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Old 01-09-2009, 09:29 AM   #11
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Although I believe you will see no impreovement from the weight of the wheels/tires, I do believe that diameter and width can have a (small) affect on MPG.

-Bob C.
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Old 01-09-2009, 06:13 PM   #12
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I have tires that are .0635 percent larger than the car came with. With it being winter and all, along with more idling, there's no way to tell how much improvement there might be. Come summer I can only return to standard driving practices and compare last year to this year. It would still be difficult. You'd have to be a very consistant driver to get good numbers. That, or you could do the a-b-a thing if the old tires are still around.

Mathmatically, it has to be better. But how much may be difficult to tell.
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Old 01-09-2009, 09:51 PM   #13
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Quote:
Originally Posted by therealtime View Post
I am currently running stock del Sol Si wheels with 195/60/14 tires.

Total unsprung rotating weight per wheel would be 24.5lb (reduction of 26.4% or 8.8lb per wheel, 35.2 total)
As mentioned, this may help your ride comfort, but has very little bearing on mileage. I suppose those surprise stop-lights will penalize you a little less, but those should be fairly rare (if you are paying attention! )

Quote:
The vehicle velocity would also change by reducing tire circumference by about 2.5% (my speedometer is currently 2.1% too slow because my tires are larger than stock and the vx tires would make it 0.4% too fast)

The frontal area would be reduced because the car would sit 0.3 inches lower and each tire is 0.8 inches narrower.
Lowering ride height is not the same as reducing frontal area. Lowering the car will cause air passing under the car will be squeezed slightly more. This may increase or even decrease drag, but at 0.3 inches I'll bet the effect is marginal.

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Seems pretty dramatic to me when you combine the reduced area, reduced resistance to rotation and reduced total mass. The only drawback is the smaller circumference which would make the engine run at higher rpms (81 more rpms to maintain 70mph).

Any educated guesses as to the mpg gains to be found?

thanks,
-tony
The narrower tires will probably reduce drag fractionally, but the other benefits seem pretty trivial, IMO. I have to agree with the mob, you probably won't see anything. (Or we could all be out to lunch. Please report what you find!)

Personally, I'm going to go the other way and install the tallest tires that will fit when my whale needs new rubber in a few months. Lowering the gearing is the main goal; the car has plenty of power and revs much more than necessary on the superslab. Stock is 195/60-15. 65 or even 70 (rare) profile tires will fit easily.
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Old 01-10-2009, 04:01 AM   #14
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Quote:
Originally Posted by Geonerd View Post
Lowering the car will cause air passing under the car will be squeezed slightly more.
I don't think so. The amount of air going under the car is based on the aerodynamics of the front of the car. The space between the bottom lip and the ground is full of air that will go under -- this is what is reduced by lowering the car. Any air coming down off the front of the car will go under -- this is not reduced by lowering.

I agree, though; in this case we're talking about marginal amounts.

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(Or we could all be out to lunch. Please report what you find!)


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Personally, I'm going to go the other way and install the tallest tires that will fit when my whale needs new rubber in a few months. Lowering the gearing is the main goal; the car has plenty of power and revs much more than necessary on the superslab. Stock is 195/60-15. 65 or even 70 (rare) profile tires will fit easily.
You're talking about raising your gearing -- a low gear is 1st gear, a high gear is 5th. Gears that lower your RPM at a given speed are described as "high" or "tall".
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Old 01-10-2009, 08:33 AM   #15
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Quick update of actual changes:

original wheels and tires weighed in at 32lb each and the VX's weighed 22lb each.

pictures here:
http://cheaptrixracing.com/delsol

FE update coming next week.
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Old 01-10-2009, 12:33 PM   #16
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I don't think so. The amount of air going under the car is based on the aerodynamics of the front of the car. The space between the bottom lip and the ground is full of air that will go under -- this is what is reduced by lowering the car. Any air coming down off the front of the car will go under -- this is not reduced by lowering.
I agree with what you just said, and I thought I said much the same in the prior post.

Well, the collection area will lose 0.3 inches, as will the tunnel area between the ground and bottom of the car. Since the tunnel height is a smaller value, the 0.3" represents a greater % change. It seems to follow that the air will indeed be "squeezed" (Ok, not the best verbiage) or forced to travel slightly faster than before. OR, it might 'backup' somewhat, increasing lower leading edge pressure, possibly causing more overall drag. Who knows??

I suspect that undercar airflow is pretty dang turbulent, with chaotic pressure and velocity gradients. How all this will behave with a slight increase in velocity and a slight decrease in chamber height is anyone's guess.

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You're talking about raising your gearing -- a low gear is 1st gear, a high gear is 5th. Gears that lower your RPM at a given speed are described as "high" or "tall".
www.google.com -> "Nitpick" "Flame Warrior"

This from the person who thinks pumping losses are highest when the throttle is closed.... (When there's nothing to pump!)

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Old 01-10-2009, 12:36 PM   #17
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Quote:
Originally Posted by therealtime View Post
Quick update of actual changes:

original wheels and tires weighed in at 32lb each and the VX's weighed 22lb each.

pictures here:
http://cheaptrixracing.com/delsol

FE update coming next week.
40 pounds is a pretty respectable dietary effort. I'm inclined to think this will have more effect than any other aspect of your mod.
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Old 01-10-2009, 03:01 PM   #18
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Quote:
Originally Posted by Geonerd View Post
This from the person who thinks pumping losses are highest when the throttle is closed.... (When there's nothing to pump!)
That's not exactly what I think, but...why would there be nothing to pump?

The amount of air it's trying to pump is based on RPM; closing the throttle restricts the air, causing loss. Imagine trying to suck a thick milkshake (or frappe or whatever it's called where you live) through a jumbo straw and a coffee stirrer...which one will require you to use more energy to get an ounce into your mouth?

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-Moo!
You have my 100% agreement there.
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Old 01-10-2009, 03:36 PM   #19
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if you drastically change the tire size, there will be a difference. I regeared my truck for 4.10s from 3.55s and my mileage went from 22 to 18. a very significant change. I could have done much the same by going from 29 inch tires to 25 or so inch tires.

that was actually suggested to me before changing out the gears. borrow some racing slicks to see if I like the feel of the gearing before changing the gears. it is much easier to change out the tires than change out the gears, especially if a friend would let you borrow them.

my take off was excellent and acceleration was much better but I was hitting 2400 rpms at 60 mph vs. 1900 rpms before the gear change.

I didn't do the tire trick because I knew I wanted more gear and didn't want to go to 4.56s because that was way too deep a gear for me on stock tires.

I know this is an extreme example but diameter will make some difference even if there is a small difference.
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Old 01-10-2009, 05:15 PM   #20
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that was actually suggested to me before changing out the gears. borrow some racing slicks to see if I like the feel of the gearing before changing the gears.
good point... I never thought about gearing change.

I wonder how many people actually do this. I assume the odometer will always be incorrect and it's just a matter of knowing the correcting conversion when it comes down to logging your miles... (and when you pass by a cop! )
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