8r19.5 vs 225/70/19.5

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
ranger magnum

ranger magnum

Well-known member
Joined
Feb 19, 2012
Posts
81
Location
Santa barbara
I need new tires for a 1987 bounder. It came with the 8x19.5's but I have been contemplating going to the 225/70/19.5. I have heard conflicting reports on this switch, but the advantsge of a slightly wider tire seems to outweigh the added drag.

Thoughts?
 
The biggest issue is the spacing between the dual tires.  The 8R19.5 don't need as large a distance as the newer 225/70/19.5.  I've heard some tire dealers say that the sizes are interchangeable to have the owner later find out that the tires are kissing each other and wearing out prematurely.  8R19.5 shows the minimal spacing of 8.8" and 225/70/19.5 shows the minimal spacing of 9.7".  I suspect your existing wheels provide a spacing near 9".  The spacing is measured from the center of the tire tread of one tire to the center of the tire tread of the next.

When I was faced with the choice, I went with the 8R19.5.  I chose not to get the newer tire size & new wheels to allow them to work together.  The other factor is that the speedometer would not read correctly with the 225/70/19.5.  There is a weigh capacity difference too, thou minimal.
 
The 225's should be about 8.9" wide while the 8R's should be about 8". That means the clearance between the duals would be just under an inch closer with the 225's (.45" for each tire or .9 total). You might want to take some measurements and make sure you have the clearance you need before making the purchase.
 
Well when I had to make that choice on my 92 Flair I did this.  I put the 225's on the front and 8R's on the back. The spare was an 8R.  It could go either place if needed.  The 225's greatly improved the way the P30 tracked down the road. 
 
I am aware of the issues as far as the spacing on the duals. My fix would be to install a spacer between them. I had success with this on my dually, and cant think of why it wouldnt also work on the rv...
 
My concern would be if you have enough stud length to allow adding a spacer and still have full thread engagement of the lug nuts.  Less than full thread length engagement and you risk that they may unzip while driving.  I've not seen any spacers for this specific application, they might have to be custom made.
 
I'm not an engineer, and I don't have hard and fast numbers for you, but common sense is telling me that spacers (especially thick spacers) would put more stress on the lug bolts and probably decrease load carrying capacity. Maybe someone familliar with loading could chime in. I would investigate it before I did it. JMHO
 
Initial measurements seem to indicate that i may just be able to fit the tires without any modifications. If I did use spacers, they would only need to be about  .250" thick. If i can find them, i will simply fabricate them in the shop.

I think the wheels are hub-centric, and that may pose a problem for the outer wheel. I do have enough stud length though.
 
What about the width of the original wheel that came with 8R 19.5 tires at 6", vs the wheels for the 225/70/19.5 which is 7-7.5" ?  I know that it is possible to run the wider tire on the narrower wheel , but is it safe?
 
vs the wheels for the 225/70/19.5 which is 7-7.5"
Click to expand...

I've not seen any 225/70R19.5 tire that comes on a wheel that wide. The largest rating I've seen for that tire size is a 6.75 wheel width, and 6.0 is the usual. Can you give an example of a rig that has a skinny 225/70 tire like that on a 7+" wheel? Are you sure you aren't thinking of a 245/70? They are typically on 7" or 7.75" wheels.

And no, it is not safe to run narrower or wider wheels than the tire is spec'ed for.  They may work well enough in normal use, but they are quite likely to pop loose at the tire bead when high lateral stress is applied, like an emergency sharp swerve or a skid.
 
You do realize that 225x70.19.5 is 2" shorter don't you?  I doubt very much if you will even notice any difference in handling. My 95 Challenger handled just fine with the 8r19.5 tire.
 
You do realize that 225x70.19.5 is 2" shorter don't you?
Click to expand...

I believe the difference in overall diameter is 1.1"  if comparing new tires with similar tread patterns.  31.1" for the typical 225/70 and 32" for the 8R.  That means the speedo and odo read about 2.5% high.  Traditional wisdom is that a change of less than 3% is "no worries", i.e. no need to change axle gearing or other mechanicals.
 
Gary RV_Wizard said:
I believe the difference in overall diameter is 1.1"  if comparing new tires with similar tread patterns.  31.1" for the typical 225/70 and 32" for the 8R.  That means the speedo and odo read about 2.5% high.  Traditional wisdom is that a change of less than 3% is "no worries", i.e. no need to change axle gearing or other mechanicals.
Click to expand...
Better look again ! 8r19.5 tires are 34".

https://simpletire.com/samson-8-r19.5-88005-2-tires

https://simpletire.com/bridgestone-8-r19.5-267775-tires
 
I think Gary has it.  From the Goodyear Tire Site:
Tire Diameter
Tires sized 8R19.5 have a diameter of 33.1 inches, and 225/70 R19.5 tires are 32 inches in diameter. Changing tire sizes will result in a speedometer that indicates faster than the vehicle is actually moving. 8R19.5 tires will rotate 628 times in a mile, and 225/70R19.5 tires rotate 644 times. The result is a 2.5 percent speedometer error with the new tires. The Goodyear website states that no gearing change is required for this amount of error.

The bigger issue is the wheel/rim width you have:
Wheels
To convert from 8R19.5 to 225/70 R19.5 tires may require the purchase of new wheels. The 8R19.5 size can fit on a 5.25-inch-wide wheels, and the low-profile 19.5 tires require 6-inch-wide wheels. Besides a better fit of tire to rim, the wider wheels will provide enough dual-wheel spacing for 225/70 R19.5 tires. Dual-mounted tires should never touch, and the 5.25-inch rim is too narrow to give proper dual-wheel spacing for the newer tire.
 
Goodyear are the only ones that I've found shorter than 34". With all the problems with Goodyear RV tires not too many people are buying them. Look at the links I provided.
 
Different tire designs will have different diameters and 33.1 vs 34 is within design variances. Tread depth is one factor, and the section height can vary a bit too, especially if designed for different pressures or load ratings.  Goodyear's own 8r19.5 appears to be 33.3".  So, using the GY an example, the 225/70 would be 1.3" shorter.

I checked Bridgestone's Truck Tire Data Book and its shows 33.6 for their R187F in the 8R19.5 size. Simple Tire shows it as 34, but I have to trust the tire industry Bible vs somebody's advertisement. And that is still 1.6" difference, which begins to become significant.
 
You must log in or register to reply here.

Latest posts

Members online

Total: 951 (members: 23, guests: 928)

Forum statistics

Threads
131,925
Posts
1,387,579
Members
137,674
Latest member
SPK550
Back
Top Bottom