Fuel mileage

[prepowerstroke]

How hard is it to change the speedo gear? And I've been really wanting the obs idi bc of the electronics, digital odo and easier to change tires size and have correct speedo. And I just like the newer style dash lol


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[79jasper]

Not bad with a 4x4 truck, as you just pop off the output housing.
Even a 2x E40D should be easy.

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[Black dawg]

Multiply that by 2 and you'll get a real tire diameter reading.
Good for choosing the right speedo gear, or calculation for us obs owners with the psom.

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That isnt entirely true, the circumference is the same regardless of how flat the tire is on the bottom.

 

[laserjock]

That isnt entirely true, the circumference is the same regardless of how flat the tire is on the bottom.

Correct. The circumference is what you care about. It just so happens its proportional to the diameter so you can compare the change directly by ratioing the the tire diameter and not calculate the circumference. C=pi*D

On my gasser, I just went up from 235 60 16 to 245 70 16 and it threw my speedo and odo off by the math about 10% and sure enough checking mile markers (about 10 miles worth) I gained about a mile. Couldn't figure out why my mileage had went from bad to even worse all of a sudden. Thought surely the tire size change didnt hurt the performance that bad...

 

[79jasper]

That isnt entirely true, the circumference is the same regardless of how flat the tire is on the bottom.

Eh, that's arguable.
Even by a minute amount, it's still a change.
No the actual circumference isn't changing, but the rotational circumference will. IMO.

If I'm wrong, show me, so I can learn something new.


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[laserjock]

Strictly speaking you need to measure the roll out of the tire. I doubt you would get a percent or so difference from the straight up math unless the thing is really flat especially at speed but really I think it's splitting hairs.

 

[79jasper]

Ah yeah. I get what you're saying.
I may have been thinking of something else.
Put a paint line on the tire, roll forwards till you get two on the ground, then measure between them, right?

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[laserjock]

Yep.

 

[79jasper]

Would tire pressure affect it?
Like say 80 psi vs 40 psi?

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[Black dawg]

or just measure around the tire. It is funny, that people forever have measured from the ground to the hub (including myself) when it isnt actually accurate.

 

[Black dawg]

Would tire pressure affect it?
Like say 80 psi vs 40 psi?

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very little, I would think, only one way to find out.

 

[laserjock]

Tire pressure really shouldn't matter much unless it is enough of a change to cause the tread to be bulged out in the middle effectively causing the tire to be taller. Think top fuel cars and the force pulling the center of the tire up changing the gear ratio with speed. That's an extreme case which results from tire deformation (a change in aspect ratio). In any event, on a truck tire, I highly doubt that tire pressure would make a measurable difference.

 

[79jasper]

Good to know.
I may have to play a little. Just have to do it on the front tires, since I'm drw.

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[prepowerstroke]

Okay maybe I missed it but hoe do you come to the conclusion if which new gear to get? Do that say like what size tires they r for? Or like how many teeth they have?


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[79jasper]

There's a cart on the interwebs somewhere. Lol
Someone here may have it.

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