Was thinking about gas mileage earlier today and was wondering if someone could help me out. I'd like to know if anyone could give me a hint on what the most efficient speed is for gas mileage(or even rpm) 6th gear obviously. Allow me to explain myself. Say you're driving 400 miles. Would it be more gas efficient to drive at say 65 for the duration, or would it be better to drive at 80(since you're getting there faster but burning gas at a higher rate) Alright, I'm re-reading this and it seems a bit convaluted. I apologize, but if anyone understands me, I'd appreciate any input. Thanks.

 

You would get better mileage at 65. Typically for cars, the magic number is somewhere near 55-65 mph. If you go any slower, you burn more gas due to the amount of time the engine is running vs how far you've traveled. Remember that it only takes a certain amount of horsepower to hold a steady speed. Going faster compounds this. The faster you go, the more power it takes due to aerodynamic forces acting against your car. You are also at a higher rpm which isn't helping either. When cruising on the freeway, drive as slow as you can without going too slow. Use your best judgement here. I typically hold 70 mph on the freeway.

While I don't have an RX-8, I do have 2 RX-7's and a Civic. Even though the Civic gets fantastic mileage, I noticed a gain by changing my cruising speed. I used to hold 80 mph. I would average around 34-35 mpg freeway. Now I do 70 mph and get around 37-38 freeway. The RX-7's respond in the same way too. You can usually affect it by a mile or two per gallon just by changing your cruising speed by 10 mph. I have also noticed that I can get better gas mileage without cruise control than I can with it. I have found that cruise control will floor it in order to hold a certain speed up a some hills. I'll just let the car lose a few mph rather than give it full throttle. This adds up quickly. Even where it is flat, it will still give more throttle than necessary going up overpasses. Cruise control is good for being lazy but it isn't the best for gas mileage. The biggest thing that affects mileage though is self control. the rotary just loves being revved up and it is so hard to drive one without opening it up.

 

Damn that was a quick response, thanks man. I was never really sure about that, I'm glad you understood my gibberish. I also noticed the same thing with cruise control, every now and again I'll be pushed back in my seat by it trying to maintain speed up a hill. Again thanks a lot for the info, much appreciated.

 

well I read somewhere here that 3750 rpms is when 2nd set of ports open. that is about 72-73mph and what I cruise at if I can keep my foot outta the gas.

got 23 mpg last time I did.

 

^^I've heard that same thing, so I try to keep it under 3750 while on the highway. I haven't managed to get a full tank of highway miles to measure what I'm getting yet, though

 

I tried to drive sedately and see how far I could go on a tank of gas. Unfortunately, RG hit the nail on the head: self control is the real limiting factor. I made it out of the garage and around the corner before I decided I didn't really care. I'm sure I got relatively great mileage for that 1/4 mile.

Seriously though, if you just barely tap the accelerator, it rev's to around 2500 RPM effortlessly. Guess what, that's 60 on the freeway, the speed at which MOST cars are DESIGNED to get best mileage. On average it requires 5HP maintain 55mph. Just remember to roll up the windows and close the moon-roof if you want best mileage. Despite the added rolling resistance in the rain, that's when I get my best mileage 'cause that's the only time my windows are closed.

 

I know from personal experience, that 70mph vs 80 mph, with the same route and same gas, similar weather, was significantly different. at 70mph cruise i got nearly 26 mpg, at 80mph it was 22.5. This was a 450mile higway trip using shell v-power gas. 4AT. 70mph is slooooooow though . And it takes 48 more minutes to travel 450 miles when going 70. Going less than that on the higway will get you runover. :p

 

Most of us don't always drive 450 miles on the freeway at a time. Usually I drive around 30 or so which is only 3 to 4 minutes slower at 70 vs 80. Traffic plays a larger role than that so I really don't get anywhere usably faster at 80. I just use more gas. Has anyone ever noticed that you might be cruising at a steady speed but there is always one person who is trying so hard to weave in and out of traffic only to get there no faster in the end? Those people are everywhere. They are using far more gas in the process.

I know there are many different formulations of gas around the country so this really only applies to me but other areas may have similar effects with other companies. When I use Shell, gas mileage goes down noticably. I don't feel any difference in power. I just lose mileage. Chevron and Exxon seem to be very consistent here and I usually average a couple more miles per gallon with them vs Shell.

 

I have an 05 and my MPG continues to increase.
I just hit 12k miles today and my car is 7.5 months old. (ouch)
My MPG seems to go up in stages.
I was getting 14-15 up to around 5k.
Then 16-17.. then 18.. then 19.5.
Today: 254.1/12.431 = 20.4 = my best ever.
95% of my driving is the same route to work.
I use the same gas station/pump/method and I pump myself.
I get almost exactly the same MPG every single fill-up.
I got between 19.4 and 19.7 on 8 straight tanks.
Then it goes up a little and I get almost that exact same amount for a while.
Call me crazy.. but this is fact.

I did change my habits around 8k. I do not upshift unless I am near the 3750 mark.
I used to upshift at lower RPM when just cruising.
I read where some ppl said this helped them. Other said it did not.


I hope this trend continues longer.

-B