Understanding the dynamics of tire size?

[Brian McVickers]

Ok, question for all the brainiacs! (You know, those of you who can rattle off equations and scientific theory!)

Given the scenario below, how will a vehicles driveline / transmission / transfer case / differentials... react to the larger tire size, would it need to work harder or easier and to what if any significant measure? Furthermore, can a system that was originally paired with the stock tire size accomodate the larger tire size?

Stock tire size: 255-65-16
Sidewall: 6.5"
Radius: 14.5"
Diameter: 29.1"
Circumf: 91.3"
Revs/mi: 694.0

New Tire size: 265-75-16
Sidewall: 7.8"
Radius: 15.8"
Diameter: 31.6"
Circumf: 99.4"
Revs/mi: 637.0


Edit --- Let me know if you need more information...

[Robthebrit]

The rolling circumference of the new tire is around 9% bigger so you'll be going the same speed with 9% lower RPMs, you'll also have 9% less torque at the same setting.

Rob

[pskhaat]

And of course radius & diameter are linear against the circumference, so you can simply calculate the % difference using the total tire height.

[Brian McVickers]

Will the 265/75 stress the system significantly more or less than the 255/65?
Is there a way to measure this?

[goodtimes]

And of course radius & diameter are linear against the circumference, so you can simply calculate the % difference using the total tire height.

almost. Don't forget that tires are not "round" once they have weight on them.

[pskhaat]

almost. Don't forget that tires are not "round" once they have weight on them.

You know, I've heard so much debate on this I don't know what to believe anymore. One side says that the quantity of tire still must be turned underneath regardless of the pressure and eventual shape, thus one doesn't actually see fewer/more rotations per mile aired down than they do at full pressure. The other side of the coin maintains the opposite.

But for the sake of our quick and dirty calulations I think we could claim a linear relationship?

[pskhaat]

Will the 265/75 stress the system significantly more or less than the 255/65?
Is there a way to measure this?

Mostly more, sometimes less. The computer and shift points in an auto are tuned for the stock tire size, as well as gearing and people claim that all the time. However, many vehicles with the same tire size are sold every day with varying differential gear options so mechanically speaking I feel that the drivetrain is certainly up to par with the new larger sizing.

[Robthebrit]

some of those ECU programming gadgets can adjust can for none stock tire sizes.

Rob

[david despain]

some of those ECU programming gadgets can adjust can for none stock tire sizes.

Rob

i dont think there are any available for the DII. ( i dont think that even the test book can do this but im not sure)

this seems to be one of the most popular tire size upgrades for the disco. i bet you will be fine. drive line stress seems to be more of a function of weight and driver style than tire size.

[Brian McVickers]

Now we're talking, at least in words that I can understand!
Thanks everyone for the input, keep it coming inf you can think of more...