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Old 04-17-2011, 11:46 AM   #1
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Default Is there any MPG increase from 6-speed/3.63?

I have a spare 6-speed and spare 3.63 and I'd like to increase fuel economy on my daily, what can I expect? I'm going to drive from Dallas to Houston about 2x per month for the next few years to see the GF who's going to Rice.

I have a significant fuel economy improvement on my turbo car, but there are lots of variables there. I'm getting 28mpg in the daily at 75mph, 34mpg at 85-90mpg in the turbo car.

I'd like to hear from the NA crew more than anyone else because of considerations for the 4000rpm open-loop switch that I'm subject to at 75mph. Now that some Texas country highways will be 85mph, I want the gearing.
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Old 04-17-2011, 12:27 PM   #2
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NB2, 1.8 N/A, 6-speed, 3.63 gears. 33mpg doing 75-80mph. AFRs at 15.5 in the 40-65kPa areas. Try to stay under 60kPa and you should be golden.
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Old 04-17-2011, 12:39 PM   #3
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Not sure if it will help, but I made a spreadsheet (attached) with max speed and revs at certain speeds with different transmissions and diff ratios. Change the stuff in blue to compare. I would have liked the miata 6speed to be a little taller for street driving.

EDIT: Also make sure to change the RevPerMile (tire size) to be the same if you compare two columns.
Attached Files
File Type: xls miata_gear_ratios_v3.xls (22.5 KB, 145 views)
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Old 04-17-2011, 12:43 PM   #4
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Quote:
Originally Posted by orion4096 View Post
Not sure if it will help, but I made a spreadsheet (attached) with max speed and revs at certain speeds with different transmissions and diff ratios. Change the stuff in blue to compare. I would have liked the miata 6speed to be a little taller for street driving.

EDIT: Also make sure to change the RevPerMile (tire size) to be the same if you compare two columns.
Check this out

http://www.flyinmiata.com/tech/gearing.php



Hustler can you disable the open loop switch? I've never heard of that before.


I have NB 6sp/3.63rp and get about 17mpg but I drive like an angry rhinoceros.
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Old 04-17-2011, 12:59 PM   #5
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I appreciate the input, but unless you have a stock car or switched gearing without changing your tune, the data doesn't mean much.
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Originally Posted by Reverant View Post
Try to stay under 60kPa and you should be golden.
The factory computer goes open-loop and ~12-ish AFR at 4000rpm I believe.

I have a stock computer on a stock car and I'd like to know if the new gearing will make a dramatic increase on MPG.
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Old 04-17-2011, 01:08 PM   #6
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theoretically it should have some impact.

when you shift to a lower ratio (higher # gear), your motor runs at lower RPM (less injections per time unit). but since the throttle must be opened further to maintain equal wheel torque you do increase load slightly BUT you reduce throttling losses and the engine runs more efficiently.

generally speaking, if you short shift your daily driver you gain the same benefit. spend more time accelerating in higher # gears insted of winding it out and you'll get better gas mileage. that's why "efficiency" models of cars often have super tall gears combined with low RPM torquey motors.
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Old 04-17-2011, 01:09 PM   #7
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Yeah the stock ECU does go open loop at 4000rpm, but if you stay in the low load areas, it should still be in the low 14s. Certainly not ~12ish.
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Old 04-17-2011, 01:19 PM   #8
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Quote:
Originally Posted by y8s View Post
theoretically it should have some impact.

when you shift to a lower ratio (higher # gear), your motor runs at lower RPM (less injections per time unit). but since the throttle must be opened further to maintain equal wheel torque you do increase load slightly BUT you reduce throttling losses and the engine runs more efficiently.

generally speaking, if you short shift your daily driver you gain the same benefit. spend more time accelerating in higher # gears insted of winding it out and you'll get better gas mileage. that's why "efficiency" models of cars often have super tall gears combined with low RPM torquey motors.
I understand all that...I'm trying to see if the MPG increase is worth it. I don't know if I should expect 1mpg or 10. The open-loop **** is a big variable of course.

