going to larger tires

[Motorhome05]

We have a Vectra 36 gd. We will be purchasing tires for the rig next year. We are thinking of putting a larger tire on so we can handle more weight. Has anyone increased the tire size? If so, to what size, and what other things needed to be taken care of?

[Motorhome05]

We have a Vectra 36 gd. We will be purchasing tires for the rig next year. We are thinking of putting a larger tire on so we can handle more weight. Has anyone increased the tire size? If so, to what size, and what other things needed to be taken care of?

[RVJeeper]

From what I've read, going to larger tires won't necessarily increase your weight carrying capacity. It's also the suspension, frame and all the other parts that carry the weight that more limit the capacities. Most folks that do go to larger tires do so because of a bit better ride quality and that's about all they gain. Additionally, larger tires will change the speedometer reading and consequently transmission shift points and a whole bunch of other computer controlled stuff. Bottom line: Generally not worth the expense and trouble.

[Tom N]

Larger tires will decrease your fuel mileage, if that is of any interest to you.

-Tom

[historyljc]

Your present size tires should handle all the weight your axles can carry. If they don't, you might want to go to a higher load range in the same size tire. One of the things you must verify on larger tires is the spacing between the duals. The flexing of the sidewalls on the larger tires may cause them to rub together. You do not want that.

[ricere]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tom N:
Larger tires will decrease your fuel mileage, if that is of any interest to you.

-Tom </div></BLOCKQUOTE>

Please explain. I would figure in some cases it would increase fuel mileage instead of decreasing, to a point.

Example: 1 have 4.88 gears and 2.75/70 x22.5 tires. I plan on replacing them with 2.75/80 x22.5 tires for a softer ride and better mileage due to larger circumference tires. I know people who have done that and they improved their mileage up to 1/2 mpg.

[Tom N]

If you increase the tire footprint you'll increase rolling resistance.

A large tire size usually results in an increased footprint.

-Tom

[Brigadoon]

I did change my tire size from the 275/70's to the 275/80's and had an engine rpm drop of 100 rpms at 62 miles per hour. In my case that was close to a half mile per gallon gain in mileage.

[MrTransistor]

I agree with Terry, I changed from 235/80R22.5 XRV Michelins at 556 revolutions per mile to 255/80R22.5 XZE Michelins at 538 revolutions per mile when we were in the 2003 Journey. Much better handling, decreased tread wear, and a slightly increased MPG as a result. (A little stiffer ride)

In theory, the contact patch with the road should be the same if the RVs weight doesn't change and the PSI pressure of the tire is the same. Hence a larger tire will have less sidewall flexing and lower engine RPM, for lower overall drag resulting in increased MPG. Recalibrating the CAT engine computer is a snap.

[NO1RAINH20]

before chaning tire size on your coach I would reccomend you look at body cleance with the suspension dumped it is very close on the Vectra,and might require some wheel lip mods.

[Baby Bear]

I feel the most important item has been overlooked. Check to see if the existing rims will take a larger tire and will there be adaquet clearance between the rear wheels.
I went up one size several years ago and got better handeling,ride and MILEAGE.( you see the diameter is larger, hence fewer revolutions per mile)

[dreamer]

MrTransistor,

Could you explain about recalibrating the CAT engine computer for the larger tires? Is this something an owner can do or must I take it in to a CAT dealer? Thanks.

dreamer.

[MrTransistor]

Hi dreamer,

The CAT engine (and I would figure the Cummins as well) runs software that is accessible via a Palm Pilot running a program called PocketTech. It connects to the engine port under the dash or at the engine. I have both cables, one for the older CATs and one for the newer C7s. There is a calibration value that can be set for, basically, revolutions per mile in the engine computer. It's computed a little differently between the two engines and accounts for tires, differential ratio, and transmission output pulses per revolution of the output shaft. Makes it easy to set very accurately.

[dreamer]

MrTransistor,

Thanks for the reply. I have a 2000 Endeavor with a 3126 CAT. I also have a VMSpc which connects to the diagnostic (engine/transmission) port to my laptop computer. I can monitor alot of parameters but can I reset the callibration for the larger tires with this or do I have only a monitor for engines/transmission functions? Thanks.

dreamer.