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Old 06-12-2008, 08:10 AM   #1
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Hey scangauge owners - teach me some stuff!

I was thinking of purchasing a scangauge for my OBDII cars even though my pre-OBDII car is the one I am working with right now to get high MPGs:
http://www.lightweightmiata.com/mpg/mpg.htm

Anyway, I was going to use it to learn a few things about driving techniques and how much fuel is used for a few things but I figured I could just ask you guys and save the bucks. So, here it goes (there may be some posts in here somewhere so please point me in the right place if I missed them).

1) How many MPGs does running the AC or opening the windows cost me?
2) Is running slowly through the gears at a high vacuum save me gas compared to getting into 5th gear as quickly as possible?
3) How many MPGs does turning on the headlights, foglights and kicking the stereo up full blast cost me?

My car is a '90 Miata with a programmable ECU that will probably get a max of 35-40 MPGs with a lot of hard work.
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Old 06-12-2008, 09:18 AM   #2
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You can use the fuel rate monitor linked in my sig for telling you relatively how much fuel is used for various things.

1) I can't tell you how many MPG, but it's slightly better to use AC than open windows in my VW at 70mph. YMMV, for sure.

2) There are differing opinions here, but for me, the best results come from shifting as low as possible and using WOT or near-WOT while accelerating. The result is that I don't accelerate any faster or slower but I keep the RPM lower.

3) Can't help you with that one. I just do what I need for safety and comfort and accept the minor cost.

I suspect that NOBODY give you absolute numbers for MPG value of such questions for your car, in your situations, with your programmable ECU.
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Old 06-12-2008, 09:42 AM   #3
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there comes a point in my car where if I have the accelerator pressed too far that my AF ratio goes from 14.6 to 12.5 which is richening the mix for more power. that is also something the scangauge will tell you air fuel ratio. I also believe that getting to top gear quicker is better as long as you don't enrich the mixture.

my car is also an automatic so that may also be the reason my response is different than others. I try to accelerate 3/4 the way or so but then let off so it will upshift and let it shift at normal places. at WOT it often wants to shift from 4th (my overdrive) back into 2nd along with the richer fuel ratio.

this may not apply at all to you since you have a programmable ecu.
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Old 06-12-2008, 02:17 PM   #4
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Originally Posted by theholycow View Post
2) There are differing opinions here, but for me, the best results come from shifting as low as possible and using WOT or near-WOT while accelerating. The result is that I don't accelerate any faster or slower but I keep the RPM lower.
So, you stomp on it and shift very quickly. I figured that may have been the best way but I wasn't sure. I've been barely pressing down on the accelerator (keeping my vac gauge as low as possible) and taking 30 seconds to go through each gear (not letting my RPMs get above 3000). Using my ECU, I can pretty much set my A/F to what I want. I have not changed it much but will in the near future as soon as I get my WBO2 sensor working again.
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Old 06-12-2008, 02:30 PM   #5
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You can use the fuel rate monitor linked in my sig for telling you relatively how much fuel is used for various things.
Thanks! I took a look at that and saw it was a DIY duty cycle meter. My programmable ECU actually displays/records that information (see link below) and I hadn't really thought of using it until seeing your post. I'll have to come up with a way to use this.

I thought WOT was the way to go. Since I can tell my ECU exactly how much fuel to inject based on vacuum and RPM I should be able to get the A/F ratio to work for me vs against me when at WOT.

Link to what my "Link ECU" will show me:
http://www.lightweightmiata.com/turbolink/turbolink.htm

The screen shots are pretty small but you can get a good idea of what it does. Displays graphs on what the fuel injector duty cycle is and how to set it.
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Old 06-12-2008, 05:17 PM   #6
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Most vehicles with OEM default programming go into open loop when you go WOT, dumping as much fuel as possible (based on pre-set tables) to make power. In such vehicles, the best place is shortly before open loop, maybe 90% throttle. My VW is equipped with wideband O2 sensors and does NOT go into open loop, so WOT is good for me.

One of the major reasons it helps is to reduce pumping losses. You're putting the same amount of work onto the pavement either way, but having to suck air past a closed throttle saps energy. WOT reduces that waste.

Fuel injector duty cycle tells you in nearly absolute terms your fuel flow rate. That isn't yet crossed with speed to make MPG, but it tells you, at a given speed, the difference between two options -- so, 45mph in 4th or 45mph in 5th; or A/C vs. open windows at 70mph; or 80% throttle vs. 100% (quite a surprise in my VW, WOT uses LESS fuel [measured absolutely] than 80% throttle).

If I could have a system like yours in my truck, and also be able to program the automatic transmission with it, I could get at least another 20% increase in my FE there.
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