Tire Size Increase

[GBLandau]

Hi folks,

It is time to buy new tires and I am considering increasing the tire size from LT245/75R16 to LT265/75R16.

We have a 2015 Roadtrek 190 Popular 4x4 with a Quigley 3" lift. We prefer out of the way camping spots and don't mind traveling on gravel roads/unimproved roads to get there.

I am considering increasing the tire size to LT265/75R16 in order to gain some additional safety margin based on our usage and the fact that these vans run reasonably heavy. I realize that wheel size and offsets are also critical but I would like to discuss those in another thread. It looks like the LT265s are approximately 1.5" larger in diameter.

I have read that some of you have already made this change. I am interested in your experiences with the tire size increase as well as any impacts (both positive and negative). I expect that the speedometer and odometer are effected but I don't know by how much.

I appreciate sharing your experiences.

Thanks, Mike

[booster]

Originally Posted by GBLandau

Hi folks,

It is time to buy new tires and I am considering increasing the tire size from LT245/75R16 to LT265/75R16.

We have a 2015 Roadtrek 190 Popular 4x4 with a Quigley 3" lift. We prefer out of the way camping spots and don't mind traveling on gravel roads/unimproved roads to get there.

I am considering increasing the tire size to LT265/75R16 in order to gain some additional safety margin based on our usage and the fact that these vans run reasonably heavy. I realize that wheel size and offsets are also critical but I would like to discuss those in another thread. It looks like the LT265s are approximately 1.5" larger in diameter.

I have read that some of you have already made this change. I am interested in your experiences with the tire size increase as well as any impacts (both positive and negative). I expect that the speedometer and odometer are effected but I don't know by how much.

I appreciate sharing your experiences.

Thanks, Mike

We were very early on in the switch to that size in our 07 190P rear driver somewhere around a decade ago.



No negatives except being a bit heavier, partly because of the tire and partly because of the steel wheels needed to get a wheel wide enough and at the right offset.


We would never go back to stock size as the peace of mind alone is worth it and they also handle better, IMO.


It is really hard to separate the offset from the size increase though because if you increase the tire diameter and it gets combined with aluminum wheels Roadtrek or other non correct offset wheels used, you can get a fender lip rub at the front or rear of the front wheelwells. The offset error is about 1 3/8" per side and that makes the wheel move for and aft more when turned, creating a potential rub.

[Deano]

In addition to Booster's post.
The speedometer will read 2 mph less than actual.
A minimum wheel width of 7" is required. The correct offset is important.

[N147JK]

You will most likely see a drop in fuel economy (MPG).

[Deano]

I saw an increase in MPG. Mine has a 4.10 gear ratio, your result may be different.

[booster]

We found the tire size didn't make a difference in mileage that I could find in our tests. Any variances we saw were were within the error of the Scanguage settings resolution limits in the areas where you can correct the fuel use to actual. Our longest fuel mileage test was 5000 miles continuous collection of fills and miles before calculating so should be pretty accurate, though as that was tank fill vs corrected miles all added together to determine the correct setting for the Scangauge.

[GBLandau]

Thanks for the responses. Mike

[booster]

Originally Posted by Deano

In addition to Booster's post.
The speedometer will read 2 mph less than actual.
A minimum wheel width of 7" is required. The correct offset is important.

Correct on the speedo, and the Chevy garage cannot reprogram that tire size as it is not in their optional tire list. You can get it done at a speed tuning shop or sport truck tuning place and would probably cost about $100. I have a tuner for our van so I was able to do it myself. The HPtuner I have required two different settings to be changed in the correct amounts to do the change. Make sure you do some careful mileage checks over long distances before you get it done so you can tell them exactly what % to change the settings. I use the a 100 mile drive on the freeway and log sign vs odometer vs Scangauge when I do at because even the markers are not perfect and you need to average a bit. I tried using the GPS and it's readings were not nearly as consistent as the milemarkers are.


AFAIK, there are only two wheels available now that are wide enough and the correct offset for the Chevies. hey also have to have adequate load capacity. One is a late 2000s Silverado pickup steel wheel at 16X7 size (6 cutouts make it easy to identify). You can fit the 265-75-16 tires on that rim and gain about 350# capacity per tire.


The other wheel is a 17X7 or 8 Vision aftermarket aluminum wheel that will take 265-70-17 tire which will give about 150# extra load capacity per tire.

[GBLandau]

I understand that wheel offset is important. I am trying to understand the relationship between wheel width, offset, and backspace.

The stock Chevy Express wheel specs are 6.5" width with an offset of +28mm. This equates to a backspace of approximately 4.85".

