Is there any MPG increase from 6-speed/3.63?
04-17-2011, 10:46 AM
#1
Tour de Franzia
Thread Starter
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
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.
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.
Reply
0
0
04-17-2011, 11:39 AM
#3
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.
EDIT: Also make sure to change the RevPerMile (tire size) to be the same if you compare two columns.
Reply
0
0
04-17-2011, 11:43 AM
#4
Elite Member
iTrader: (2)
Join Date: Jan 2007
Location: Los Angeles, CA
Posts: 8,682
Total Cats: 130
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.
EDIT: Also make sure to change the RevPerMile (tire size) to be the same if you compare two columns.
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.
Reply
0
0
04-17-2011, 11:59 AM
#5
Tour de Franzia
Thread Starter
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
I appreciate the input, but unless you have a stock car or switched gearing without changing your tune, the data doesn't mean much.
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.
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.
Reply
0
0
04-17-2011, 12:08 PM
#6
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,338
Total Cats: 573
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.
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.
Reply
0
0
04-17-2011, 12:19 PM
#8
Tour de Franzia
Thread Starter
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
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.
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 wish I could score another 6-speed, I really like holding a spare in the garage just in case.
Reply
0
0
04-17-2011, 12:41 PM
#9
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
Reply
0
0
04-17-2011, 06:01 PM
#11
Supporting Vendor
iTrader: (33)
Join Date: Jul 2006
Location: atlanta-ish
Posts: 12,659
Total Cats: 134
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.
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.
Reply
0
0
04-17-2011, 06:03 PM
#12
Tour de Franzia
Thread Starter
iTrader: (6)
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,085
Total Cats: 375
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.
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.
Opening up a 5-speed is too much drama.
Reply
0
0
04-17-2011, 08:44 PM
#15
Elite Member
iTrader: (9)
Join Date: Jun 2006
Location: Chesterfield, NJ
Posts: 6,893
Total Cats: 399
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.
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.
Reply
0
0
