New tires on DP - 2" shorter than the old ones

Hi All, based on feedback on this forum I purchased 6 new tires on my 2003 Fleetwood 37U.  the old ones were 8 years old.  I purchased Toyo's for $2,600 total.  They are 2 inches shorter than the old Michellin's and I went with the recommendation of my tire store. 

Can someone help me understand how significant a 2" reduction in height might impact things?  I went on a trip last week and didn't notice a difference.  I assume RPM's would be higher at a given speed but I wasn't tracking that very closely to notice a difference.

Thanks in advance - and thanks again for all the advice this newbie has received.  It has made a significant difference for us.

I do not know for sure, but if you had the exact number off the old one, and the exact number off the new one, when the smart ones come along, it will help them give a better answer.  They will be something like 265/75R 22.5.  But those numbers are what others will really want to see. 

I will say, it certainly helps that you replaced all of them, and not mixing sizes.  But 2", I don't feel knowledgeable enough to give advise, I don't know if changing the size, changes the load rating, etc. 

Hopefully no big deal, I know you are trying to get on the road.

I assume 2" shorter means the outer radius of the tires is reduced by 2 Inches. this will cause your Speedometer to be off by as much as 10%.(On the low side) so whe your neeele points to 60 you will be doing 55 more than likely.

IN my case when I replaced ther XRV's with Toyos the difference was less than 1%.

Good Heavens! Why were such smaller tires installed?  Was that the recommendation of the tire dealer?  In my opinion, that borders on being irresponsible!  I hope nothing that was said here encouraged you to go with smaller diameter tires!

You probably didn't notice rpm vs speed difference because the speedometer and odometer are reading incorrectly.  Your actual speed was lower than what the odometer read, so you probably thought the other traffic was  moving fast (and so you probably sped up to stay with them)

While Toto's are a good tire, I hope the tire store didn't recommend that size only because that's what they had in stock.

Or because they could sell that size at a more attractive price. Some sizes are in high volume production and more competitively priced than others.

I have followed this and the prior thread with interest as I am replacing all the tires on our 40ft Itasca. I'm not an expert by any stretch of the imagination, but have spent a fair amount of time researching replacement tires. I have run Michelin tires on all my vehicles for at least 25 years. On our RV I replace every 5-6 years. The last two sets of Michelin tires have not given good service, mainly cracking. So like the OP I am switching to Toyo Tires. Tōyō does not offer a tire in my size 275/80/22.5. So I'm replacing with a 295/75/22.5 the difference in circumference in very slight, in fact as John pointed out less than 1 percent 220 cm v 221 cm. I intend to have the tires spin balanced, I never have used beads so don?t know much about them. The tire cost is $2400 v $4200 in Phoenix. BTW I searched several threads on this forum and quickly disqualified most other brands.

This will give you some idea if by chance you have the size tire that came off.

https://www.discounttiredirect.com/learn/tire-size-calculator

Here's another Tire Size Calculator.  Enter the new and old tire sizes and it will tell you the speedometer error caused by the difference in tire diameters.

https://www.tacomaworld.com/tirecalc

It also tells you the difference in tire widths, which can be a factor with dually tires.  You don't want the tires to be too wide for the rim spacing or the inner sidewalls will rub against each other.

Most importantly, are they the same ply rating and the same weight rating as the old tires?  Or at least what the manufacturer recommends for your vehicle and its weight?  Could be dangerous if the smaller tires are not capable of carrying the same weight at the same speed, etc.

thelazyl said:

Hi All, based on feedback on this forum I purchased 6 new tires on my 2003 Fleetwood 37U.  the old ones were 8 years old.  I purchased Toyo's for $2,600 total.  They are 2 inches shorter than the old Michellin's and I went with the recommendation of my tire store. 

Can someone help me understand how significant a 2" reduction in height might impact things?  I went on a trip last week and didn't notice a difference.  I assume RPM's would be higher at a given speed but I wasn't tracking that very closely to notice a difference.

Thanks in advance - and thanks again for all the advice this newbie has received.  It has made a significant difference for us.

Click to expand...

