going to larger tires

Motorhome05 · 05-04-2008, 05:44 AM

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 · 05-04-2008, 05:44 AM

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 · 05-04-2008, 06:41 AM

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 · 05-04-2008, 08:55 AM

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

-Tom

historyljc · 05-04-2008, 10:37 AM

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 · 05-04-2008, 12:33 PM

<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 · 05-04-2008, 01:49 PM

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

A large tire size usually results in an increased footprint.

-Tom

Brigadoon · 05-04-2008, 03:42 PM

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 · 05-04-2008, 10:25 PM

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 · 05-05-2008, 11:13 AM

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 · 05-05-2008, 05:35 PM

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 · 05-06-2008, 04:45 AM

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 · 05-06-2008, 05:43 AM

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 · 05-06-2008, 07:54 AM

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.

Tom N · 05-06-2008, 12:50 PM

When I stated you may lose fuel mileage I did not realize you were going to a narrower series tire.

If you went from a 70 series to a large size 70 series you'd have a larger footprint.

-Tom

MrTransistor · 05-06-2008, 01:26 PM

Hi dreamer,

I don't know if VMSpc allows you to change the settings in the engine or not. There are quite a few parameters that can be changed for the better (IMHO).

There are others here that have the VMSpc (John Canfield comes to mind) and may be able to advise you. I have helped others to reset settings on their rigs in the past. If you will be passing through Albuquerque any time soon, I'd be glad to show you these settings and how they change the operation of the engine and transmission.

You can also have an authorized CAT dealer download the latest and greatest software to the computer and make any changes that you want to the functions.

This link explains most if not all the nifty things the engine computer can do and what you may want changed like ˜Latch' mode for the engine brake function. (leave Engine brake switch on and Engine Brake doesn't come on when you let off the gas but when you hit the brake for ½ second or more then stays on till you hit the gas again)

http://ohe.cat.com/cda/files/363929/7/Progrramming%20Pa...05%20LEXT0023-01.pdf

Here are other threads with info on the Pocket Tech (adapter and software for Palm that is no longer available from CAT), tires, and MPG.

http://irv2.infopop.cc/eve/forums/a/tpc/f/2486094452/m/...291090212#7291090212
http://irv2.infopop.cc/eve/forums/a/tpc/f/2486094452/m/...881016831#7881016831
http://irv2.infopop.cc/eve/forums/a/tpc/f/2486094452/m/...291036891#8291036891

MrTransistor · 05-06-2008, 02:01 PM

Hi Tom,

I agree. If the revolutions per mile remains the same by going to a larger section, lower profile tire at the corrected inflation pressure for the wider footprint then the MPG should be slightly lower. i.e. a 235/80R22.5 to a 255/70R22.5. On the other hand if a larger tire with the same height ratio is substituted, i.e. a 255/80R22.5 for a 235/80R22.5 then the increased diameter at the correct tire pressure should yield a reduced engine RPM and will offset the increased tread contact drag for a net increase in MPG.

The drag caused by sidewall flexing should remain the same if both tires are inflated to the correct pressure for that tire and location.

Another factor is the design of the tire itself. Michelin makes specific tires for specific application. There is the XRV, which has a very soft ride. The XZE, has a very deep tread and harder rubber for long life and better handling. And the XZA3 for reduced tire drag and improved fuel economy.

I choose the XZE 255/80R22.5 for the Journey when we had it and I think they returned the price of the tires when we traded up to the Horizon. The Horizon came with XZE 275/80R22.5 and after installing the ˜Super Steer Motion Control Units' this coach handles like a Jag and rides like a Rolls. And at just over 9 MPG average I figure we may be a little limited in our journey's but we can still afford a few hundred miles a month in our retirement.

Fakrwee · 05-06-2008, 03:26 PM

I do a lot of 4X4 offroading and we're constantly installing taller tires. Usually if we go to taller tires we install lower gears as to not lose the power band of the motor.

I'm attaching a link to an article that even though it's for 4X4 it applies to all vehicles. Hope it helps.

This is an excerpt in case I can't link it.

Whenever larger-than-stock tires are installed on a truck, it will have a direct effect on the truck's performance. Why? It has to do with the effective gear ratio. Your truck comes from the factory with the optimum axle gear ratio to work with the truck's engine, transmission, and stock tire combination as well as provide a good balance between acceleration and fuel economy. When taller tires are installed on a truck but the axle ratios stay the same; the effective gear ratio is reduced. This means the engine is forced to operate below its power band, and performance and fuel economy suffer as a result. In order to restore the effective gear ratio (and the truck's performance), you'll need to have the axle gears swapped to lower (numerically higher) ratios.

Tires vs gears Calculation

Brigadoon · 05-06-2008, 06:30 PM

It is probably that effective gear ratio change, caused by the larger diameter tire, that increased my miles per gallon.

The coach came equipped with 275/70's, and 4.88 gears. This had my Cummins ISL turning 1800 rpms at 62 mph. When I switched to the 275/80's, which are about 2 inches in diameter larger than the 70's, the effective gearing was altered when compaired to the original tires. Therefore, at 62 mph my engine now turns 1700 rpm's.

Cummins has power/fuel economy charts showing the "sweet" spot for fuel economy. The low end of the rpm scale escapes me as I write this, but the upper limit was 1700 rpms. So in my case the larger tires dropped my 62 mph cruising speed rpms to the top of the sweet spot for fuel economy.

Now if I slow down a little more, maybe I can raise my 8.25 mpg average.