Tire size issues: clearances, diameter, etc.

[dooz]

My Type A pusher has 22.5 rims. OEM tires were 755/80R22.5, but only Michelin has this size. Tires were replaced before I got it with 755/70R22.5. These are about 5% smaller diameter, which means at an indicated speed of 60 mph, I'm only going 56 mph (and thus the distance I go is less than indicated and fuel mileage is less than indicated).

I want to move to a tire size that is closer to OEM, but I worry about such factors as clearance between the duals. (For example, 765/75R22.5 has 0.8 in less clearance.)

I also don't have a source for what sizes actually exist or are common. (Again, Michelin owns the OEM size, and my current tires are an uncommon size.)

Any experts on here, or does anyone know a good Web reference?

[bm02tj]

go to tiresize.com  

You should be ok  as I assume you mean   255 and 265  not 755 and 765

[wildebill308]

2 hours ago, dooz said:

My Type A pusher has 22.5 rims. OEM tires were 755/80R22.5, but only Michelin has this size. Tires were replaced before I got it with 755/70R22.5. These are about 5% smaller diameter, which means at an indicated speed of 60 mph, I'm only going 56 mph (and thus the distance I go is less than indicated and fuel mileage is less than indicated).

I want to move to a tire size that is closer to OEM, but I worry about such factors as clearance between the duals. (For example, 765/75R22.5 has 0.8 in less clearance.)

I also don't have a source for what sizes actually exist or are common. (Again, Michelin owns the OEM size, and my current tires are an uncommon size.)

Any experts on here, or does anyone know a good Web reference?

Welcome to the forum. It will help you to get better answers quicker if you put your RV information in your signature. Se mine and others on the forum.

Ok lets find out what your OEM tire should be. As I don't know what RV we are talking about you will need to look several places. Look on the wall by the driver's' seat or in the cabinets there will be a sticker showing what size the OEM tire was. I doubt it was a 755 or 765. 

Bill

[dooz]

I don't know WHY I keep doing that! Those would be some super-fat tires!

Yes, 255 and 265.

And yes, I was believing the 0.8 inches less wouldn't be a problem, but I still don't know if this is a common/commonly-available size.

Thanks for your input

[dooz]

My RV is:

2004 National TropiCal T396, Cat/Allison.

OEM tires were Michelin 255/80R22.5. Can't determine (w/o measuring rim) the rim width.

Thanks.

Oh, and I found a chart of sizes, this one for Goodyear, which helps:

https://www.goodyearrvtires.com/tire-selector.aspx

 

Edited October 27, 2022 by dooz
Add info.

[wildebill308]

Yes those would be big fat tires. This should help you find tires that will be close enough to work. https://tiresize.com/comparison/

On my last coach I went from 255 to 275 and they worked great. Also gave me a 6% overdrive.

Bill

[dooz]

Thanks again, Bill!

I got most of what I know from the site you suggested. I have two problems that site did not solve:

1. Ensuring enough space between duals. I haven't found any site or anyone who can tell me what that spacing is or should be, but I have in the past seen something about how there needs to be a minimum space/gap.

The Goodyear site does list appropriate rim sizes (widths) for each tire size, but I don't know what size rims I have (old spec sheets don't say, and I don't know right off how to measure with tires still on--andTBH I'm trying to learn as much as possible while sitting at my computer-lol). (My logic: Compatible rim width ensures adequate spacing/gap.)

(OEM size could fit on 6.75, 7.50, or 8.25, but 265/nn or 275/nn seem to need 7.50 or 8.25. So if I have 7.50 I'm good, but if I have 6.75, not so much.)

2. Which sizes that I can plug into the tiresize page are actual/common sizes. For example, I can input 275/75 (which gives me the closest match on diameter), but Goodyear doesn't list it. (Already had fun getting a tire while traveling to replace 755/70R22.5.)

Anyway, hoping some retired tire person will jump on here.

 

[wildebill308]

I will bet your rims are just fine as far as "offset" is concerned. Go look at/measure what the gap is now. Using the comparison tool it shows the  difference between 255 80 and 275 80 as far as width is only 0.8". 

Heare is a picture of the gap in the duels from my last coach with the 275 80 tires. Never had a problem.

I bought Continentals through the FMCA and they were over $100.00 cheaper each over the Michelin discount price.

Bill

Edited October 28, 2022 by wildebill308

[rayin]

Dual tire spacing; this should be informative: https://www.doublecointires.com/wp-content/uploads/Double-Coin-Dual-Spacing-InfoSheet.pdf

Back  many decades ago I drove a dump truck hauling coal for my uncle. Back then the rule of thumb was 1.5" between tires at bottom low point so rocks/lumps of coal would not lodge between the tires. Today I'm told that has been reduced to 1" between tire sidewalls on bottom of tires.

 

[five]

On 10/26/2022 at 7:27 PM, dooz said:

... Can't determine (w/o measuring rim) the rim width...

 

My rims have the dimensions of the rims on the inside of the rims.

[rayin]

1 hour ago, five said:

My rims have the dimensions of the rims on the inside of the rims.

Yup, aluminum rims have the measurements stamped on the inner side of the rim. Steel rims are stamped on the inside of the rim, covered by the tire when mounted.

[tireman9]

Yes dimensions and tire to tire clearance are important but so is the load capacity yet no one mentioned the load capacity.

Lets look at load capacity & dimensions.

Your OE 255/80R22.5 had dimensions of  10. 4 wide and 38.84 OD on 7.5 wide rim  LR-G gave you 5205 / 4805 Lbs S/Dual load capacity   Min dual spacing was 11.30

I have a post on my blog on Min Dual Spacing  

and here is a graphic of that spacing

 

There are 265/75R22.5 tires with the same load capacity as your 255

The dimensions are 10.31 width 38.43 OD and 11.61 min spacing so these should fit as you only loose .0.31" clearance and support the load as LR-G tires