I found this tire size calculator on another site. It'll tell you if you change the size of tires and/or wheels how much your speedo will be off calibration.

Tire Calculator

Nice find

Thanks for sharing

SS Crew, I'm impressed by your rolling stock. How long have you had the Big Guys on your truck? How did you choose Cross Terrains? What did they replace? What changes, good, bad, and otherwise, did you notice?

I ask this because I drive in a very sporting manner and want to go to something a lot stickier and crisper than the OEM items(Bridgestone R265 V-Steel; anybody want a set with less than 1000 miles?) and figure to go to at least 18" wheels and, for example, Michelin Pilot LTX 285/55 VR18 tires. I'll put excellent winter tires (e.g., Revos or maybe even Blizzaks) on the OEM wheels and use those when necessary (typically several months of the year around here).

I'd prefer not to go to a larger rolling radius than stock, both because I don't want any taller gearing (the stock 3.73 is plenty tall with the OEM rolling stock) and because I'm concerned about upsetting the QS system's calibration. Your 305/50-20 tires are about 5% taller than OEM; have you recalibrated your speedo, ABS, QS, etc.? Have you noticed any ill effects from changing effective gearing?

Something is odd about this cool tire calculator on that mazda miata site...

In our case, the calculator cites that the 245/75R-16 run at 662 revolutions per mile.

This is quite contrary to the 683 revolutions per mile as stated by Bridgestone's company website, General Motors on techII data TSBs, the TireRack.com website, and other collaborating sources.

If the basic data for our tire size is so off (by as much as 21 revolutions per mile), then the convenience of this calculator is muted by it's inaccuracy.

Quadrafacts, the Miata calculator is indeed inaccurate concerning revs per mile, and the reason is that it assumes that the tires are circular.

In the real world of finite tire pressures, tires are flattened on the bottom, and the rolling radius (distance from the center of the tire to the pavment) is less than the radius of the circle formed by the rest of the tire, so the real RPM figures are higher than what the calculator says. I ran many sizes thorugh that calculator, and all departed from tire makers' RPM figures in this way.

But that's not all: if you look up RPM figures for many brands and models of tires all of the same nominal size, you'll find small variations. These reflect different tire pressures, tread depths, test vehicle weight, and other specifics of each tire and each maker's measurement technique.

If you really want accurate speedo calibration, or accurate estimates of road speed at red line in each gear, you need accurate RPM numbers for the drive wheels.

If the certification sticker on the 4wd QS Burb states that the front tires are to be run at 50 psi, and the rear tires are to be run at 80 psi, then would this 60% increase in pressure from the front tires to the rear tires affect the rolling radius as much as the 21 RPM difference between the miata site calculator and the published specification?

If so, then how does the "Auto" 4wd driveline cope with two different effective final drive ratios, where it is said that the front and rear differential ratios should match exactly, regardless of road surface "slip" compensation?

This message has been edited. Last edited by: Quadrafacts,

Relax, Quadrafacts. Going from 50 to 80 psi makes a negligible difference in tire revs per mile in these very sturdily steel-belted (and thus fixed circumference) Load Range E tires, certainly much smaller than the discrepancy between reality and the Miata calculator's figures.

That difference is also much smaller than the rotational rate differences experienced by all tires as a vehicle steers. Conventional two-wheel steering has each of the four wheels traveling a curve of different radius, so there will be four distinct (and significantly different) wheel rotational rates even if all the tires have exactly the same revolutions per mile.

For that reason, full-time AWD systems (e.g., Yukon Denali, Audi Quattro) have center differentials so the front and rear wheels can run at (at least somewhat) different speeds, which permits tight steering on grippy pavement.

Engaging 4WD on a typical truck whose transfer case has no center diff (or locking the center diff on AWD systesm that permit you to do this) will result in stuttering or other jumpy behavior when steering on high-traction surfaces, so you should not use conventional 4WD unless things are somewhat slippery.

The Autorac auto-4WD system on the non-Denali Yukons and Subs engages the hubs on the front wheels but does not connect the front drive shaft until slipperiness (of the sort produced by rain, snow, sand, etc.) is sensed.

This message has been edited. Last edited by: Kong,

Kong, since you are shopping for wider tires of the same diameter (or same loaded radius) and same load capacity as stock, and since by your posts so far it seems that you might undertake a scorched earth approach to your research, which tire sizes have you determined so far to be equivalent (or close enough) to the stock loaded radius and load capacity?

245/75-16 Stock size, 30.5 inches diameter, 682 revolutions per mile, LRE.

