I'm currently without 5th gear and I get ~12mpg around town with AC. Does anyone know if I will see an improvement if I repair 5th, if so how much?

 

I doubt your MPG go up much, if any, driving in town.

I don't go into 5th gear till about 55mph; that way my rpms will be above 2k.

 

+1

I don't do fifth unless I'm above 50. from what I understand, rotaries don't like to be lugged below 2k RPM.

 

why are you "without" 5th gear? if it's the synchro, just double clutch.

 

yea, i never really use 5th unless im oon the expressway. 5th gear at say 60 mph leaves me very little power so i just leave it in 4th so i run about 3k rpm and get around 16 mpg

 

Wow, 12's still pretty bad, no matter which way you slice it. Normally you don't see that bad of gas mileage unless you've gone single.

 

Is my poor milage cause by using AC? Most cars have a ~2mpg penalty for using AC, is it worse for the FD3S or do I need to change my spark plugs and fuel filte, etc?

 

If you are on the stock ECU, then you are getting about the right mileage for your driving. It wouldn't hurt to do the normal maintenance of plugs, O2 sensor, air/fuel filter if you don't know the last time they were done.

 

I usually don't shift into fifth until about 62 because most of the time my speed varies too much. I did a little test and seemed to get better mpg. Mine is already bad, I'm lucky if I get 17 mpg. anything helps.


It's "somewhat" simple math.. if you use 10% throttle in 4th gear to maintain speed, and shift into 5th and have to use 25-30% throttle in order to maintain that speed, your throttle input dictates your ecu's fuel output, not your gearing... not to mention it requires a larger amount of input to make a change because you have less torque at a lower RPM....

 

Throttle percentage, or position, doesn't have anything to do with gas mileage. Gas mileage relies on the RPM of the engine. The harder your engine has to work, that is the higher RPM you maintain, the lower your gas mileage will be. You won't ever have a higher RPM by switching into a higher gear, because of said gearing.

 

isnt keeping the rpms down gonna give you better mileage even if its a little more? like 1-2?

 

Man I hardly ever see 5th. Unless I'm in the 80mph + range (4500rpm and above) I just drive around town in 4th.

 

ineffective

 

Yes.

 

What?? If you think this works ride around with your e-brake on at full throttle and 3000rpms and tell me if you get the same milage as riding around at 3k normally. Your inj. duty is going to be alot more at 3k at full throttle than at part throttle. And greater inj. duty= more fuel= less mpg.

Gas milage has to do with all these factors throttle position, load, rpms, speed(air resistance), etc, not just rpms.

 

This is an incorrect statement. Fuel is not only dependent on engine speed. While engine speed is an indicator of how much the engine is working, it is not the only factor that goes into calculating how much fuel is being used.

 

Yes. One of the best things I ever did was tune my car so it will run low for drivability. 5th gear is usable at 1500 to 1800 RPM (about 40 to 45mph) for cruising. I am able to get about 18mpg in mixed, spirited, driving.

As for engine efficiencies, reduced RPM always favors mileage; throttling it does not; running an engine at a low RPM at WOT and injecting just enough fuel will yield the best results. Sound familiar?

 

Agreed.

This contradicts itself. WOT and Just enough fuel, do not go together, unless for some reason you tuned it to dump just enough fuel to run at an efficient constant speed.

Anytime you are giving the engine more gas than needed to stay at a constant speed you are using more fuel than needed at ANY rpm until you hit the drag coeffient at 170+. If it didn't dump more fuel when you pressed the gas, it would never accelerate.

The key to best mpg is finding the most efficiend rpm in 5th gear and running at constant throttle, given you are on a level surface. Hence why you get better mpg by using cruise control than you do by accelerating and letting off constantly even though your avg. rpms may be the same.

 

Actually, it does make sense. If you're not throttling the engine then airflow is proportional to RPM and VE, and fuel can be injected accordingly. I think you have overlooked the obvious point: that this is the premise of operation for diesel engines. And it is the reason why they have superior fuel efficiency.

 

I understand that. It is just that WOT and injecting only enough fuel to keep a constant rpm doesn't work out. When you are runing a constant rpm, cruising down the road, you are not using all of air nor fuel that the engine can take in at that given rpm. At WOT you are basically maxing that out.

They way you it was stated, or as I understood it is: If I were on a load bearing dyno running 3k rpms and only using 1/4 throttle to keep it at 3k rpms I would run out of fuel at the same time as if the resistence were increased and I had to use WOT to keep it at 3k rpms. That just isn't true.