What have you done to your JK this week? 2.5M Views

[Macca2801]

Originally Posted by PapaSmurf

The circumference of the tyre doesn't change, but the radius does. If you let enough air out of a 33" tyre to lower the axle by 1/2", the 1km distance requires 391 revolutions rather than 380 - and the speedo is based on the revolution count.
You can think of the circumference of the tyre as a caterpillar tract rather than a perfect circle. Inside it there's a drive wheel that changes the size as you let the air in or out.

While this is technically correct, it is VERY extreame example! The OP was about changing the speedo readout by changing tyre pressure a few PSI not 15-20 as you need to bag out (ie significantly alter the diameter) a SB Radial.
This is totally insignificant in correcting speedo from going from a 31" tyre to a 33" tyre as the OP was attempting to do. For this to be correct the 4 PSI (36-40)difference would have to equate to a 1" difference in radius...not going to happen.

[Wallyd]

Although the radius is lowered when deflating the tyre, there is no difference on the rpm the tyre does as it still has to roll the entire circumference of the tyre, and in respect to speedo, that shouldnt change either as the circumfirence of the tyre, again, has to do the entire revolution on the road, so therefore the rpm on tyre to length of distance covered over a period of time, will always be the same.

Where as, when a smaller diameter tyre is placed, there is less circumference therefore less distance gained on the the same rpm the axel is doing.

The car will struggle more to do the same speed/distance which will only effect the fuel used to do the same speed/distance

[colingo]

I wish I had this much time on my hands.

[PapaSmurf]

Originally Posted by Macca2801

While this is technically correct, it is VERY extreame example! The OP was about changing the speedo readout by changing tyre pressure a few PSI not 15-20 as you need to bag out (ie significantly alter the diameter) a SB Radial.
This is totally insignificant in correcting speedo from going from a 31" tyre to a 33" tyre as the OP was attempting to do. For this to be correct the 4 PSI (36-40)difference would have to equate to a 1" difference in radius...not going to happen.

Agreed - just wanted to illustrate the principle. Since the dependency is linear, to get 1.5% speedo error you need about 6mm variance in the tyre radius. No idea how much pressure difference it corresponds with: that depends on several factors. In practice I have never noticed any speedo variance based on tyre pressure.
But this is not what this thread is about, so I'll shut up now.

[Banshee]

Well, it's more 32" to 33" in my case, as the originals were the 255/75R17s not the 245s.

As to the difference in height, I could see the change in height as they went up. Slow & small change, yes, but noticeable.

oh yes, the footprint is still 15-20cm long at 40psi, so tyre adhesion isn't affected that much. At least not by lack of contact area, the increased pressure making it stiffer & bouncier, however...?

Aaaaaaanyway... on the 50km trip to work today, 50km on the GPSr equated to 49.5km on the odo, so I have made ~0.6% difference in accuracy, (and I guess rolling diameter). At 40psi, I had ~1.6% error over 100km, at 38psi, ~1% error (over 50km). Will see how the trip back affects the estimate, tonight.

DBOR bullbar due to arrive tomorrow.

[Macca2801]

Originally Posted by Banshee

Well, it's more 32" to 33" in my case, as the originals were the 255/75R17s not the 245s.

As to the difference in height, I could see the change in height as they went up. Slow & small change, yes, but noticeable.

oh yes, the footprint is still 15-20cm long at 40psi, so tyre adhesion isn't affected that much. At least not by lack of contact area, the increased pressure making it stiffer & bouncier, however...?

Aaaaaaanyway... on the 50km trip to work today, 50km on the GPSr equated to 49.5km on the odo, so I have made ~0.6% difference in accuracy, (and I guess rolling diameter). At 40psi, I had ~1.6% error over 100km, at 38psi, ~1% error (over 50km). Will see how the trip back affects the estimate, tonight.

DBOR bullbar due to arrive tomorrow.

4 PSI is a noticeable difference in side wall height????? NOT A CHANCE, sorry...maybe if it was 0-4psi not 36-40 PSI.
But hey goodluck with it all, as pointless as it is, what you are probably reading is the difference in distance from one side of the road to returning on the other hahhah

You do know its shorter to travel around Aust anti-clockwise dont you hahah!!

Matt.

[Gravel]

You may be wasting your time here...

For about every 7 degrees the temp goes up or down the tire pressure will change also by about 2 PSI.

So if you set your pressures at say 15 degrees outside temp; then start driving around in 30 degree temps the tire pressure could have increased as much as 4 or 5 degrees... Doh....

In the US and other very cold countries with heaps of snow etc, drivers are warned not to set their pressures in the garage but at a servo because they can see as much as 30 degrees diff in temp from inside the garage and day time running temp and the tires will be substantially under inflated... because the tire cools when they venture down the street.

So in short... if you’re stuffin about over a couple of PSI you’re wasting your time...