Tire size and gear ratio

[Agaveguy]

Not sure where to post this so I put it here in the class A forum.

I have a 40' DP with 275/70/22.5 tires. I want to use 275/80/22.5 tires for the better ride.

For those who have made such a switch, did you notice a big difference in drive axle ratio change? The taller tires will drop the cruise RPM a bit but I am wondering if it will hurt performance much?

Do any of the 40' DP owners know what their drive axle ratio is? What tire size are you using?

[LJowdy]

Your new tire size will be 2.1 inches taller, Speedo error is between 1 to 2 MPH difference. IN other words, if the Speedo says 60 MPH you're actually speed is 63.3 MPH. You'll see a small drop in performance but probably not much.
The width of the tires is the same but you may want to investigate your wheel well openings and see if a taller tire will cause any "up/down clearance issues. Although the tire is 2.1 inches taller, you're only dealing with half that amount on the top of tire where it rides withing the wheel well.

You can compare tire sizes here: https://tiresize.com/calculator/

As far as ring and pinion ratio's, do some Google searches and you should be able to find the information you need

[Dsallach]

On a 275 tire, changing from a 70 to 80 will raise the sidewall height 1.08". My first concern would be if your coach has the clearances to the frame and fenders to accommodate that change without potential issues. If my math is correct, that would increase the circumference of the wheel by a factor of roughly 1.06 which will affect the accuracy of you speedometer, etc. I do not know what transmission, engine drive axel you have so I cannot comment on the performance.

[Fiesta48]

Larger radius tires will help correct most speedometers over statement of speed. 1-3 mpg.
For me I've already did minor damage to a front wheel well fender when the air was out of the air bags. No to larger tires.

[Agaveguy]

I have not specifically looked at the clearance above the tire in the wheel wells with the bags deflated, but I do know that some of the other floor plans and powertrain options with my coach had the larger tires, that kind of makes me think there is adequate room, this weekend I will go out and take a look.

My engine is the 6.7ISB 360/800, my drive axle is a Detroit but I do not recall seeing any ratio information, I was just under the rig a few weeks ago too. I'll have to get underneath again and take a closer look at the ID tags on the axle.

I may give Freightliner a call and see if the taller tire is even a possible upgrade without extensive reprogramming to make it work.

[Woodman018]

Have you weighed your coach and inflated the tires accordingly. If you have 2 axles you should get 4 weights, one for each side of each axle. If your tires are over inflated they will ride rougher than need be and they will also loose road contact somewhat, they will also wear faster in the middle of the tread.

Dave

[Gary RVRoamer]

You probably won't notice a difference in performance. And you probably won't notice a difference in ride either. You can likely achieve as much (or more) by fine tuning the air pressure in your existing tire size. Get the coach weighed and then consult the tire load-inflation tables to get it to the optimum but still safe amount.

[jimmy braden]

I don't think the ride will improve. It might degrade. A taller tire has more of the sidewall (as opposed to tread) devoted to its total size. That will likely transmit more of the road irregularities directly to the cab.