Doing a (futile) MPG test - check my math for me...

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e9999 said:
FWIW, I don't think that low air pressure affects the odometer at all. A flat tire still has the same circumference as an inflated one. And high air pressures would need to be so high that the steel/rubber tire would have to physically stretch which I don't think you can readily do.
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As others have said the flexing sidewall is what makes the difference. Ever see the diameter of a dragster tire change at RPM? And the effective diameter of a flat tire is rim+tread (little to no sidewall). Inflation pressure controls the middle range of values.
 
A flat tire does have the same circumference, but the effective circumference (distance per rotation) changes because we are rotating relative to a closer point. I think... :hhmm:

rAZyGGI.png
 
1407driver said:
Respectfully, I believe this is partially incorrect. Yes, the circumference of the tread is the same, but a lower pressure (or flat) causes the tire to scrub somewhat. Not as much as just riding on a rim, but you can see this very easily if you've ever had a flat on the highway. The vehicle pulls in the direction of the flat partly because of the reduced diameter. Actually, there are a lot of physics at play(excess drag, etc.). Softer tires will even fold. At 100+ it can get interesting... You could also test this with lockers. Lock an axle and drain one tire. Drive straight on pavement. You will get some binding.
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I was not talking about scrubbing and folding and slipping on the rim, of course. Those are extreme situations.

re_guderian said:
^ what he said. Tires under load are not truly "round", nor static in shape, thus a rolling diameter is more useful, based on the radius from the center of the axle to the ground. Versus the radus from the center of the axle to the top of the tire. Or maybe I'm whacked in the head... :meh:
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I don't think the shape matters.

hoser said:
The TPMS system on one of my cars uses the ABS sensors rather than individual pressure sensors in each wheel. ABS sensor detects different rotational speeds (left to rear, compared with opposing axle) and says, "hey, your tire is low on air."
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Interesting, don't know what to make of that

OregonLC said:
As others have said the flexing sidewall is what makes the difference. Ever see the diameter of a dragster tire change at RPM? And the effective diameter of a flat tire is rim+tread (little to no sidewall). Inflation pressure controls the middle range of values.
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??

yuji said:
A flat tire does have the same circumference, but the effective circumference (distance per rotation) changes because we are rotating relative to a closer point. I think... :hhmm:

rAZyGGI.png
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Mmmm.... I don't think so
 
AHHH I'm going back and forth on this one.

No matter what the shape is, one revolution should move you exactly the circumference unless there's slippage on the wheel. But a flat tire is running on the wheel / rim, so its diameter IS decreased.. how exactly the tire deflects to make that possible is the part that's confusing me.
 
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yuji said:
A flat tire does have the same circumference, but the effective circumference (distance per rotation) changes because we are rotating relative to a closer point. I think... :hhmm:

rAZyGGI.png
Click to expand...

I don't think this is exactly right, either. The truth is somewhere in the middle. What e9999 is trying to get at (I think) is that the tread length, or tire circumference, doesn't change. It's also tied to the center of rotation, as long as the bead doesn't slip, so that theoretically one rotation of the axle would coincide with one rotation of the tire. Since the tread is what is contacting the ground, you would have the same distance traveled with a flat tire as an inflated tire in a theoretically simplified situation. In this regard, the outer edges of the rim will actually slide forward on the inside of the tire tread. In reality, this definitely happens, but there is also some forced slipping of the tread against the ground. Another example of tire slipping when you would not expect it is in the following comparison:

Situation 1: You idle forward in your LC on flat, dry pavement with the steering at full lock. Your turning radius is X.

Situation 2: You drive forward in the same circumstance, except this time you are driving just a bit faster. Not skidding or extreme acceleration, but just a bit more throttle, steady-state. You will need a larger area (or radius of turn, X+) to turn around due to the tires scrubbing/flexing, etc. Try it.
 
Someone just needs to chalk a tire and roll it at 10 PSI and then again at 40 and we'll all be the wiser either way. Pics or it didn't happen. ;-)
 
yea, yea, there can be a bit of deformation, but my point is the main misconception that many/most? people seem to have, namely that if you have a flat tire, the "radius" of the tire below the center is much less than with an inflated tire and then the "effective diameter" is smaller and the odo is fawked. That is flat (:)) out not true IMO.
 