Wouldn't it be important to maintain the factory backspace when increasing wheel width? For a 7" wheel this would equate to an offset of approximately 22mm and an 8" wheel would be approximately 9mm.

It seems that maintaining the factory backspace would preclude potential tire rubbing issues on the suspension parts in a tight turn.

I suspect that I am missing something here. I am just trying to understand the necessary parameters in my considered tire upgrade.

Thanks, Mike

[booster]

Originally Posted by GBLandau

I understand that wheel offset is important. I am trying to understand the relationship between wheel width, offset, and backspace.

The stock Chevy Express wheel specs are 6.5" width with an offset of +28mm. This equates to a backspace of approximately 4.85".

Wouldn't it be important to maintain the factory backspace when increasing wheel width? For a 7" wheel this would equate to an offset of approximately 22mm and an 8" wheel would be approximately 9mm.

It seems that maintaining the factory backspace would preclude potential tire rubbing issues on the suspension parts in a tight turn.

I suspect that I am missing something here. I am just trying to understand the necessary parameters in my considered tire upgrade.

Thanks, Mike

In a nutshell, backspace should really only be used as a comparison so you can get a rough idea off your current setup by looking to see if you have the extra space needed, IMO.


Offset counts more as it is a critical part of the suspension design because it locates the center of the tire/wheel in the desired place to get the scrub radius correct when turning.



In the case we are looking at, increasing the wheel width up to 7" increases the backspace 1/4" compared to the 6.5". Almost all Chevies are OK with that much and it is actually a bit more than that because the tire itself is what really matter and the tire width increase is more than the rim width increase. If there is a rub it probably would come on the swaybar and only at max turning so very slow speed and not really an issue in most cases. Getting a rub in a Roadtrek is more likely to be on the OD of bigger tire at the lower outside corner of the front skirting or the far inside of the rear of the front wheelwell. A lot of the OD rub is influenced by how far forward or back the wheel sitting in the wheelwell and changes with tolerances and wheel alignment settings. Having less positive offset also increases the chances of getting a rub in that area because less positive offset makes the tire and wheel move more fore and aft when turning more than stock offset does.

[GBLandau]

Thanks for the inputs Booster. Mike

[GroupB]

Back spacing is important - must be assured of no interference with the fixed parts in the wheel area.

When I replaced my wheels, I had to move the speed sensor cable to keep adequate clearance. The wheel width was 7.5". No other issues.

Here are some details:

https://www.classbforum.com/forums/f...0-a-13064.html

[algunbato]

I have a 1998 Pleasureway on Dodge chassis.
Stock tire was a 245/75-16. I went to a 235/85-16, for a ride height increase of 0.5 inches. The switch to Michelin XPS Traction was a game changer. Stiff steel carcass tires handle tight, ride smooth, devour dirt roads.

[GroupB]

Something to note when looking for wheels - Chevrolet changed the truck 8 lug wheel bolt circle diameter sometime in the early 2010's. They're either 165.1mm (6.5") or newer 180mm.

[Dave0718]

i made the same tire upgrade on my 2006 GMC Quigley and had no clearance problems. I found that my real speeds were 4% over the speedometer statements.

[phantomjock]

This thread is "Timely!"

I am just finishing up a number of upgrades, and tires are next on the list.

My 2008RT170P is in need of new shoes. The Michelin Defenders are past their life and even though we were careful on our 4 day trip from New England to FL, we factored the price of tires in our offer.

On the trip I remarked to my wife how well the van handled. In fact she note it was easier handling than her Nissan Xterra. Our plans include towing a trailer and race car and when not at the track venturing off grid and semi-remote. As such we will be looking for "On-/Off-Road All-Terrain Tires." We will also be up-sizing to 265/75R16 E-rated. These will "just" fit on the AR wheels, with all the attendant "dimensions/issues" previously noted here on the forum. BTW, we noted little effort due to scrub radius with the 245 Michelin tires, and with the 265s would expect the same, with the contact patch being in the same position, just wider.

Our considerations are (as of today):

1. Falken WildPeak A/T3W
2. BFGoodrich All-Terrain T/A KO2
3. Toyo Open Country A/T III
4. Goodyear Wrangler Workhorse AT

I have done the measurements, and yep, they'll be a bit wider - 1/2" inside and out, and raise the van about 1/2" too. Not too dramatic. May need Mud Flaps or Fender Flares...maybe a bit of under fender trim, we'll see.

Any thoughts - or experience on these particular tire selections? I note a number of members have upgraded their tires to this range along with other suspension mods. These mods will be in our future I suspect too.

Cheers - Jim

[booster]

Originally Posted by phantomjock

This thread is "Timely!"

I am just finishing up a number of upgrades, and tires are next on the list.

