How bad do you want to get better mileage???

pasadena_commut said:
Here is a little table showing the factor (1- x/R)^2 for various
values of x in inches:


Code: 
 R-x         x  factor
   0     11.85  0.000  (contact at the ground)
   1     10.85  0.007
   2      9.85  0.028
   3      8.85  0.064
   4      7.85  0.114
   5      6.85  0.178
   6      5.85  0.256
   7      4.85  0.349
   8      3.85  0.576
   9      2.85  0.577
  10      1.85  0.712
  11      0.85  0.862
  11.85   0.00  1.000 (at the axle)
  23.7   11.85  2.000 (at the top of the tire)
Click to expand...

I measured the distance from the ground to the bottom of the car in front of the the front left wheel and it was 8". So just below that the factor is about .576. Assuming the Cd of the tire is 1 (a flat plat perpendicular to the flow is 1.28, but the tire is at an angle and not quite flat, so presumably a slightly better Cd) the air deflector would need to have a Cd of << .576. If the tire's Cd is actually higher then the potential gain from the deflector is increased. Looking at the table of Cd for different shapes here:

http://en.wikipedia.org/wiki/Drag_coefficient

I think it should be possible to do much better than Cd .576 with a smooth U shaped deflector in front of the tire, and perhaps a "boat tail" placed behind it. From that table, 0.25 is probably a realistic goal, which would indicate a deflector only extending downward about 2". In terms of lowering the car to achieve the same effect, the Cd of the car is .32 so lowering it by about an inch would be a net aerodynamic plus, but more than that is probably of limited use.

This is anecdotal, but about halfway down in this link there is a mention of the effect on gas mileage on a Prius when one of the air deflectors broke off:

http://answers.yahoo.com/question/index?qid=20080309093555AAOWcwg
 
Last edited:
Just put three different fiberglass wings on it in random spots and you'll get much better gas mileage and power. +HP too. :D
 
It's great to see some of them oldtimers here! I thought y'all were gone.

I, too, have been driving more cautiously. I found that coasting towards a light can potentially screw over the guy behind me, so I typically try to balance that out if someone's behind me and I'm coasting. I've done 55 on the highway before (in a 55mph zone). People were passing me left and right. BUT... I think if more people start doing it and seeing others slowing down, it'll become more common and accepted. Remember that the reason we have low highway speed limits is because of the original oil embargo in the 70's.

One time my ex and I were each driving home. Both of us went on the highway for about 10 miles. The difference is she did 80 or so, and I just did 60. By being smart and weaving around slower traffic, I actually managed to say somewhat close to her. The funny thing is that I was right behind her when I hit the first stoplight after exiting the highway.

Many of the things we can do to save money are more socially awkward. For example, I was waiting for gas the other day and turned my engine off. When the car left, I got out and pushed the car forward to get the gas. Some guy in an old 60's or 70's boat started laughing and yelled "PUSH HARDER"! Funny thing is he sat there for over 5 minutes idling his car. I wasn't about to turn on the car and waste the gas to start the engine for literally one carlength of space!

Also, by putting the stock air intake back on my car (had an AEM SRI), I was actually able to INCREASE gas mileage by a good 1 or 2 mpg. Check your wheels. Are they stock? The stock wheels are rather light compared to most aftermarket rims. You'll lose a good 1-2 mpg from having heavier aftermarkets, especially if they're wider than stock...
 
For information on car aerodynamics the best source I have found online so far is

http://autospeed.com/

The articles "Aero testing", "Undertrays, Spoiler & Bonnet Vents", and "Modifying under-car airflow" were very informative.

I have been looking at the bottoms of cars (sounds like form of perversion, doesn't it?) and notice that there are two opposing trends in common use for under body aerodynamics. In one camp there are cars like the Mazda 6, with a flat tray behind the bumper and at least up to the front wheels, and some sort of air deflector in front of the wheels. In the other camp there are cars like the Yaris and the Corvette, where a curved spoiler is placed just below or very slightly behind the bumper. It is my impression that it is much harder to do the former well than the latter. The problem is that the air deflectors in front of the front wheels, or for that matter the existing air dam on the P5, tend to cause lift at speed because pressure builds up in front of them. The spoiler under the bumper doesn't cause lift, but it tends to smack on things when driving, and does increase the frontal area.
 
maxek said:
Has anyone seen move: " Who Killed Electric Car" ???