Hypothetical Examples of equivalent alternates:

16" Wheel

255/?
265/70-16
275/?
285/65-16
295/?
305/60-16
315/55-16

17" Wheel

275/?
285/60-17
295/?
305/55-17
315/50-17

18" Wheel

275?
285?
295?
305?
315/50-18

etc. etc. to the extent that you have looked into this...



(Note to anyone who may stumble upon this post out of context: The tire sizes listed above are hypothetical examples to illustrate the hoped-for answer to the posted question. Nothing listed above has been verifed to be available in an LT service designation, nor in a 10 ply LRE rating, nor at any particular weight bearing capacity. In fact, some of the tire sizes listed above may not even exist.)

quote:

Originally posted by Kong:
SS Crew, I'm impressed by your rolling stock. How long have you had the Big Guys on your truck? How did you choose Cross Terrains? What did they replace? What changes, good, bad, and otherwise, did you notice?

I ask this because I drive in a very sporting manner and want to go to something a lot stickier and crisper than the OEM items(Bridgestone R265 V-Steel; anybody want a set with less than 1000 miles?) and figure to go to at least 18" wheels and, for example, Michelin Pilot LTX 285/55 VR18 tires. I'll put excellent winter tires (e.g., Revos or maybe even Blizzaks) on the OEM wheels and use those when necessary (typically several months of the year around here).

I'd prefer not to go to a larger rolling radius than stock, both because I don't want any taller gearing (the stock 3.73 is plenty tall with the OEM rolling stock) and because I'm concerned about upsetting the QS system's calibration. Your 305/50-20 tires are about 5% taller than OEM; have you recalibrated your speedo, ABS, QS, etc.? Have you noticed any ill effects from changing effective gearing?

I've had the "big guys" on my truck since it had about 4000 miles on it. I chose the Cross Terrains because they came with a 65,000 mile treadwear warranty. I had Kuhmos (ku-mo) as the first set of 20's. I got almost 30,000 out of those, but they got quite noisy the last 5,000 miles. I wanted a tire that was quiet and would last. Michelins have always been a long lasting truck tire, so I blew my wad and got the Cross Terrains.

I do like the Cross Terrains, they're quiet, they grip, but the tread wear sucks. I've got a little over 25,000 on them now and I'm about at the wear bars. The good thing is I get the next set of Michelins at over 55% off the purchased price. So for a little over $400 I'm in a new set of 20" tires.

When I put the 20's on my truck, I got a Hypertech programmer to calibrate the increased tire and wheel size and maybe pick up a few ponies on the way. The Hypertech is crap! It did recalibrate the speedo, but the performamce tuning just made the truck ping, and the increased shift firmness made it slam into 4th gear, that was it.

I have always thought the 6.0 with 3.73 gears was a dog, I'm sure the 20's didn't help, but when I drove a Yukon XL with a 5.3 for a loaner, man, that thing hauled oss compared to my truck, now I knew my truck was a dog. I went on GM-Truck.com and noticed how many people were running a Wester's custom tuned computer. Wester's is even a sponser on this site. I asked a few questions and then decided to try to get more power that way.

Lemme tell you, the Wester's Hot Tune is the hot tip! At first I couldn't tell any difference, then after a few hundred miles it was really noticeable. It still won't break the tires loose unless it's wet, but it pulls like crazy. My towing mileage went from 9.5 to 11 mpg. That's towing a 5000 lb boat over hills at about 70 - 75 mph. My cruising mileage went up a little from 12.5 to 14.5, but that's running about 80 - 85 mph over the same hilly terrain going to the Colorado River.

The truck is still no hot rod, but it sure performs much better than stock.

I hope in all my rambling I have answered your questions.

Later -
SS

This message has been edited. Last edited by: SS Crew,

Quadrafacts and SS Crew, I'll respond to both of you in a moment, but first some news: I now have the new wheels and tires on my Suburban, and have some baby pictures for you. Yes, I'm a happy daddy!

Details: Jax HD 18x8.5 rims, 25 mm offset, mirror machined (not chrome); Michelin Pilot LTX 285/60-HR18/RF (rated for sustained 130 mph and 2805 lb load at 50 psi; 667 revs per mile)

This message has been edited. Last edited by: Kong,

Left front wheel closeup:

Left rear wheel closeup:

Looking good!

You state that the rims are Jax 16 x 8.5. I hope you've got 18" rims, right? 'Cause you've got 18" tires.

This message has been edited. Last edited by: SS Crew,

Thanks, they not only look good but work very well indeed; smooth, quiet, grippy, and precise.

Meanwhile, through the magic of the editable post, I correctly identify the wheels as 18s. Thanks for catching that.

Sweet rims, Kong

And what happened to the baby pics ?? Congratulations First one ??