I still say chalk it and try it. I'd do it but truck is immobile for 90k.
 
e9999 said:
yea, yea, there can be a bit of deformation, but my point is the main misconception that many/most? people seem to have, namely that if you have a flat tire, the "radius" of the tire below the center is much less than with an inflated tire and then the "effective diameter" is smaller and the odo is fawked. That is flat (:)) out not true IMO.
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Hmmm.... :hhmm: How then would you explain a "contact patch" with the ground? And how it changes with tire pressure? e.g. airing down? I acknowledge there is also a change in the tire's cross sectional width when airing down, but also a significant change in the length of a contact patch.
 
re_guderian said:
Hmmm.... :hhmm: How then would you explain a "contact patch" with the ground? And how it changes with tire pressure? e.g. airing down? I acknowledge there is also a change in the tire's cross sectional width when airing down, but also a significant change in the length of a contact patch.
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I think of it kind of like a tank track. The track (tire tread) length remains the same length regardless of the path it travels above the ground, or how much of it is on the ground(contact patch) at once. The differences for this discussion, I believe, are the added tire scrubbing with a flat and the fact that with a flat, large contact patch the tread block walls are parallel instead of diverging from the center of rotation, and therefore less ground would be traveled with each axle rotation. I've had more flats than I could count on ten hands, most at high speed. It has sure felt to me that there's a lot more than the added drag of a flat when it happens. A flat or blowout with something like an old posi rear end could get very interesting, very fast! Theoretically, of course!

What is this thread about?
 
Oh, and my experience with tpms sensors in different vehicles is that they work very quickly and are quite sensitive. Annoyingly so, sometimes. But, I have no experience with the 100's tpms, and I don't know how it works or if it works the same as other cars. My Silverado showed each tire's pressure on the dash, and it was always dead on.
 
hoser said:
The difference would be really minute. It probably takes the indirect TPMS using wheel speed sensors several miles to distinguish a difference.
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I'm sure it would take several revs and a surveyor's tape to get a meaningful result. But you bring up a good point: if the rollout didn't change with pressure then indirect TPMS would fail.
 
1407, there are basically two different types of TPMS. The one you are referring to on your Silverado (direct) has individual sensors in each tire. The one I'm talking about (indirect) is more of an early system when manufactures tried to use existing technology/components (ABS sensor) to identify a low tire condition. It has no way of telling the individual tire pressure, only that one is off.

For more info:
http://www-nrd.nhtsa.dot.gov/pdf/esv/esv19/05-0082-O.pdf
 
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hoser said:
1407, there are basically two different types of TPMS. The one you are referring to on your Silverado (direct) has individual sensors in each tire. The one I'm talking about (indirect) is more of an early system when manufactures tried to use existing technology/components (ABS sensor) to identify a low tire condition. It has no way of telling the individual tire pressure, only that one is off.

For more info:
http://www-nrd.nhtsa.dot.gov/pdf/esv/esv19/05-0082-O.pdf
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Thanks for that info. Makes total sense. I have seen the low tire warning on crown vics a gazillion times. I'm only assuming they are the indirect kind. They do seem to work quickly. You hear the bump/thump/curb/bolt-click,click,click/whatever, and the light comes on right away. I've also noticed they work well with a slow leak, sometimes showing fairly minor pressure differences- which also leads me to believe the tire doesn't need to be totally flat to have a change in rolling circumference. A drawback, though, is with aggressive driving they do cry wolf quite a bit as well...
 
OP here... Just did a full tanks' worth of MPG study:
- 275/80/18 BFG ATs
- Tires at 35 psi (no tank treadin')
- Torsion bar cranked to level
- Yakima/Thule 4-bike roof rack with fairing
- 4 people + gear for family vacation (not too much)
- 60-70 degrees
- 68-70 mph (that was painful)

16.22 MPG. Given the heavy/big tires, rack and leveled stance, I can't complain. But the key to that number is clearly the 70 or under mph.
 
Manhattan said:
OP here... Just did a full tanks' worth of MPG study:
- 275/80/18 BFG ATs
- Tires at 35 psi (no tank treadin')
- Torsion bar cranked to level
- Yakima/Thule 4-bike roof rack with fairing
- 4 people + gear for family vacation (not too much)
- 60-70 degrees
- 68-70 mph (that was painful)

16.22 MPG. Given the heavy/big tires, rack and leveled stance, I can't complain. But the key to that number is clearly the 70 or under mph.
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I assume you meant 275/70-18 Load Range E BFG AT's. :whoops:

Now crank that tire pressure up to 40psi where it's supposed to be and watch your mpg get even better.

:cheers:
 
gaijin said:
I assume you meant 275/70-18 Load Range E BFG AT's. :whoops:

Now crank that tire pressure up to 40psi where it's supposed to be and watch your mpg get even better.

:cheers:
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Yep - 275/70s - oops; thanks

A while back, I did some tank to tank (or back to back) tests and the pressure had an insignificant impact on the mileage. Plus, shouldn't E-rated tires on a relatively light (5,600 lbs) truck be fine at 35psi?

Yes, I just highjacked my own thread.
 
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