tire size question

[challengerkid09]

Hello all, I am new to the forum but i have a question i need answered. My in-laws recently purchased a 2012 qx56 loaded to the gills, however my mother in law is a little handicapped and had trouble getting in and out of the vehicle. After a few trips back to the dealer they decided to lower the running boards and they also decided to remove the 22" tires that came stock from the factory and replace them with 20's. Sounds brilliant right? well it was until they took a recent drive to florida. My father inlaw being in his 70's likes to use the cruise control on his car to give his legs a little rest when making such a long drive, however now the cruise control wasnt working. they called the dealer and he told them it was probably due to the smaller tires and then informed them that their odometer reading was probably wrong as well. They brought the car to a dealer in fla and he said the cruise control issue was definitely due to the smaller tires but informed them there was nothing they could do and recommended they switch back to the 22's. My question is..... isnt there a way to reprogram the computer and tell it what size the tires are so the odometer will read out properly? or is this something that is set at the factory and cant be reset?

Thank you in advance for any light you can shed on our problem.
JIM

 

[goblue]

yes, the vehicle should be able to be reprogrammed, as I understand it (haven't done it). there are a few people on this forum that had the dealer switch wheels prior to delivery and haven't noted any problems.

Also worth noting, the difference between the tire sizes is so minimal it's ridiculous. The 20" are actually BIGGER by 2-tenths of an inch (making the vehicle 1-tenth of an inch taller). With the 20 inch tires, the speedometer is off by 0.5% When your speedometer says 60mph, you are actually going 60.3mph. (bigger tire makes your speedometer read slower than you are going).

I cannot imagine that this would make the cruise not work. Something else is going on.

Hopefully the dealership did not screw your in-laws over by charging them extra to move them into BIGGER tires. They should have gotten a substantial discount as the 20s are cheaper than the 22s. One consolation is that the 20" stock tires are much better than the POS 22" tires for the vehicle.

I run 18" winter tires rather than the 22" stock tires. My winter tires are SMALLER by 2-tenths of an inch. I did not re-calibrate the speedometer and I have no problems with the cruise.

welcome to the forum.

 

[ilikebeer]

Sorry, but this sounds like a scam to me. Changing the wheel size doesn't change the ride height, changing the tire size does. There are so many factors that would come into play reducing tire diameter that I can't imagine any dealer actually assisting in that endeavor because of potential liability.

What is the new tire size?

 

[manwagon]

Run, don't walk to another dealer. This dealer is either ignorant or worse when it comes to tire knowledge. A quick visit to a tire calculator website such as:

http://www.net-comber.com/tirecalc.html allows the user to input the stock tire specs and also the proposed optional tire specs. When I put in the stock tire size: 275/60R20 and compared that to the optional 275/50R22 the results were interesting.

The overall diameter of the stock tires was 32.992" versus 32.827" for the 22's.
Wheel Circumference for stock was 103.648" versus 103.128" for the 22's.
Wheel revolutions per mile are 611.301 versus 614.38 for the 22's.
Wheel revolutions per second @ 65mph: 11.037 versus 11.093 for the 22's.

The optional 22's will cause the speedo to read 64.674mph when going 65mph

Finally, the ground clearance of QX will actually be .083" LESS with the 22" wheels compared to the 20's!

All of these dimensions/specs are so comparatively close that they are essentially the same and would have no practical affect on speedo reading or the operation of the cruise control.

It is time to have a conversation with the dealer.