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salvadoran300zx Wrote:
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> The overall diameter of the tires is what matters,
> the diameter of the wheel doesn't matter at all
>
> it does matter when it comes to weight. metal will
> always be heavier than rubber. the more metal the
> more weight, the more weight the more rotational
> mass, the more rotational mass the more gas you
> use up.


25lbs additional weight due to wheels isn't going to change your gas mileage.
 

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Discussion Starter #44
Alright guys, went home from college today in anticipation of a snow day tomorrow.

I checked my tire pressure. 3 tires were at about 30psi and my front left was at about 25psi

Put them all to 36psi.
 

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90ttauto Wrote:
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> you can get on the highway and reset your odo at a
> mile marker.

I have found out by using my GPS that the mile markers on the highway aren't very accurate.
 

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salvadoran300zx Wrote:
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> so multiply your total tank miles by 0.081 and
> then add that to the original number, then divide
> by the number of gallons and voila! you got your
> new more acurrate MPG


You'd mutliply it be 1.081, not .081 Sal. ;P

But Kylekidd is smart enough, he figured it out on his own. *clap clap*
 

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hmmm... numbers is not my real specialty =P hahaha!

but it works! its much more complicated.. but it works!

my kindergarden way: 14.6 x .081 = 1.18 + 14.6 = 15.78

your middle school way: 14.6 x 1.081= 15.78

It works just the same... through a much longer process though =P but it works! =D
 

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Discussion Starter #48
Both of your math works. A+ for both of you.

But still I feel my MPG is low in comparison to everyone else's.

We'll see how things turn out after i fix my vacuum leak and after i measure MPG again after this tank. (after increase in tire pressure)
 

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dumbestone Wrote:
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> Plus you now have that extra air cushion in the
> tires to roll easier, but that extra diameter may
> make your speedo be off a little more. Catch 22.


Calculate that one Sal!!! ;)
 

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Radial tire stretch? Increase in diameter? Maybe, if you are talking from 5 psi to 40 psi, but we are talking less than 10 psi. The increase if any, would be minute, and not enough to significantly alter mileage figures. Yes, there would be less rolling resistance due to a decrease in sidewall deflection and tread distortion.
 

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racerbud wrote:

> Radial tire stretch? Increase in diameter?
> Maybe, if you are talking from 5 psi to 40 psi,
> but we are talking less than 10 psi. The increase
> if any, would be minute, and not enough to
> significantly alter mileage figures.
> Yes, there would be less rolling resistance
> due to a decrease in sidewall deflection
> and tread distortion.

That depends on whether the tire was at recommended pressure and whether the sidewall was deflecting and the tread wash mush-bottomed before going to 36 psi. If my tires on xxx-car are 5 pounds low from recommended 33 the flat-spotting and drag is obvious. If the tires at 36 psi are being "over-pressured" above recommended then the effect is much less.
 

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The rolling resistance can definitely have an affect on mileage.
 

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racerbud asked:

>And? What effect does that eight psi have on diameter?

As the tire pressure is lessened the flat spot on the ground is increased with the "collapse" of the tire; that is, unless you have a tire pressurized to the point of being ready to explode, the weight of the car will cause a flat contact spot on the pavement and the flex of the tread and sidewall as it revolves into contact with the pavement and forms the flat spot will do two things: 1) reduce the effective tire diameter at the point of contact and 2) cause more rolling resistance due to the energy spent in forcing the tire to deform to a flattened shape of the tread and sidewalls.

That said, it would appear that if the tire is rolling a smaller contact diameter Kyle's mileage would not be effected as much as the tire would be closer to the 16" stock rim/tire OD, but the rolling resistance also takes more fuel/power. Probably, the only true test is to do some tire/pressure testing on a dyno with fuel, engine speed and tire rpm monitoring. The automatic also has an overall effect on the results due to the slippage curve of the converter vs. speed.
 

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1. If there is no slippage of the tread the diameter per revolution remains the same, per radial tire design. (Belted tread with a balloon inside).

2. The energy spent in deformation of the sidewall and tread, results in heat, and less overall life of the tire. We all learned early on that energy produces heat. The diameter did not change, it just takes more energy to turn the assembly resulting in poorer fuel mileage.

3. I don't connect what a 24" tall tire has to do with 16" stock rim/tire OD?
 

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I think we are saying the same thing and I'm not trying to be an a$$ either. I read straightdope and agree to a point. I'll explain.

All my experience with tires comes from a competition background and working with tire engineers at the racetrack. In the past, as a driver, I have run bias ply and radial racing tires, DOT radial, (showroom stock), and of-course street and trailer tires. The important things I have learned from it all is high pressure is good, and heat kills tires.

I suppose my best high pressure example would be when I raced a 1993 Nissan Sentra SER, in SCCA Showroom Stock "B". For the FWD Sentra the starting air pressures were front 25 psi, rear 55 psi. Yes, 55 psi. At the end of the race or practice session, the fronts would be 38/42, psi and the rear, our Moroso gauge only went to 60 psi, and the engineers wouldn't say.

We also took tire temps, and the front tire temps would read, inside 180/220, center 190/230, outside 160/190. The rear readings, 190/220, 160/180, 140/150. The inside temps generally read higher due to the amount of camber.

Getting the FWD to 'rotate' is the secret to going fast and 55psi in the rear meant there was just enough rubber on the road to do just that. The engineer was right, but told me to never mention the brand, so I won't.

As far as passenger car tires, I always exceed the "Recommended Pressure", for better control and longer tire life. The more weight the more air, at least 6 and up to 10psi, cold.

Another interesting fact; I used 944 tires on my 1990 NA 300ZX during it's racing career.
 

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the "Recommended Pressure" is there to also improve braking. I would consider going over it a bad advise.

After all, it was not a Jack of all trades who designed the car and tested it at various conditions to come up with the "Recommended Pressure"
 

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Recommended pressure is a fit all for people who can't figure anything out for themselves. It is just like MSRP.

I'll stay with my recommendation considering my E-150 has 116k on it's Michelin's and not to the wear bars yet at 48.psi. Recommended pressure is 41.
 
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