I wish I could score another 6-speed, I really like holding a spare in the garage just in case.
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Old 04-17-2011, 01:41 PM   #9
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speaking only at cruise. calculate the pulse with at your current rpm and afr. assuming it targets same afr with new gears, calc the pulse width of cruise rpm with new gears. you can equate a pulse width to a volume of fuel. you ca probably calc the exact quantity of fuel saved at cruise.
.
A quick and dirty estimate that should ballpark it would be to just take the ratio of the number of injector pulses at both rpms. This assumes that the injector pulswidth remains the same, which isn't correct, but at cruise I would think that the MAP values for both gearsets would be close, resulting in a similar pulswidth for the same AFR
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Old 04-17-2011, 02:56 PM   #10
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Thanks for that spreadsheet, orion.
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Old 04-17-2011, 07:01 PM   #11
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The 6 speed works against you if your goal is to keep revs down, and the cost of a 6 speed + 3.6 gear swap will have a very poor ROI in terms of fuel savings.

My recommendation for economy would be the 5 speed with S4 RX7 5th gear. If you want to go further then add a 3.9 rear. That will still be reasonably quick in the lower gears, but put the car in a coma in 5th--well at least relatively speaking.
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Old 04-17-2011, 07:03 PM   #12
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Quote:
Originally Posted by Ben View Post
The 6 speed works against you if your goal is to keep revs down, and the cost of a 6 speed + 3.6 gear swap will have a very poor ROI in terms of fuel savings.

My recommendation for economy would be the 5 speed with S4 RX7 5th gear. If you want to go further then add a 3.9 rear. That will still be reasonably quick in the lower gears, but put the car in a coma in 5th--well at least relatively speaking.
I have a spare 3.63 and 6-speed both in the garage.

Opening up a 5-speed is too much drama.
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Old 04-17-2011, 07:06 PM   #13
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Just sell me your 3.63 and you'll have a bunch of extra money for gas.
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Old 04-17-2011, 07:10 PM   #14
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Quote:
Originally Posted by Ben View Post
The 6 speed works against you if your goal is to keep revs down
Maybe in light traffic, how so on the highway?
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Old 04-17-2011, 09:44 PM   #15
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Quote:
Originally Posted by Faeflora View Post
Check this out

http://www.flyinmiata.com/tech/gearing.php



Hustler can you disable the open loop switch? I've never heard of that before.


I have NB 6sp/3.63rp and get about 17mpg but I drive like an angry rhinoceros.
Wait...soooo why go with a 6 speed if the 5 speed puts revs lower in high gear?
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Old 04-17-2011, 10:02 PM   #16
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Quote:
Originally Posted by TurboTim View Post
Wait...soooo why go with a 6 speed if the 5 speed puts revs lower in high gear?
103whp is why. I'd do the 3.63 on the 5-speed but I'm worried it would be dreadfully slow...that's approaching vette gear ratios.
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Old 04-17-2011, 10:23 PM   #17
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So ******* boost it WTF?!

seems like a lot of work for a little benefit if then the 5speed/3.90 combo is too much "benefit" (and less work).
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Old 04-17-2011, 10:33 PM   #18
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Have you considered a MS? A used PnP is reasonably cheap, and will probably net you more MPG than the drive train swap would. I picked up about 6 MPG on a mostly stock motor with just a good tune. In for results regardless.
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Old 04-17-2011, 10:39 PM   #19
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Quote:
Originally Posted by baron340 View Post
Have you considered a MS? A used PnP is reasonably cheap, and will probably net you more MPG than the drive train swap would. I picked up about 6 MPG on a mostly stock motor with just a good tune. In for results regardless.
buying MSpnp takes dollars, the gear mods are free.
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Old 04-18-2011, 01:05 AM   #20
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Quote:
Originally Posted by hustler View Post
buying MSpnp takes dollars, the gear mods are free.
...

why not just do it then?
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