If not very important to see!!!
I will never ever buy GM in my life.

And Bush did ruin American dream completely. I do not know how old you are but If you are young I feel for you and I feel very bad for my kids.
Click to expand...

Well, not to get too political, but I am sure the american dream is still achievable. And it hasnt killed it for his oil buddies... ^_^
 
so i've been trying this 2500 rpm and under thing and i think im seeing a huge difference. how i usually drive, i shift between 3-4k and when my gas needle hits the halfway mark, it usually reads 180-200miles. i hit the half tank mark today with 250miles. im waiting for the next refill to see how many MPGs i get. trying to beat my best of 27mpg
 
pasadena_commut said:
Just for kicks I worked out some drag figures for the tires hitting the oncoming air. Picture a line drawn perpendicular to the road passing through the center of the axle. The tire has radius R and the car is moving at velocity V. Let the center of the axle be position zero, and let "x" measure the distance below (positive) or above (negative) the axle. The velocity at the axle is V, at the contact patch zero, and at the top of the tire 2V. Consequently along this line

V(x) = V( 1- x/R)

This is also the velocity at the leading edge of the tire at position x below the axle. (The horizontal component of the velocity of the tire for all points at "x", not just along the line we started with, is the same.)

The drag due to air hitting the tire is proportional to V(x) squared, so

V(x)^2 = V^2 * (1- x/R)^2

For a P5 with 195/50R16 tires the diameter calculated here

http://www.miata.net/garage/tirecalc.html

is 23.7 inches.

Here is a little table showing the factor (1- x/R)^2 for various
values of x in inches:


Code: 
[FONT=Courier New]R-x         x  factor[/FONT]
[FONT=Courier New]  0     11.85  0.000  (contact at the ground)[/FONT]
[FONT=Courier New]  1     10.85  0.007[/FONT]
[FONT=Courier New]  2      9.85  0.028[/FONT]
[FONT=Courier New]  3      8.85  0.064[/FONT]
[FONT=Courier New]  4      7.85  0.114[/FONT]
[FONT=Courier New]  5      6.85  0.178[/FONT]
[FONT=Courier New]  6      5.85  0.256[/FONT]
[FONT=Courier New]  7      4.85  0.349[/FONT]
[FONT=Courier New]  8      3.85  0.576[/FONT]
[FONT=Courier New]  9      2.85  0.577[/FONT]
[FONT=Courier New] 10      1.85  0.712[/FONT]
[FONT=Courier New] 11      0.85  0.862[/FONT]
[FONT=Courier New] 11.85   0.00  1.000 (at the axle)[/FONT]
[FONT=Courier New] 23.7   11.85  2.000 (at the top of the tire)[/FONT]

As you would expect, there is not much to be gained by keeping the airflow off the bottom of the tire. A more interesting question becomes the Cd of the air diverter used. An air diverter attached to the body in front of the tire will be moving at velocity V. That's a faster velocity than any part of the tire currently exposed beneath the car. So for an air deflector in front of the tire to have any positive effect it must have a lower Cd than the tire cross section it covers. That suggests to me that a flat plate diverter (perpendicular to the flow, similar to the front air dam) would cause an increase in the overal Cd. A U shaped (low Cd) deflector a few inches tall might be useful though. Lowering the car two inches displaces the draggiest part of the exposed tires up into the body and so out of the airflow, and consequently lowers the overall Cd.
Click to expand...

I agree with most of this - but for all practical purposes - it is the wider tire increased rolling resistance that hurts fuel econmy *way* more than the increased width/drag penalty.

Quickly change final drive ratio? Get larger diameter wheels/tires. But only if they weigh the same as the stock combo does this 'buy' you any fuel economy (~3%).

Any money spent for fuel economy would have to pay for itself in two years. So depending on how much you drive - most changes may not be practical. Estimate your annual fuel useage X $4.25/gal and use '10%' of that number as a target. Use no more than half of this as your budget for economy mods (if you use the whole number - you don't save anything;))

Reality? Just change your driving habits FTW!(flash)
 
Lord_Zath said:
One time my ex and I were each driving home. Both of us went on the highway for about 10 miles. The difference is she did 80 or so, and I just did 60. By being smart and weaving around slower traffic, I actually managed to say somewhat close to her. The funny thing is that I was right behind her when I hit the first stoplight after exiting the highway.
Click to expand...



