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Old 09-22-2010, 06:57 PM   #15
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There is an evening's worth of reading on this forum about tires[use search option]. I also think we are mixing up some terminology here. Not familiar with the Bridgestone naming conventions [eg, 250s, 260s or 280s] but generally speaking, tire size is described as follows--295 80R 22.5 inch rim or 11R 22.5s; where 295 is the tread width, 80R is the profile or sidewall height and 22.5 is the rim diameter. Prior to 2004, Alpines came with Toyo(110s) 295 75R [22.5]. In 2004 and after, they came with Toyo(120s) 295 80Rs???; and finally they came with GY670s--295 80Rs.

Ref earlier posts, if your coach was originally equipped with 295 75Rs and you go to 295 80Rs or R11s, your ECM [speedometer, and mileage will run slow]--80R has a slightly larger circumference than the 75R and I think the R11 may be even a bit larger than the R80 or perhaps they are the same. Ive heard Cummins can adjust their ECM to account for the larger circumference. Ive also heard the avg 275 80R is an exact match for the 295 75R but cant prove it.

As for tire shopping--enjoy reading the forum on preferred brands and good luck on price.

PS--with all deference to the Georgetown 325 fellow on the F53 frame, I doubt you are running 295 80Rs on a gasser--my apologies in advance if I am wrong.
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Old 09-22-2010, 10:36 PM   #16
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Old Scout --

The Bridgestone naming conventions for 250, 260 and 280 don't refer to the tire size; rather to a type of tire. My 250's are 295/80 22.5. The 260 and 280 typically are a different size. As I said earlier, the 295/80's are almost a perfect match for the factory settings on our 2006 speedometer and Silverleaf, and we had 11R22.5 Toyos on it before.

The 295/80's in the R250 series have a higher load rating than the 295/75's in the R260 or R280 series, which is one of the main reasons i use the 295/80's.

Here's a link that explains the difference -- pick a series -- 250,260,280 and you can see what's different in the tire and the different load ratings for each size.

Untitled Document
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Old 09-23-2010, 07:56 AM   #17
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Wow--that is certainly a lot of information about Bridgestone. Since the initial question was from a 2000 owner, I was trying to explain that his coach was originally equipped with 295 75Rs. Accordingly, if he goes with any 80R profile tire, his ECM will probably not match the slight difference in circumference. The difference is about 5% lower: for miles and for MPG calculation; and on avg, about 100 RPMs lower on the tach for the same measured speed/MPH.
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Old 09-23-2010, 12:05 PM   #18
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I didn't realize the 2000 came with 295 75's, since our 2006 came with 11R22.5's Toyos that probably explains why we haven't seen much difference switching to 295 80's.
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Old 09-23-2010, 11:08 PM   #19
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Old Scout:

I've been trying to find out the exact difference in MPH for my VMSpc program. I switched from the 75R to 80R and have been trying to figure out the ratio to input into the programming. It was suggested a couple years ago that it was either 5% or 2-3%.

I have been using mile markers to see how far off my speedo is. It seems like 3%, but I'd hardly call my technique scientific. Speed monitors with radar seem to be all over the place with my actual speed. Meanwhile, if I want to be traveling 55 mph, I set the CC at 52.5 which is somewhere around 5% lower. The upside? No tickets.

Don't make me bring out my math skills and actually calculate the rolling distance of each tire and contrast them! (Probably take less time than this post, though.)
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Old 09-23-2010, 11:30 PM   #20
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I have given up on my mph on the dash. I use my gps and works great as far as I can see it is right on.
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Old 09-24-2010, 07:52 AM   #21
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I haven't gone looking but suspect there is some website somewhere that has tire circumferences measured to the last millimeter. Same as Matt, I first noticed the difference with my co-pilot's gps software. Seems to be about 3mph slower at 60mph, or about 5%. Assume its about the same for mileage but its hard to time mile markers and the trip-o-meter. Anyway, in addition to no tickets, it helped explain a "coincidental" reduction in MPG calculation--that an a leaking CAC. So 5% is about right--IMHO.
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Old 09-25-2010, 01:20 AM   #22
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I found this post from RMMPE from 9/23/07 regarding how to figure differences in tire circumfrence. I remember using it at the time to do the calculations. Seems accurate (thank-you Bob for this).

Rule of thumb:

The 1st 3 numbers are the width of the tread in millimeters (25.4mm/inch).
The 2nd 2 numbers are the "Aspect ratio", meaning the % of sidewall height as related to the width.
The 3rd numbers are the rim size.

