gas mileage: RPM's VS. Engine Vacuum
 

im not really sure what way to go with this. its basicly the 4th and 5th gear debate when city driving. lets say this is all at 40 mph. driving in th gear is at a lower rpm than 4th gear, but the vacuum is higher, causing more fuel to be added, right? in 4th gear the rpms go up abour 300 and the vacuum goes down about 5Hg. which would give you better mileage?

im curious about this, any one have any imput?

I'm also very curious, anyone?

apparently, one of my tuners says he gets the same mileage in 4th as he does in 5th. Test it out yourself. Fill up a tank to the click at a gas station and reset the tripmeter. Drive normally and cruise in 5th gear until you have less than 1/8th tank left. Fill up to the click again and note the number of gallons it took to get there. Divide the number of miles travelled by the number of gallons used to fill up and you'll get the approximate mileage. Repeat this process and cruise in 4th instead of 5th and calculate mileage again.

My guess, you get maximum fuel effinciency at around 2500 rpm. I can drive 40 in 4th at 2500 like I said, and if I shift to 5th my rpm drops but my tailpipes also light up like christmas. It's the only time they fire up too, so my guess is that it's using A LOT more fuel.

I drive by how much work I think my engine is doing.

As an example, you can cruise at 2500 rpm in 5th just fine... until you get to a hill. Depending on the hill, I can floor it in 5th and keep a steady speed. However, that's making my engine work a lot harder than if I shift into 4th and go up to ~3500 rpm in which case I only need (probably less than) 1/2 throttle.

If you are supposed to be driving in 4th at around 2300-2500 and you shift into 5th, you have to press the accelerator more DECREASING the amount of vacuum. My fuel pressure gauge tells me this, since the stock FPR runs off of vacuum, the less the vacuum, the higher the fuel pressure, the more fuel.

I find between 2000-2500 is a good cruising rpm in 3rd and 4th.

Driving in 5th at RPMs lower the 2000 will basically be lugging the motor. More chance of detonation. I hear the timing is a little to far advanced for the higher gears and lower RPMs.

Higher vacuum means the engine is under a lighter load, and LESS fuel is used. You will almost always get better mileage by driving for the lowest RPM with the highest vacuum reading.

Go one step further and add a narrowband A/F gauge so you can monitor when the car is in closed loop. You'll learn to drive as if an egg was under the pedal...

Why don't you just get a meter that gauges how much power is going to your fuel pump? Shouldn't that give you a good idea of how much fuel it's pushing? ...maybe not I guess since there's a fuel return line. But there might be another way to meter the fuel.

There should always be 13-14v, no less at the fuel pump. I think T2s are the only ones that vary the voltage for idle or cruise, someone else im sure can step in here with the right answer for T2s.

Its the Fuel Pressure Regulator that regulates the fuel pressure. Not the fuel pump its self. You can easily install a fuel pressure gauge to monitor this. Still, this won't tell you how much fuel is being delivered. Only a AFR gauge will be able to tell you that and you will still only have a slight guess.

No. If you really want to measure the exact fuel flow, you need to use two flowmeters. One in the feed and one in the return. Then the difference between the two is the fuel that has been consumed.

Sounds like something Steve, owner of the GV site, should start building as another project.

ive found that i get the best mileage in any overdrive gear within reason. Obviously, 5th gear going 25mpg is just funky. but still. that is why there IS an an overdrive in all cars (reasonably speaking)

There is no over drive in standards and there is not an over drive in ALL cars.

http://en.wikipedia.org/wiki/Overdrive_%28mechanics%29

overdrive = engine spinning slower than transmission. unless your limited to a 1:1 gear, then you have an overdrive gear. When i mean reasonably speaking, i mean cars that you would actually care a little about having mileage on. i bet if you walk outside to your parking lot at work. 99% of those cars have an overdrive gear.

best way to monitor gas mileage with something you can buy off the shelf is with an injector duty cycle gauge

What Aaaron Cake said is true I do notice better gas mileage driving in closed loop on the highway by just monitoring my narrowband A/F gauge. With or without the AC on I get better mileage. I love it when im on the highway just cruising on a hot sunny day with the AC pumping cold, and im still getting awesome mileage. Thats one of the reasons I have it in my car to monitor the closed loop mode, and the other is just for the bling,bling effect Lol. It surprises me that people actually use and tune there car relying on these cheap A/F gauges.
They rely on it to see if there running lean or rich, hilarious

I really wish you would stop posting misinformation

Overdrive means that the final drive turns faster then the engine. Every modern car has an overdrive gear, some have more. For example, my Insight has 2. The FC has overdrive in 5th.

I could make some injector p/w logs with the rtek and find out..

Ah yes you are right, I was thinking on different lines.

And let me know of this OTHER misinformation.

whats a good narrowband to get? i have an autometer A/F guage, but it isnt really connected.

All narrowbands are the same.

yeah that's what I was beginning to suspect when I thought about it more.