Originally Posted by fxdave
was that the size tire you had on it ? some are talking load capacity ect i was looking to just replace the tires i dont know enough about going bigger any info id love to hear thanks
Long post warning.
When I needed to replace all 6 tires early last summer I spent a lot of time on the web trying to educate myself. What follows is my understanding.
I am not a tire expert and I don't play one on television. I've posted this elsewhere and no one ever pointed out any error - but I don't guarantee that I'm offering good advice. I may know just enough to be dangerous.
1. Do a 4 corners weighing. You need to find a truck scale that has enough room around it to position the coach so that each corner is the only one being
weighed. The scale will be a flat platform, level with the ground.
Try searching Yellow Pages for Public Scales, Moving companies, and the one that works best for me -- Grain Elevators. My local elevator did it for free
since I didn't need any paperwork, they just wrote down the four numbers and waved me on. Nice folks.
The two critical issues are tire diameter (revolutions per mile) and tire width. Tire width is important for steering clearance and for proper dual spacing
at the rear. You need a minimum spacing, or larger, to get air flow between the duals for cooling. Its a critical measurement.
For tire width steering clearances you get a good picture of the situation by turning your fronts to each extreme, slide under the coach (engine off, brakes
set) with a steel tape rule and eyeball all around for clearances, and visually and tape measure see what additional width will do at the turn extremes.
For the rears you can use the a simple method. Engine off, brakes set, slide under the coach at one dual position and look between the tires front to back
(or back to front) to measure the closest distance between the tires.
If the new tires are 1" wider each, use just 1" decrease in the spacing estimated decrease in distance between tires. You only use 1/2 the tire width because
half the increase is on each side of the tire. If the new spacing looks reasonable (on the order of around 3" +/- 1/4" take the coach and your measurements
and the tire size you are considering to your truck tire dealer and get his opinion.
You do not have to have exactly the same size tires as came with your coach, you do have to have tires that will give you full clearance in turns and between
the duals and are within a few percent of the revolutions per mile of your current tires.
BTW, going to a higher load rating can give you a softer ride! You don't have to inflate to as high a pressure as you would with a lower load tire. That's
straight off the tire load/pressure tables if you look for it comparatively.
2. Here's how I found a replacement size for my tires. I had 9R22.5s as the OEM tire. They are becoming harder to find as most tire makers have moved
towards metric sizing.
Toyo has both standard sizes (9R22.5, 10R... etc) and metric (270/70R22.5). You need to look at each Toyo tire model for sizes to find comparables.
Basically the metric numbers are tire width/aspect ratio (sidewall height as a proportion of tire width) Radial 22.5, or 20.. (wheel
Once you locate a Toyo tire model (medium duty truck - all position) with comparable measurements you need to check the tire specs. There is a link on the
manufacturer's website that takes you to a table that includes a variety of special information.
I was able to find a metric substitute for my 9R22.5 OEM tires with a Toyo 275/65R22.5. The metrics are 1" wider, but no taller, have the same revolutions
per mile (no need to have speedometer re-calibrated) and were a grade higher (from F to G).
3a. The critical issue is tire clearances if you go to a wider tire. For the fronts I turned the steering wheel to extreme left and extreme right and
measured clearances between tires and any nearby structure, checked to make sure that the additional width wouldn't hit anything.
The backs get a little more technical. You need to be sure that the wider tires in a dual configuration are far enough apart to allow ventilation between the
tires. The specification tables usually give minimum dual spacing.
Simple method. Measure the spacing between the side-walls of the duals at the "fattest" part of the tires. On my coach that was around 3-3/4". If you go with
a 1" wider tire, as I did, you have to realize that 1/2 of that inch increase is on the right side of the tire and the other 1/2" is on the left side of the
Two tires each 1" wider, means the gap between is decreased by 1" (1/2 from tire A and 1/2 from tire B. So my dual spacing was reduced from almost 4" to almost 3". Three inches should give plenty of ventilation.
3b. The technical (and a little more difficult) method is to look up your Wheel Offset provided by the wheel manufacturer.
You need the wheel model number from the rim and then apply the following formula:
Min. dual spacing = 2x(offset) - width of one tire
My offset was 6.44". I wanted to use a 10" wide tire.
Mds = 2(6.44) - 10 = 2.88.
Two point eighty-eight was virtually the same number I got from the simple method.
4. I made up a spreadsheet to keep track of all the tires I looked at and a simple calculator for the Mds computation. You can see it here:
Tire comparisons: http://home.roadrunner.com/~kwildman/tire.jpg
5. Also, you need to pay attention to Revolutions per Mile since large differences between old and new tires will show up on the speedometer and odometer.
When I started this search I really knew nothing about the issues. I did a lot of reading on the internet and now I know just enough to be dangerous.
You need to look at the tables, talk to your tire supplier, and make your own decision.