Without knowing the specific tire size of the old tires and size of the new tires we really can't make a determination if there is a problem or not.  This would be a number such as 265/80R22.5 LR-G.  If you don't see the "LR-G" look all around the tire sidewall for something like "Load Range G" or other letter such as "F" or "H".

There should be a placard on the wall of the RV next to the drivers seat which shows the tire size the chassis manufacturer required for this vehicle.  Compare that tire size to your new tires.  If you can't find the placard by the drivers seat, then look in all the cabinets for a placard.  Or look in the owners manual for the tire size.

NOTE: If the place you bought the tires installed a smaller tire size than what is on the placard then you have tires the are not rated to carry the load of your RV and are at risk of having a blowout.  If the tires are a smaller size that what is required then take the RV back and demand they put the correct tires on the RV.  Any cost to correct the problem should be covered by the tire shop.

Is this a used RV that you bought with the old tires already on the RV?  If so the previous owner may have put on heavier (bigger) tires than the original equipment and now you are back to the original equipment sized tires.

The only way for one tire to be 2 inches shorter than another tire is for the two tires to be different sizes. 

It doesn't who makes a tire sized such as 255/80R22.5.  They will all be very close in size.  You might see a 1/4" or 3/8" difference, but you won't see 2 inches.

To clear up some of the smoke here. First a two inch difference in height (diameter) for 22.5 inch tires will probably result in an error of around 6% in the speedometer reading with a higher reading for smaller tires. e.g. If your speedometer says 66 mph you are actually going 60 mph. Note that this also means your engine speed, in drive, is 6% higher and the torque under acceleration is increased by the same amount.

Secondly the load rating is not necessarily too low. A smaller tire may have the same load rating as a larger one, however it is likely to have a lower speed rating and may well require higher pressure at a given load. Since wheels are designed for a given maximum operating pressure you could have a problem there. Alternately, the shorter tire may be wider and thus have the same contact area at a given pressure. The downside here is that it may be too wide to be used as a dual on your vehicle (tires could contact each other).

Given the above facts, an inch or more difference in height may be acceptable from a safety or performance standpoint. That is, within reason load range G is equivalent to load range G regardless of tire height. Clearances between tires, pressure ratings of wheels and similar aspects may well also be acceptable between applications.

Ernie

Why does this happen? Part of the reason in that Michellin sidewalls for a given tread width are taller than most other manufacturers . They use 80% of tread width for sidewall height while most manufacturers use 75%. That means that the tire overall height for a Toyo equivalent will be about 10% of the tread width shorter than the Michellin's all else (tread thickness and pressure for a given load) being equal. This could be a difference on the order of a half inch in height. Tread thickness could easily account for another 1/4 inch or more.

Thanks for that "smokeless" explanation, Ernie.

The only thing we know for sure is that a 2" lower tire height means the size is different in one or more parameters and that the speedometer and odometer are affected.  Anything else is conjecture.

Hopefully thelazyl will soon respond with the actual size data.

One other thought is that you also lose a little ground clearance. May not be a big deal, depending on how much there is to start with.  The other thought is that if the load range of the tire is the same as the old one, the tire side walls may be a little stiffer to handle the same weight. Could affect the ride a  bit.

When I changed my rears to Michelin XZAs, I gained 4 percent in rolling diameter.. When traveling at an indicated 64 I am actually doing 66.5..  I LOVE MY MICHES!!, smooth, quiet, and at 47K miles show no wear, no fractures or any abnormalities.  In qualification: My coach is always in a heated garage when not in use.>>>Dan

Gary,
I apologize for not getting back sooner. 

The new tires are 255/70R  22.5.  I didn't keep the size of the old tires.

Thanks for your help on this.

Mark (LazyL)

gas milage will go down.

Did anyone at the tire shop happen to look at the chart on your coach for correct tire size?  Like someone said, the chart should be near the drivers seat.  Something doesn't sound right.

thelazyl said:

Gary,
I apologize for not getting back sooner. 

The new tires are 255/70R  22.5.  I didn't keep the size of the old tires.

Thanks for your help on this.

Mark (LazyL)

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What RV model did you have, couldn't one just find out what tires is supposed to be on it?