Front and Rear Tires are Different Sizes

[rjproctor]

I just bought a 38-foot used 2000 Newmar Dutch Star a couple of months ago. I just checked the age on my tires and found that most of them are older than 6 years, and one is actually 15 years old. So, I am looking to buy some new tires.

I discovered that the two tires on the front axle are 275/80R22.5 and the four tires on the rear axle are 255/80R22.5. The manufacturer's plate in the motorhome indicates that the tire size is 255/80R22.5. Is it a common practice to have larger tires on the front? I can't find anywhere in the Birmingham, Alabama, area that can do individual wheel weighing, so I don't know if the front were increased because of additional weight safety.

Any ideas would be greatly appreciated. If someone knows of a place nearby that can weigh my motorhome wheel by wheel, that would be nice also.

[wolfe10]

rjproctor,

Welcome to the FMCA Forum.

No problem having different size tires on front vs back. Suspect a prior owner upgraded the front tires for more capacity or actually same capacity at lower PSI.

[BillAdams]

The tires do need to be the same diameter (or should I say, should be) so the 22.5 is the constant. The 275 tire and 255 tire are different widths with different rims size requirements but it's often possible to mount 2 different width tires on the same rim. It appears that in your case this is NOT an issue. You could either keep the current configuration or put all 255 tires on all axles. Moving to wider tires on the drive may not be a good idea as the wider tires could be too close, or even touch, which would be a bad thing.

[wolfe10]

Indeed, all tires on an axle MUST be the same (actually very close to the same rolling radius/circumference).

But, no reason the fronts must be the same as the backs.

And, as Bill said, be careful changing rear tire size, as it affects final drive ratio.

[rjproctor]

Thanks for the replies. The tire dealer suggested that I put 255's all around but I am tempted to change the front to something a little smaller rather than the larger size currently on the front. I currently have to use ramps to get into and out of my drive because the rear end drags on the concrete severely. Since the tires on the front are slightly taller than the one on the back it contributes to the rear end dragging. Maybe if I move to a slightly shorter tire on the front compared to the rear I can get closer to getting into and out of the drive with shorter or possibly no ramps at all. Thanks again.

[wildebill308]

As Brett said all tires on an axle should be the same size. This is especially important on the rear duals, even the air pressure is important on the rear because if one has more air it is carrying more load and dragging the other tier because the rolling diameter is different.

Having each corner weight is the best practice but having each axle will tell you what you need for inflation or if you are overweight on that axle.

A 15 year old tire is really unsafe that is more than twice the recommended age to replace. What position is it?

Bill

[rjproctor]

The really old tire is on the inside passenger rear.

[huffypuff]

One reason you don't want to mix different size tires front to rear is if the coach has ABS, traction control and/or stability control. If it kicks on in an emergency you want to be sure that it does what it's expected to do. ABS saved me from a crash once during my Christmas holiday trip when a woman changed lanes and stopped in front of my coach.

[tireman9]

Not sure if I would be concerned with different size front & rear. I agree with Brett in post #2.

RE ABS I would contact Newmar and confirm if the unit has ABS. If it has ABS, is there a computer connection between the Front & Rear tires/brakes?

RE rear end dragging. The radius difference is only 0.8". If you know the wheelbase and the distance from the rear axle to the rear bumper we can calculate the increase in clearance to the road at the rear.

[JohnJill]

I just replace mine on Saturday had the 255/80.22.5 on had a hard time deciding if I was going to go back to Michelin due to the price and they expire before they wear out. Graham International in Sherman Texas helped me find the right size tire example same width, ply, weight capabilities and found the 265/75/22.5 TOYO. We put about 300 miles on them that weekend and could not tell a difference in ride. plus they were $ 2,700.00 all 6 mounted and computer balanced.

John

[wayne77590]

Thanks for the replies. The tire dealer suggested that I put 255's all around but I am tempted to change the front to something a little smaller rather than the larger size currently on the front. I currently have to use ramps to get into and out of my drive because the rear end drags on the concrete severely. Since the tires on the front are slightly taller than the one on the back it contributes to the rear end dragging. Maybe if I move to a slightly shorter tire on the front compared to the rear I can get closer to getting into and out of the drive with shorter or possibly no ramps at all. Thanks again.

RJ

I'd be careful regarding smaller tires on the front. the Mfg indicated 255/80R22.5 and I suspect that is the smallest tire for the load they anticipated. Putting a smaller tire on could be overloading the tire. Since there is no four corner weighing available, just get the front axle weight. At least with that you will have the total weight of the axle, still not knowing if one side is heavier than the other. However, with the entire weight you will have a better working knowledge on tire size. If a smaller tire is under the 1/2 axle weight it would not be a wise move. Stick with the 255 and inflate it to the max 110 PSI until you can weigh the corners.

[rjproctor]

I was thinking of changing the front from 275/80R22.5 to 275/70r22.5 which is slightly shorter than the 255/80R22.5 which is on the back but has a much better load rating. Changing the front from the 89 to the 70 trims a little more than an inch off the diameter. The 255/80R22.5 which is the recommended tire has a max load of 5,205 lbs max. The front axle portion of the GVWR is 10,400, or 5,200 lbs per wheel. I just don't feel comfortable with the weight being that close to the max for the tire since the weight distribution per wheel is not the same. Of course maybe this will be moot if when I have my axles weighed this Saturday I may not be close to the max. Perhaps I should have weighed my MH before I started posting.

[wayne77590]

No problem in trying to find answers before buying or doing things. None at all.