Another tire question / suggestion request

[Sh0rtBus]

Measure from the center of your wheel to the ground and x2. Tell me that you still get 36.5". Most every 37" tire I have seen has an actual measurement closer to 35". But hey, what do I know.

I didn't think to measure like that, that makes a lot of sense considering only the bottom side of the tire is being compressed at any given time

I actually measured the diameter across the face of the tire from the floor to the top of the tread. Wouldn't taht make the most since, as it's a true measurement, rather than measuring half the tire and then multiplying it by two? WHy owuldn't you measure from the floor to the top of the tire?

 

[JakeJK]

I actually measured the diameter across the face of the tire from the floor to the top of the tread. Wouldn't taht make the most since, as it's a true measurement, rather than measuring half the tire and then multiplying it by two? WHy owuldn't you measure from the floor to the top of the tire?

Well the top of the tire doesn't have an weight on it but when it rolls around to the bottom it will so it's more accurate to measure it halfway then multiply it by 2

 

[jeeeep]

keep in mind a tire actully rolls in an oblong shape, the sidewall flex keeps it on the rim. this is why most calibrations are calculated based on the mounted tire measured from the center to the ground x2, not the tire specs. The actual amount inflated will also affect the measurement/calibration which is why all speedos can vary up to +/- 5mph on any stock car with OEM equipment.

you can also get a close measurement by measuring from the ground to the top outer tread (which would be the road contact spot once it rotates)

 

[Sh0rtBus]

Well the top of the tire doesn't have an weight on it but when it rolls around to the bottom it will so it's more accurate to measure it halfway then multiply it by 2

Regardless of whether or not there's weight on it or not, the most accurate measurement of anything is a straight measurement.....not a fraction of a measurement multiplied by some number to get the total. So you're telling me if I measure the distance from the center of my wheel to the ground and end up with 17" and multiply that by two to equal 34"....that's going to be more accurate than me measuring the total distance from the ground to the top of my tire? The top of the tire will never be compressed so it remains a constant. Really, it seems to me the most accurate way to get the true size of a tire is to measure the total diamater across the face of the tire while it's laying down on the ground before it's ever inflated. I couldn't say whether or not the "advertised" size compensates for inflationor not and if so, what pressure? Too many variables there. Granted I'm still on my first Jeep and probably don't know anything after being a service advisor for 8 years, but seems to me the simplist measurement would be the most accurate because there are no variables.

 

[JakeJK]

Regardless of whether or not there's weight on it or not, the most accurate measurement of anything is a straight measurement.....not a fraction of a measurement multiplied by some number to get the total. So you're telling me if I measure the distance from the center of my wheel to the ground and end up with 17" and multiply that by two to equal 34"....that's going to be more accurate than me measuring the total distance from the ground to the top of my tire? The top of the tire will never be compressed so it remains a constant. Really, it seems to me the most accurate way to get the true size of a tire is to measure the total diamater across the face of the tire while it's laying down on the ground before it's ever inflated. I couldn't say whether or not the "advertised" size compensates for inflationor not and if so, what pressure? Too many variables there. Granted I'm still on my first Jeep and probably don't know anything after being a service advisor for 8 years, but seems to me the simplist measurement would be the most accurate because there are no variables.

The literal tire size yes but not for what we need, for the speedo and such. And clearance increase etc. No the top will never be compressed but for the sake of calculations you want the compressed diameter of the tire. (Equal to the bottom tire size plus half the wheel size multiplied by two). Think about when the wheel rolls it won't roll oblongly it will compress as the tire touches the asphalt so basically for computations sake you can have a tire that is that compressed all the way around to give you a perfect measurement granted it doesn't look that size nor is it actually that size. That diameter is used for calculations it is the best way to get the speedometer as close as possible

I'm in school for mechanical engineering and in my mind that made perfect sense haha I may not have explained my train of thought very well though

 

[JakeJK]

I don't know if this makes any sense but this is what speedo calculations would be made off of

 

[Sh0rtBus]

Now after 45k miles, my speedo is off about 5 mph above about 50, but when my tires were new (4 years ago), I entered 36.85" into the appropriate spot on my AEV ProCal chart and my speedo at that time was spot on, as measured by GPS. I still don't see how even the compressed value of the tire would be twice the measurement from the ground to the center of the wheel. The tire, rolling or otherwise, will never be compressed on the top of the tread. So the distance from tne cener of the wheel to the top of the tread seems like it would be a more constant measurement, as so many things can affect how much the bottom of the tread is compressed. Heat and ambient temperature, weight transfer under acceleration/braking, road surface (hard vs. soft), fuel....and that's not even accounting for the driver, passengers, payload, etc. The distance from the center of the wheel to the top of the tire; however, will never change aside from tire pressure affected by temperature.

I say let's agree to disagree on this one and get back to the OP.

