Other than by brute force, is there a way to predict/find the optimum cruising rpm (in this case, for the RX-8) to get the best mileage?

 

on the old ones (rx7's) you could watch the o2 sensor and it "turns off" at a certain rpm (3200ish) right before that rpm is the best economy spot. the rx8 has a wideband though so it might not ever "turn off"

 

The manual has shift instructions for maintaining optimum MPG. Essentially, stick under ~3k RPM (ya rite).

 

it's a pretty simple concept:

lowest engine speed and throttle angle in the highest gear possible.
so in a real-world situation, you're going to get the best fuel economy with your foot nearly off the throttle completely, just cruising at 40mph in 6th (2000 rpm).

but, cruising isn't what really drinks the fuel, it's the acceleration, so always accelerate slow, keeping rpms low, in the highest gear you can find while keeping the car moving. that's all there is to it.

 

I find 69 (mph) to be just fine at 3500 RPM

 

but he asked for "optimal".
lower speeds have lower aero losses, and lower fuel usage per minute because of the lower engine speed: you'll save a lot more gas at 60kph in 6th than at 110kph.

 

the easiest way to find optimal cruising speed is to hook up a diagnostic tool and observe the fuel rate at different speeds/gears/rpms ... I was planning on doing this but it appears my friends OBDII datalogger won't work

I have a hunch its probably around 3-3.5k RPM

 

...what you have to realize is that there is a difference between efficiency (thermally, volumetrically, or any other metric you care to think of) and actual rate of consumption.

even though the engine might use that fuel slightly more efficiently at 3.0k rpm than at 2.0rpm, it's also fighting a whole lot more resistance because of the higher speed, and thus you may need to use more fuel to maintain that higher speed.

the lowest speed possible in the highest gear you have is the most efficient way you can drive your car. this is even more true in modern cars which tend to run fuel lean at low rpms (at 1:16 ratios if at all possible), and not at a mechanically fixed ratio between throttle angle and amount of fuel pumped out.

 

I'm talking about the most efficient specific fuel consumption (SFC) RPM which is the amount of fuel needed per horsepower produced.

Basically, for every fixed speed there is a fixed amount of resistance you have to overcome - which is related to the amount of power you must produce. With a SFC curve, you have a curve that tells you what RPM you need to be at to use the least amount of fuel to make 1 hp, so that is the best cruising rpm for any given speed. This curve is what OEM's use to setup cruising RPMs for cars with CVT tranny's.

From experience of doing lab tests, it usually comes out the low point of the SFC curve is about 1/3 of the redline... hence my 3-3.5k RPM remark



If you want to talk about raw consumption, the best cruising speed is probably 20mph

 

personally I found that keeping the rpms between 2000-3000 rpm is a good ideal area for shifting and driving prudently. 2 gear, 3rd, 4th, and 5th this is ideal.... 6th too if you like driving 60-70 mph on the freeway.

 

I thought this was a sports car. it makes me mad to think that I have to drive it like I'm on some kind of some national economy run just to get barely acceptable mileage.
The book said 30.2 highway, the sticker said 24 highway and I can't get anything like even driving like an economy run. There is something wrong with at least soem of the cars. I hope Mazda addresses this problem real soon.

Also, what are they going to do with the upcoming RX-7 whcih will have a turbo-charged Renesis rated at 300 HP?

 

I tend to treat the car like an Olympic sprinter. Sure they are capable of running at unbelievable speeds, but have you ever seen them going through their daily lives? I've never seen one sprinting down the aisle of the grocery store, or racing down the sidewalk. They save it for when it counts - in training and on the track.

So I drive my 8 carefully for the most part, shifting at low rpms to conserve my gas. Then, when I *want* to I let it out for that overtake on the freeway, or to merge, or to get around the moron who won't let me over to get to my exit. Or just when I feel the need, the need for speed! I get my fun, I get decent mpg... no complaints here

 

Interesting range of responses ....

I'm not looking to drive 30 mph down the freeway in 6th just to get the best mileage. I'm just trying to figure out if I should be in 5th or 6th gear when I'm going 60 mph. In Houston traffic, it's probably best to stay in 5th because of the periodical acceleration needed to avoid the drunk/stupid drivers. If I get out of town, I figure it's probably best to be in 6th and keep the rpm down.