So:

A 275/80/22.5 tire will have a sidewall height of 220mm/8.66". This, times 2 plus the rim diameter of 22.5" puts the total diameter at 39.83"(125.1" circumference or 10.43 feet/axle rev).

A 295/80/22.5 tire will have a sidewall height of 236mm/9.29". This times 2 plus the rim diameter of 22.5" puts the total diameter at 41.08"(129.06" circumference or 10.76 feet/axle rev).

You'll have to do the math to see what size most closely matches the Toyo size you are running on now.

Hope this helps.
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Old 09-25-2010, 01:37 PM   #23
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With respect to the speedo and odometer readings, the number of interest is Revolutions Per Mile. This number can be found on all of the tire spec sites like the Bridgstone referenced above. This is the number that is placed into the engine ECM to give the correct output to the tranny, speedo and other devices like Silverleaf. It can be changed in the ECM firmware by any Cummins, Allison, and other repair facility with the proper computer programing and a laptop.

For the 295R75 size tires the number is 518. The number is the same for all manufactures of this size tire. For the 275R80, it is the same number. Likewise, 295R80 and 11R80 share a same number, something like 512 I think.

Futher, a little thinking will tell you that at 60 MPH, that is 1 mile a minute, given 518 Revs per mile for the tire, then the tire is traveling at 518 RPM. I just know you were wondering how fast your wheels rotated while traversing the interstates.
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Old 09-25-2010, 11:24 PM   #24
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Quote:
Originally Posted by Louie & Linda View Post
With respect to the speedo and odometer readings, the number of interest is Revolutions Per Mile. This number can be found on all of the tire spec sites like the Bridgstone referenced above. This is the number that is placed into the engine ECM to give the correct output to the tranny, speedo and other devices like Silverleaf. It can be changed in the ECM firmware by any Cummins, Allison, and other repair facility with the proper computer programing and a laptop.

For the 295R75 size tires the number is 518. The number is the same for all manufactures of this size tire. For the 275R80, it is the same number. Likewise, 295R80 and 11R80 share a same number, something like 512 I think.

Futher, a little thinking will tell you that at 60 MPH, that is 1 mile a minute, given 518 Revs per mile for the tire, then the tire is traveling at 518 RPM. I just know you were wondering how fast your wheels rotated while traversing the interstates.
Louie & Linda, Jeff & Cheryl,

Something doesn't quite compute. If take the calculations from each of you and multiply, I don't get the right answer:

(1) Jeff & Cheryl say, "A 275/80/22.5 tire will have a sidewall height of 220mm/8.66". This, times 2 plus the rim diameter of 22.5" puts the total diameter at 39.83"(125.1" circumference or 10.43 feet/axle rev)."

(2) Louie & Linda say, "For the 295R75 size tires the number is 518. The number is the same for all manufactures of this size tire. For the 275R80, it is the same number."

Using these two statements gives:

10.43 feet/axle rev TIMES 518 rev/mile = 5402.74 feet/mile.

However, there are 5,280 feet/mile. So, one or both of these numbers is a bit off.
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Old 09-26-2010, 01:43 AM   #25
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Plus the percentage of change in MPH is negligible. A bit more than 1/2 MPH at 60. Doesn't seem to match the less-than-accurate mile marker test, either.

I'm calling for an Engineering Confab.
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Old 09-26-2010, 11:40 AM   #26
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Looking at the Bridgestone Data Book, 295/80R 22.5's in R250 have 499 revs per mile (RPM) and 11R22.5's have 501 RPM, which is probably why I couldn't tell the difference when I replaced my 32000 mile old Toyo 11R22.5s with the new Bridgestone 295/80's. If the Toyos had the same RPM, the difference would be 0.2 mile at 60 mph. The 295/80's would be traveling at 60 when the 11R22.5's would be going 59.8 at the same engine RPM. But since they were worn, the Toyos were probably putting on a few more RPM.

The 295/75's in the same data book, depending on the tire, R260 or R280, have 512 to 517 RPM. This would be a difference of 2.5 to 3.6 %, or between 58.1 mph to 58.7 mph compared to the 11R22.5's at 60 mph.

So the 295/80's are a close match to 11R22.5's but the others are off more noticeably. I'm curious as to when WRV switched to Toyo 11R22.5's, since our 2006 came with them, and the earlier coaches had metric profile tires. The 11R22.5's don't have as high a load rating so maybe that came with ours because it was a 36' and perhaps the 38's or 40's had metric sizes?
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