5th gear - less MPG?

[FCRotary]

My friend and I both have noticed that in 5th gear we get worse gas milage than when we stay in 4th, but don't the 2nd injectors kick in at 3,800 RPM or so?

If running in 5th gear at 2.5k RPM's wastes more gas than being in 4th at the same speed, what is the problem?

[FerociousP]

what speed?

injectors are also load based

[TehMonkay]

Yeah theyre load based. I found when i go 80 i get better fuel mileage, i assume there may be better fuel atomization or the fuel may burn better at a higher speed.

[VinnyV81]

My car gets horrible gas mileage pretty much whatever I do! LOL

[josh18_2k]

i find i get the best mileage in 6th.

[Whizbang]

i have a theory on what might be the issue.

if your in 5th at say at 60, there isnt a lot of load on the engine. And generally, your foot on the gas pedal can vary a lot but not really change your speed. Whereas in 4th there is more load and you are aware of how much pressure your applying to the pedal. Because the TPS sensor will control fuel based on rpm as well as throttle position, you have to be pretty aware of what your doing with your foot.

Generally speaking im usually in 5th while cruising over 30 mph. that is were i find the best mileage. I had a lot of time to play with this idea at freeway speeds driving cross country in the vert and 5th got the better mileage but you couldn't really rest your foot on the secondary butterfly spring resistance like you can in forth.

[josh18_2k]

try it with cruise control on the entire time. that should give you more reliable results.

[Ender]

I know in my TII I get significantly better mileage at or under 75mph in fifth than anything above that speed (I also have a lower secondary staging RPM due to my Rtek 1.7 ECU).

If you're below ~45mph in fifth, you very well might get better mileage in fourth gear. If I understand it correctly, most gas motors (most 4cyl, 6cyl, but especially rotaries) don't make great torque below 1,000-2,000 RPM. The rotary probably doesn't do so hot under 3K either. Somewhere in between 2,800 to 3,800 RPM for a stock FC is where I'm guessing the mileage 'sweet spot' is for cruising. Any other thoughts on that?

[My5ABaby]

How are you measuring gas mileage? The proper (or at least best that we can easily accomplish) is to fill up several times, measure how much gas consumption you've had, and then calculate MPG. Do this for both 4th and 5th gear.

Also, 2.5k sounds kind of low for 5th gear. I usually don't go to 5th gear until at least 55 mph (not sure on what rpm that is).

Finally, why do you have a TII exhaust on an N/A?

i have a theory on what might be the issue.

if your in 5th at say at 60, there isnt a lot of load on the engine. And generally, your foot on the gas pedal can vary a lot but not really change your speed. Whereas in 4th there is more load and you are aware of how much pressure your applying to the pedal. Because the TPS sensor will control fuel based on rpm as well as throttle position, you have to be pretty aware of what your doing with your foot.

Generally speaking im usually in 5th while cruising over 30 mph. that is were i find the best mileage. I had a lot of time to play with this idea at freeway speeds driving cross country in the vert and 5th got the better mileage but you couldn't really rest your foot on the secondary butterfly spring resistance like you can in forth.
For the S4 it's more based on the AFM, rather than the TPS. Although it is still useful for cruising, the TPS on S4's only goes to about 30% throttle.

[FCRotary]

I do switch into 5th at 55 or so, if you switch it in any lower you feel like you're bogging out, at least my car.

And I bought it with the t2 exhaust on it, didn't know it till a buddy looked under the car and recognized it, hahaha...

[JunpoweR]

I drove back from LA to Oakland (5hr 45min) drive and about 480 miles.
I had an engine installed and they forgot to put transmission oil into my tranny.
(Rotary Power) Blew all my gears and left with 4th only.
Waited for the lady to come from long beach with some transmission oil while I was stuck at the bottom of the grape vine.
Filled her up and started driving another 380 miles in my only gear, 4th!
Seemed to get way better gas millage then in 5th.
I also made it up my driveway doing a 7000rpm clutch burn in 4th to get it moving.
I believe MPG greatly depends on throttle position regardless of what gear you are in.
Try putting a brick under your gas pedal so you can only step on the throttle about 1", I am sure your gas millage will soar! lol
It just depends how much throttle you need to use to hold a certian speed. Sometimes 4th will hold 75-80 mph better than in 5th gear, meaning less throttle = less gas.

[NoDOHC]

Awesome discussion! I did some research on instantaneous fuel economy when the '86 was still N/A and I had the Haltech in it.

With some quick math, I discovered that instantaneous fuel enconomy in MPG can be computed easily with the injector energized time (assuming operation below staging setpoint. Please bear with the numbers as I was running 50 degree BTDC timing and 16:1 AFR.


5th gear, MPG = 110/injector on time in ms.
4th gear, MPG = 80/injector on time in ms.
3rd gear, MPG = 58/injector on time in ms.
2nd gear, MPG = 39/injector on time in ms.
1st gear, MPG = 23/injector on time in ms.

At various continuous speeds in 5th gear I had:
23 mph: 31 mpg
46 mph: 28 mpg
55 mph: 32 mpg
61 mph: 36 mpg
66 mph: 32 mpg
71 mph: 36 mpg
75 mph: 38 mpg

These numbers were extracted from the Haltech data log and entered into a spreadsheet to compute the instantaneous mpg. I tried to pick spots where the throttle position was constant (level patches of road).

In fourth gear I had:
20 mph: 24 mpg
30 mph: 28 mpg
35 mph: 30 mpg
40 mph: 32 mpg
50 mph: 26 mpg
55 mph: 24 mpg
60 mph: 21 mpg

So what I can conclude is that you are actually better off in fourth gear up to 50ish mph

(The relative engine efficiency should not have changed between a stock ECU and the Haltech, even if the efficiency is better overall).

edit: If I entered data above 75 mph, it might indicate that I was speeding, which of course I wasn't.

[My5ABaby]

38mpg? I know you were running a standalone but...

[NoDOHC]

That is instantaneous fuel economy. I usually got 30ish driving to work.

[Ender]

That is instantaneous fuel economy. I usually got 30ish driving to work.

Oh yeah, I kinda forgot for a second that an instantaneous number wouldn't figure in stops/starts and all the fun accel/decel portions :willy_nilly:

[Fidelity101]

I rock 4th gear 3krpm at 60mph, around 65 and higher is when I use 5th.