My 2008RT170P is in need of new shoes. The Michelin Defenders are past their life and even though we were careful on our 4 day trip from New England to FL, we factored the price of tires in our offer.

On the trip I remarked to my wife how well the van handled. In fact she note it was easier handling than her Nissan Xterra. Our plans include towing a trailer and race car and when not at the track venturing off grid and semi-remote. As such we will be looking for "On-/Off-Road All-Terrain Tires." We will also be up-sizing to 265/75R16 E-rated. These will "just" fit on the AR wheels, with all the attendant "dimensions/issues" previously noted here on the forum. BTW, we noted little effort due to scrub radius with the 245 Michelin tires, and with the 265s would expect the same, with the contact patch being in the same position, just wider.

Our considerations are (as of today):

1. Falken WildPeak A/T3W
2. BFGoodrich All-Terrain T/A KO2
3. Toyo Open Country A/T III
4. Goodyear Wrangler Workhorse AT

I have done the measurements, and yep, they'll be a bit wider - 1/2" inside and out, and raise the van about 1/2" too. Not too dramatic. May need Mud Flaps or Fender Flares...maybe a bit of under fender trim, we'll see.

Any thoughts - or experience on these particular tire selections? I note a number of members have upgraded their tires to this range along with other suspension mods. These mods will be in our future I suspect too.

Cheers - Jim

The AR wheels hang out almost 1 3/8" further than the stock wheels, so inside rubs won't happen. But because of the larger negative offset, the swing of the wheel around the balljoint is much larger so the tire moves for and aft more. That might cause a front or rear of tire tread issue depending on how well centered the wheel is in the wheelwell.


The scub scoffing is definitely there with the Chevies and AR wheels, but in normal driving it is not overly noticeable. It does, however make them push more in corners than with stock wheels.


The most blatantly visible test of the scrub sucuffing is to do a series of fairly sharp turns going backwards on a concrete slab. This will magnify any scrub you have. I found this out while backing our van in the first few times to get it into the lower garage at home, as it requires a bit of maneuvering.

When I got done it looked like we held a burnout contest in the driveway there was so much rubber laid down on the concrete. As soon as we changed to the correct offset wheels, that scuffing went away and no skid marks at all, and the van doesn't push much anymore. The noise turning on gravel, even frontwards got way lower. Lots of people are driving around with the AR wheels, but we are very glad we got rid ours as everything got better with stock offset.

[dicktill]

Originally Posted by booster

The most blatantly visible test of the scrub sucuffing is to do a series of fairly sharp turns going backwards on a concrete slab. This will magnify any scrub you have. I found this out while backing our van in the first few times to get it into the lower garage at home, as it requires a bit of maneuvering.

When I got done it looked like we held a burnout contest in the driveway there was so much rubber laid down on the concrete. As soon as we changed to the correct offset wheels, that scuffing went away and no skid marks at all, and the van doesn't push much anymore. The noise turning on gravel, even frontwards got way lower. Lots of people are driving around with the AR wheels, but we are very glad we got rid ours as everything got better with stock offset.

Perhaps PJ is better off on this since he has a 170 rather than a 190, and the wheelbase is 20" shorter (135" rather than 155"). Don't know for sure but I suspect that GM didn't change the steering geometry with the longer wheelbase/extended van. YMMV.

[phantomjock]

Dick -

I think you are spot on there. The WB is a very important factor to consider, and that may well account for the feel, and the differential comparisons between vans.

And, push is not as bad as loose. These are not vehicles we drive and handle at track speed. In fact, automakers build understeer in as it is reasonably predictable and can be handled by the majority of the driving population. And, coincidentally a (slightly) shorter WB will have less understeer, assuming similar gross weights, etc..

Guess I'll just have to make the switch and cope with the results. It is not as dramatic a change as could be, and I suspect should be just OK. Really don't want the rig to get loose on a highway exit!

Cheers - Jim

[booster]

Originally Posted by dicktill

Perhaps PJ is better off on this since he has a 170 rather than a 190, and the wheelbase is 20" shorter (135" rather than 155"). Don't know for sure but I suspect that GM didn't change the steering geometry with the longer wheelbase/extended van. YMMV.

Interesting thought but I thought that scuffing of this sort was more related to bad ackerman caused in the front by the offset change to scrub radius. I know wheelbase can influence turning radius, but never heard it being a problem with scuffing, but it also very possible it might as ackerman also does have the rear axle angle included.


I just looked and the vans all use the same pitman, idler, knuckles, and drag link.


All I know for sure is that the right offset got rid of all noticeable scuffing, even going backwards, with our extended version. It must be something like the caster helping going forward and not when going backwards, maybe.