B_Swapped93

Posted this waaay back, I'm now posting it again which is ideal for increased fuel economy. A higher RPM lets say cruising at 3.5-4K(in 4th) with a little throttle load OR cursing at 2.5K(in 5th) rpms with an increased throttle load. Lets say this is flat city/town driving so no need to down shift, which wins out are you dumping more fuel while at a higher RPM or at a lower rpm but your foot is notably more on the throttle. In theory the more your foot is down on the throttle the more fuel you're dumping right?(bringing the injectors closer to max duty cycle) or does that not count when you're in the lower RPM band vs flooring it at 8.5K rpms. If someone could explain it that would be great I tend to be at the lowest RPM possible, I don't bog it but there are times I feel like it is too low a gear for my speed.



by load in mean throttle input aka foot further down on the pedal.

nicknack25

I too ponder upon this. I feel at higher rpms with little throttle load, you'rstill using less fuel because of exactly what you're saying. The higher throttle load, the fuel output increases. And i feel like if you're at a lower rpm with a higher throttle load, then a lot of fuel is being used. I think you just gotta find/listen to that perfect rpm. Well at-least that's what i try to do lol. I just try and keep a constant speed with little throttle load. Which is why mpg is increased on freeways. Throttle doesn't fluctuate as much as in city driving.

But i'm not well educated on fuel injectors or the topic so i kinda just came up with my own "theory". Might just be a load of bs lol. But i am interested in a more intelligent response.

tony_2018

Android phone? Look to the app torque and buy a bluetooth obd2 reader. I've used this to calculate mpg and I notice if I'm high revving (not super high, but around 3500 -4000rpm, low load ( light on the gas pedal), the mpg did increase. But if your on hills, going up and down, this will vary. I also test this vise-versa as well and it gave mix results.

B serious

iTrader: (1)

Around town, you're not going fast enough for load to be a factor. Shift to a higher gear when cruising.

Creator

iTrader: (1)

I'm no engineer but I've also put a little thought into this dilemma. I've concluded a couple of things.
My first conclusion might be a little out of date as many of the highway speeds have changed. I had assumed that honda would try to get the best milage out of the top gear at 55 mph. So if your in a stock car check your RPMs under this condition. Maybe I'm excluding a lot of factors, but my presumption was that depending on your speed you should stay in the gear that gets you closest to that same RPM.

The other thing I deduced based on current vehicle manufacturer trends. Ever notice how cars are coming with 7-8 gears now. Most of these vehicles you wouldn't put into 8th gear and expect to get much acceleration out of the car, but it does offer lower RPMs at higher speeds. I'm almost certain this isn't just for the comfort of lower vibration out of the engine, it's for increased gas milage while cruzing on the highway. So that would lead one to believe that as long as your not accelerating you would want the lowest RPMs possible to save on gas.

B serious

iTrader: (1)

Efficiency has a lot to do with tuning and engine design. Cars with more gears have the advantage of having the correct gear for more situations. Highway MPG has increased, but tuning the entire driveline is the key.

For what we're discussing here, know that the LS/GSR/ITR all get virtually the same MPG listed by the EPA. In my experience, shorter gearing doesn't hurt highway MPG for B18C engines. But definitely hurts the B18B/B20 engine. This has to do with a million factors.

I've owned an ITR, GSR, and LS at the same time. All were 2001 model years. The ITR got the best MPG everywhere. The GSR got better MPG around town when I put a B16 trans in it...but lost about 1/2-1mpg highway.

Weight and the engine's efficiency were the winning factors, I'm sure. Honda's stock gearing for each motor is tuned for the dynamic of the engine and use of the car.

Around town, you will NEED to keep rowing thru gears to get the most efficiency. I feel like I get better MPG around town than most people. Shift when you need to. Use common sense. Try different combinations of gears/speeds/techniques.

tony_2018

Gearing is important but so is the tune, as B-serious stated. The d15z7, d15b 3 stage vtec, is capable of giving good gas mileage even with a 4spd automatic (my only package with custom wiring) but is best optimal with a 5sp manual.

Dagobah

I've heard that for good gas mileage you want 1 a small engine 2 high torque at 3 low rpm.

Supposedly, the small engine produces less friction, and running at low rpm produces even less. The torque is needed to run the lower rpm, which is more in line with what you're asking.

I've done a lot of highway driving in my 95 GS-R, and I've thought about the same thing you're wondering. According to the manual, the shift point no matter what gear you're in seems to be around 3500rpm. I think that on a theoretical road that is completely flat, you'd get your best mileage at the lowest rpm of the highest gear you can run without stalling the engine. Or maybe for the GS-R, it'd be like...2500rpm in 5th gear. Much lower than that and the engine seems to start straining because you're running it in a non-optimal gear.

At 70mph though, I'm doing over 3500rpm, and because of the power that the engine produces at that rpm, the car seems to hardly notice the hills. Having driven a newer car with a live mpg reading, I can see how violently the mpg fluctuates on the upside and downside of hills, or on take-offs and coasting.

To date, the best I've gotten is 35mpg and that was driving cross country when I fill the tank up and don't stop again until I need gas.

Long story short, I have no solid answers. Just some observations.

B_Swapped93

I think its a bit too hard to tell on without a built in mpg calculator, the only vehicle I have with that is the Tundra and that's a big auto V8 so its not even valid. I doubt the s300 has anything like that, the car is turbo'd now making the low 340whp range so I guess mpg is no longer a worry lol. Thanks for the input tho, gearing is the most significant factor I think. Right now I'm driving my 1.6l ex and it seems that no matter how I drive I can only gain/loose 40km's(25miles) per tank with gas as cheap as it is that's not really negligible I don't know what that translates to mpg wise, but maybe I gain or loose 5 or so bucks if that not even haha.

Dagobah

I didn't know your car was turbocharged. I like those low 300hp builds. My pipe dream is to shoot for something around 350bhp.

Having a turbo, your engine should satisfy those 3 big components. Do you have any idea what torque you're producing around 2000rpm to 2500rpm? With the turbo, and I'm assuming you haven't changed the gearing, do you feel like you can shift earlier without straining the engine? Theoretically you should be able to.

I saw a turbo build once where the guy made it work out that out of boost, the turbo was just barely forcing air into the engine, the idea being that instead of having to suck air in, which reduces power, the engine can spin more freely. The guy claimed 45 or 50mpg. I've also heard about a guy with an s2000 that was turbocharged and he claimed he could get 40mpg.

Since you have tunes and maps and all that good stuff, is there a way to get variable maps, say, a conservative tune and then a race tune? I love the idea of that but I don't know if it's available. Also, maybe there wouldn't be any purpose for it on a vtec engine, since it already has two modes.

speedjunkie_g35

I went from 34 mpg on a stock gsr with full bolt ons to 27 when I added a 98+ JDM ITR transmission (hwy use, city never changed noticeably, then back to 32 mpg after I boosted my gsr to 305whp. I prob would do even better if it wasn't such fun to drive. I def think our cars have a certain rpm's range where they are the most fuel efficient. In 5th gear at 4500rpm at 80mph, there is a noticeable loss in fuel economy. Under 4k, fuel efficiency goes backup.