And Eddie....just to clarify, you said in your post that people in the know refer to "advertised size", which is always smaller and I think you actually meant "actual size" is always smaller. :beer:

 

[JakeJK]

Now after 45k miles, my speedo is off about 5 mph above about 50, but when my tires were new (4 years ago), I entered 36.85" into the appropriate spot on my AEV ProCal chart and my speedo at that time was spot on, as measured by GPS. I still don't see how even the compressed value of the tire would be twice the measurement from the ground to the center of the wheel. The tire, rolling or otherwise, will never be compressed on the top of the tread. So the distance from tne cener of the wheel to the top of the tread seems like it would be a more constant measurement, as so many things can affect how much the bottom of the tread is compressed. Heat and ambient temperature, weight transfer under acceleration/braking, road surface (hard vs. soft), fuel....and that's not even accounting for the driver, passengers, payload, etc. The distance from the center of the wheel to the top of the tire; however, will never change aside from tire pressure affected by temperature.

I say let's agree to disagree on this one and get back to the OP.

And Eddie....just to clarify, you said in your post that people in the know refer to "advertised size", which is always smaller and I think you actually meant "actual size" is always smaller. :beer:

Yeah same here I've done enough math today to be thinking this hard about this haha

 

[rob_p]

View attachment 162307

I don't know if this makes any sense but this is what speedo calculations would be made off of

Is that a wheel or a kiwi?

 

[JakeJK]

Is that a wheel or a kiwi?

View attachment 162311

Hahahaha that was good

I'm a mechanical engineering major not an art major lol

 

[jeeeep]

kiwi's are round only if you slice it correctly, they're also sort of oblong lol

 

[JakeJK]

kiwi's are round only if you slice it correctly, they're also sort of oblong lol

Oh how a thread can be so easily derailed hahaha

 

[rob_p]

Oh how a thread can be so easily derailed hahaha

Hey the diameter of a kiwi is serious stuff!

 

[JakeJK]

Hey the diameter of a kiwi is serious stuff!

Yes tis is true, especially when it is in my belly

 

[Fishrising]

Thanks all! When do I have to consider re-gearing? When hitting the 35" range? 34" range? I've got the 3.8 4spd auto with 3.73s. And again, based on the usage I stated in the original post.

 

[MR.Ty]

Thanks all! When do I have to consider re-gearing? When hitting the 35" range? 34" range? I've got the 3.8 4spd auto with 3.73s. And again, based on the usage I stated in the original post.

If you haven't seen it already here is the gearing chart for the 3.8

5.13 will get you good performance both on and off road with 35s.

 

[dillard09]

If you haven't seen it already here is the gearing chart for the 3.8
<img src="http://wayalife.com/attachment.php?attachmentid=162609"/>
5.13 will get you good performance both on and off road with 35s.

Damn! Beat me to it Mr Ty.

 

[2Cross]

From reading your original post.
I'd say GO with a really good winter tire.
I used to run the BFG KO in the 34" size and they did well. Supposedly the new KO2s do better than these . I've heard good things about the duratracs in the snow too but I've never personally used them. The Goodyear MTRs do well in the snow too (I currently have these).
As far as gearing I'd go with 4.88 to make climbing up those steep hills easier. 5.13 would be better, but you have a D30 front end and I wouldn't want it to bind on you and break while in 4hi going back and forth to to mountain. Breaking the R/P Probably wouldn't happen but it sounds like you put a lot of miles in your rig and not worth it.

 

[Fishrising]

If you haven't seen it already here is the gearing chart for the 3.8
View attachment 162609
5.13 will get you good performance both on and off road with 35s.

So...3.8 auto with 3.73s, according this chart, 32" tire = 1892. Is that 1892 the RPMs required to maintain 70mph with a 32" measured mounted and loaded tire (i.e. the whole measurement debate in this thread, or rather the compressed measurement)?

And if so, if I were to order a 285/70/17 (~32.7") tire, in theory it should compress and get somewhere close to or under a compressed measurement of 32", so I should be OK with my 3.8 auto and 3.73s?

Am I following the logic properly?

 

[Sh0rtBus]

So...3.8 auto with 3.73s, according this chart, 32" tire = 1892. Is that 1892 the RPMs required to maintain 70mph with a 32" measured mounted and loaded tire (i.e. the whole measurement debate in this thread, or rather the compressed measurement)?

And if so, if I were to order a 285/70/17 (~32.7") tire, in theory it should compress and get somewhere close to or under a compressed measurement of 32", so I should be OK with my 3.8 auto and 3.73s?

Am I following the logic properly?

You should be fine with 3.73's on 33" tires. Maybe not optimal arrocrding to the chart, but I don't think you'll notice to much of a difference (loss) at that ratio. Wouldn't bother regearing until you decide to step up to 35" tires or larger. Granted, some 33" tires can be fairly heavy, which will of course affect performance, but it's not really worth the cost of regearing for 33's if you think you may go larger in the future. With the cost of regearing being $900+, better to do it once unless money just grows on trees for you. But if that were the case, I'd suspect you'd be going larger than 33's.