Can you not just reprogram it on the screen like you can in a Tesla? Maybe with a scan tool?

 

I don't see why size downgrades would be an issue if the tire weight and speed ratings were sufficient. Smaller diameter does mean an increased motor RPM at any given speed. However, the truck has, I think, a speed limiter to keep from exceeding the speed rating of the tires. Since it would be based on RPM, all a smaller dia. tire would do is cause that limiter to activate at a lower speed.

Not authorizing smaller sizes is pretty dumb since people are going to be putting snow tires on which generally aren't available in the 35" diameter of the RST wheel/tire combo.

Anyone else have thoughts? Time to write Mary?

 

 

I'd be wary that dealerships would void your warranty - if it related to wheel/tire changes insisted on.

Costco Tires here said corporate Costco won't allow wheels/tires that the vehicle manufacturer doesn't support. Example - had an Escalade ESV w/22's that I wanted to reduce to 18's - and Costco said they wouldn't do it.

 

eh....my vehicle, my choice. I changed out my 21" rims/tires on my tesla as soon as I got home. Put 18" wheels and tires on it and it might be 1mph off, but close enough for me.

 

I’d be okay with 1 mph but I’m 5-6 mph.

 

See if you can convince your dealer to call Techline and see if they can get a VCI anyway? I need to re-reach out to mine on the same concern, forgot to bring it up when I was at service because my normal advisor wasn't there that day.

 

I can try. I don’t see what the big deal is with downgrading to 18’s. There should be a warning or something about not doing it.

 

Having the dealer swap out 18s for the 24s on the RST seems the easy solution but has the issue of diameter difference/speedo inaccuracy. A couple of forum members have expressed interest in buying your 24s and one offered $3k for the wheels and tires. Since you can pick up a good new set of 18s with tires on TireRack for $2500 or so. I would rather go that route.

285/75-18 and 295/70-18 tire sizes have weight ratings of 4080 lbs which is greater than the 24" Michelin Primacies on the RST (3195 lbs). The 285/75-18 is a 35" tire while the 295/70-18 is 34.3 in diameter, so the speedo issue is non existent or minimal. They are load range E and a higher service description rating than the stock 24" tire. The one disadvantage is that those tires are A/Ts rather than highway tires so perhaps not as efficient, quiet or good handling.

I see no reason those tires should be prohibited as they are actually an upgrade and most likely one of those will be on the Trail Boss.

 

After further discussions, we have to go back to the 24" wheels and tires. The reason is the air suspension and 4 wheel steering. The LT & WT don't have either and therefore, 18's and 22's would be okay. Just not in the RST.

 

Still doesn't make sense. Your issue was the speedo calibration. Just put a set of 285/75-18 tires on the 18" wheels and it will be fine - same diameter as the RST 24s.

 

Yep, mine work great.

But now I need break out the old Garmin to check my speedo vs actual (thanks to this thread)

 

You can use your I phone to check the speed as well. Any phone for that matter. It’s an app and is right on the money.

 

Me dealership denied me in putting on my 18 inch wheels shared a bulletin they got from GM and refused. I asked if they would put the tires on the wheels and I could mount myself and they denied me that as well. Oh well, I get the desire to deny liability, but I wasn't expecting to rejected. They made the claim that all systems, ABS, TCS, AEB in the event of a near crash, etc. would all be affected. I don't think the weight or engineering between all of the models is that different, but I could be wrong I guess. And I think for me, I get that I purchased aftermarket wheels and they can't confirm my purchase meets the GM standards, but of the OP here, and GM wheels, it shouldn't be an issue. They are overly cautious and afraid of litigation, but they've been like this for some time now.

 

They made the claim that all systems, ABS, TCS, AEB in the event of a near crash, etc. would all be affected.

The sensors for those measure wheel speed. As long as your tire diameter is close, I don't see why they would be affected. GM should have a simple way to input tire diameter into the UI and have those systems adjust accordingly.

 

I wonder if I could upgrade the 18 inch wheels to match size of rst which would be a diameter of 34.9. So 18 inch wheels with bigger sidewall and recalibrate to upgrade wheel size from WT to rst. Wonder if the dealer would do that for me

 

You do not need to upgrade the 18" wheels if that is what you have, just change the tires to 35" and have the speedo recalibrated. you should be aware though that all the 35" tires are all-terrains of some form.

 

I was running 33's on 18" snow tires in the winter time on my Denali and had no issues besides it accumulating miles faster than it should and the speedometer being about 6-7% fast