70 (3000 RPM) for this leg. Surprisingly, I got 42 MPG! Compared to the 45.2 I got doing 55ish (2400 RPM). I can only credit this to better aerodynamics on the Aspire vs. a Festy.

[nitrofarm]

Thats excellent mileage Tom. :thumbs_up: I have to clarify that my RPM is 3150 @ 77mph not 75 as previously mentioned. My speedo is @ 2mph off. I noticed that 65-70 is the sweet spot for my car with the Festy gearbox. But its really impossible for me to drive 65. Most times I'm floating along 77 mph on my 52 mile work commute.

[TominMO]

tom, if you get rid of that aspire trans, let me know....

I'll keep it as a backup actually, Arty.

Nitro, I just did a run this morning in my Aspire w/Aspire 5-speed. Driving at 55-58 average, I got 45.2 MPG over 170 miles. On another identical Aspire (white 95 SE), I got 46mpg driving at about 60 mph, but may have had a little headwind help on that trip. On my drive home today from Indystiva I got about 40, driving above 65 all the way.

[FestYboy]

tom, if you get rid of that aspire trans, let me know....

[blkfordsedan]

Good to hear that. After I ran the numbers, that stock Aspire gearing really makes it wind out. Now I see why you guys want to go to the Festy gearing! The Aspire trans would be great if you were running 14" or 15" wheels.

[nitrofarm]

I have had a Festy 5 speed in two different Aspires and have logged @ 60k total. It def helps with the mpg's 22.6 tire dia is @ max you wanna run though. @ 75mph I'm turning 3150 RPM. I drive it hard (75 mph is my normal cruise) but my wife just clicked off a 44mpg on a 120 mile run. Not bad if you ask me,but I'm partial I'm averaging 38-42 in this cold damp weather.

[TominMO]

Interesting info, thanx zoom.

BTW Brian, 3000 RPM in 4th in my Aspire comes to 55 MPH, so at the risk of annoying people I could almost reasonably drive at that speed on the hwy to check fuel consumption.

[zoom zoom]

As far as your axel question, I will bet you dollars to donuts (literally too I love donuts and I know I'm not gonna lose) the only difference between aspire and festy axels is that the inner cv cup is longer for an aspire axel, giving it that extra almost an inch worth of extension/travel. From the hub side that butts against the races to the tripot bearing that goes into the inner cv cup, it is the same distance.

[blkfordsedan]

Using the chart that Drumnerd33 posted, the drive ratios are as follows;

Aspire- 5th gear is .763:1 and the final drive is 4.058:1. This gives an over-all ratio of 3.10:1 in 5th.
Festiva- 5th gear is .692:1 and the final drive is 3.777:1. This gives an over-all ratio of 2.61:1 in 5th.

Khumo lists their 155/80R13 as 914 revs/mile....so at 60mph the tire will be turning 914 RPM.

Aspire- 914 x 3.10= 2834 engine RPM at 60mph
Festy- 914 x 2.61= 2386 engine RPM at 60mph

[blkfordsedan]

I thought about the 4th vs 5th test, but the car is set up from the factory ( gears & tire diameter ) to be driven at a highway cruise in 5th gear and still have the RPM in the acceptable torque range. In your case, about 2500 @ 55. The issue would be trying to maintain 55mph while only turning say 2000 rpm or less. Im guessing thats the whole reason that Mazda put a steeper final drive ratio in the Aspire...to compensate for the taller 13" tires and get the cruise RPM back up to where it is most efficient. Probably to also help with the added weight of the Aspire. It's no big deal.....It may not drop the RPM far enough down to be in a "dead zone" and you may still have plenty of torque at the lower RPM range.

[TominMO]

A good test would be to drive it in 4th at a given RPM, say 2500, then do the same in 5th at the same RPM. That would isolate the effect of wind resistance. In 5th would be easy, because that would be normal hwy speeds, but driving in 4th at 2500 would be slower, and potentially unsafe since people will be driving much faster around me, and might even rear-end me if they weren't paying attention to how fast they were coming up on me.

In my Aspire (with the Aspire 5-speed), 5th @ 2000 RPM = 45 MPH; at 2500 it would be 56, and @ 3000 it would be 68. But 3000 RPM in 4th might not be reasonable on the hwy; I'll see what that speed is later today.

[blkfordsedan]

My thought is that the main reason for 62 MPG @ 45 mph and 52 MPG @ 60 mph is not due to the lower engine RPM as much as it is the decreased aerodynamic drag and HP requirements at the slower speeds. My theory would be that if you drive the car at a consistent maintained speed of say 60mph with the stock wheels and tires and turning say 2700 rpm (I just made that up), and then drive at the same maintained 60 mph with taller tires and/or different gearing that drop the cruise RPM down below the engines optimal torque band (say 1800 rpm), your MPG will either decrease or at least not increase enough to warrant the changes or reduction in performance.

In physics terms, lower speeds=less work=less fuel used. If you dont change the vehicle speed, Cd or vehicle weight, you're not decreasing the work required. Given these constants, the best MPG would be achieved by having the engine operating at an RPM where engine output is most efficient and extra fuel is not being used to sustain an engine RPM in excess of that which is adequate to maintain speed based on power output. This all depends on where the engine develops its torque and is most efficient. I could be wrong, and you may be on the right track. I'm sincerely hoping it works out for you....sometime I over think things (according to my wife)!

[TominMO]

Interested to see how this turns out. My fear would be dropping the cruise RPM too low and getting it out of the optimum torque range. The result would be a decreased ability to maintain speed on hills, requiring more "foot pedal" to overtake the hills, other cars and get up to cruising speed. If you do a lot of stop-n-go driving, the decreased torque advantage combined with the taller tires and heavier weight could also hurt the average MPG and make the car more sluggish to drive.
Not trying to criticize your plans, Tom...it's just what my instincts and experience tell me. It would be great if you can actually get some real world results (and hopefully prove me wrong!)

Good insights, and I am aware of them too. My experience has been that I get the best mileage at the lowest RPMs regardless of these factors. At 45 mph for 85 miles I got 62 mpg with my stock Festy, the only mod being 155/80x13 tires, for slightly taller gearing. RPM at 45 was about 1800 IIRC. At 55 the car got 58 mpg; at 60 it got 52.5 I think, all tests done on the same hwy loop. All results seemed very consistent with rpm and wind resistance increases/decreases. In the city, using hypermiling techniques (which I don't normally do, but was curious about the results), the car got 50 mpg.

As far as power--I don't have a cat, I've increased the timing and removed the lower airbox, and it really moves. The next mod is Matt D's exhaust manifold, then his ported head and intake. He claims I'll get about 125 HP with this setup, basically a stock BP, so enough power for hills is not my concern! :-) All the more reason I want to lower the RPMs for hwy cruising, since I will suffer an estimated 20% mpg drop due to the much greater airflow, even if I can keep my foot out of it (ha!).

Edit: the best test for mileage I have done on a stock Aspire 5-speed, driving about 360 miles on the hwy, gave 46.75 mpg at 60 mph. My earlier 85-mile test gave 48.25, but of course was not as real-world. Still pretty close tho, considering.

If you do a search on my user name, you can find all the threads I started in General Discussion on my fuel mileage tests. Use "Search single content type" and "Find threads started by user".

[Pu241]

Yes, that is a possibility, but then other mods like dropping back to 13" wheels and tires or using an intake spacer to increase low end torque might need to be used.
BTW, I haven't put the spacer Skeeters_keeper made for me on my B3 yet so don't know if that will work.