Yes I do see that a lot. People speeding down the highway and than I am just with them for many reasons: lights, traffic, I take a shorter road or I have a chance to exit before hitting traffic.

I used to be one of those people but I realized that it does not get me any faster if any maybe 5 minutes. Not worth it.(five-0)
 
Best bet is to get the tallest tires on the smallest rims that'll fit. Possibly 14", but I don't know if those will clear the brakes.

Smaller tires are a no-cost option if you need to change already... smaller tires actually save you money, since they cost less than whatever you're riding on now.

But they'll mean a penalty in handling, braking and safety. I've got low-rolling resistance Michelins (175/70R13!) on my baby ute'. It stops and corners like a bicycle... which is to say, it doesn't do either very well.
 
27 MPG on the last tank with 100% city driving! (This is an automatic.)

No hardware changes, but a major change in driving style and route. This is all I did:

1. Change route from one with a mostly stop signs to one with mostly lights. There is less traffic on the previous route, and it is a nicer drive, but there just isn't anything much I can do on it to increase the MPG. Believe me, I tried. Both routes are about 7 miles, all surface streets.

2. Time the lights so as to maintain as much of the car's kinetic energy as possible. Which is difficult, since these lights are all traffic triggered with no evidence of synchronization. At least it is possible with the lights, with the stop sign route it was impossible.

3. Burn no more gas than necessary to pick up enough speed to get to the next light, coasting down to a crawl near the light. But don't bother once the speed gets below 5 mph. At that point you might as well just get there and stop. (KE at 5 mph)/( KE at 30 mph) = 3%, there isn't enough energy present to be worth saving.

4. Don't piss off the maniac driving the next car back. Drive old style if the SUV in the rear mirror acts like it wants to hump the P5.

Best results require light to moderate traffic in order to provide several cars, but not too many, ahead. Keep them there so that they will trigger the light sensors. Also, when traffic lights are obscured by trees at a distance the color of that light may be inferred from their brake lights. Sometimes the walk signals will still be visible when the traffic light isn't. Too much traffic makes it impossible to time the lights.

I've also found that even though the speed limit on most of this route is 35 that is rarely the best speed to drive to make it through the lights. 30 is usually better. Go to 35 and invariably I end up closing on a red light too quickly and end up having to brake to hit it green, or have to stop. That is not a general rule, just something that seems to hold for this route. Also, some walk signals flash red, then go solid red, THEN the traffic light changes red, whereas other walk signals go solid red at the exact same time as the traffic light does. This can throw off your timing if you don't know which type of walk signal you're dealing with.

Anyway, the modified driving didn't turn the P5 into a Prius, but 27 MPG is a lot better than 22 MPG.
 
pasadena_commut said:
Also, when traffic lights are obscured by trees at a distance the color of that light may be inferred from their brake lights.
Click to expand...

People I follow ALWAYS brake coming around a corner w/light after it. Even if it's green. So I've learned to maintain my speed until I can visually confirm that the lights are red.

Nice ideas though!
 
maybe my p5 just sucks, but sometimes cruising at like 50 mph or even 60mph i get worse gas mileage, rather than driving 70mph. Odd I know but I have a scangauge to prove it. I can barely get 30mpg on the freeway.
 
Is it better to shut down your engine at long signals or jut keep it idle for good mpg ?
 
dd123 said:
Is it better to shut down your engine at long signals or jut keep it idle for good mpg ?
Click to expand...

That would depend on how much the P5 burns idling, and how much gas it consumes when it starts. Actually there are two values for each of these, depending on whether the car is warmed up or not (open or closed loop).

I have yet to see anybody post an actual measurement, or Mazda's specifications, for any of these four values.

Also to be considered is the wear on the starter. If on a normal day the car is only driven to and from work, then it is started twice. Shut down at every light and it might be started 20 or 30 times a day. Could that wear out the starter motor?

One thing I can guarantee you - if you shut off your engine at a light sooner or later you will encounter a situation where it will not restart.
 
I have no idea how 100k would not cut it. You would be able to live very comfortably for 100k a year granted you're not an idiot.
 
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