I changed my front 20" wheels to 18" wheels Can I run 18" wheels on front and leave the 20" wheels on rear or will this damage the AWD transfer case?
I can change the rear wheels to 18" too but trying to save time and extra work
I am trying to isolate steering wheel shaking at high speeds I think it might be the front wheels/tires

 

The circumference of the wheel / tire combination has to be the same to avoid possible damage to diffs or viscous couplings. Typically variation of 10 mm or less is recommended and side to side difference is worse than end to end.

 

hmm so what if someone has to put on the 18" spare wheel

 

JaguarXJL15, Circumference is the key word here. You are taking the circumference of the rubber, not the rim. So, whether you are using a tire with an 18" rim that is only 5" wide, as long as the circumference of the tire is 28" *(factory is 27.7"), then all is good as your factory tires are going to be around 28". There are calculators out there on the internet that you can match a 315/XX/R26 to a 225/XX/R18 tire. Now, this is extremes, but with enough playing of the numbers, you can make it work. Now, the 315 tire is going to be like a 20 series tire and the 225 is going to be like 120 series tire, but it is possible. Kinda like if you look at your car, a lot of the AWD cars some with 245 tires on the front and 275's on the rear. The difference is the 245's are 245/45R19's and the rears are 275/40R19. The aspect ratio allows both tires to have a circumference of 27.7". With that, the AWD system is happy. I can tell you from personal experience that something as small as running 40 series tires all the way around on the car with 245's up front and 275's in the rear is enough to make the car do funny things at times. So, getting the circumferences the same is going to be key.

So, for example, you in theory can run 245/50R18 tires on the front of the car and then run 275/35R20's in the rear. Both have a circumference of 27.6". The car would be happy with that setup. Maybe if you give me an idea of what width tires you are looking at running, I can do some number crunching for you.

 

Tire size on the 18's 245/40R18 and the rears 275/35/R20 it will not work I will just change the rear tires to the 18's as well and not risk damage

 

JaguarXJL15, the tires you list have a difference of 7.4% in diameter. This WILL!!!! result in eventual damage to the transfer case. Not to mention your ABS and dynamic stability control are going to be constantly kicking in and making the car drive like hell. With that being said, if you run the rears at 275/35R18, then you would be good. The only "problem" in this case is your speedo would be reading off by 7.2%. So, for example. If you run the tire sizes you say with both 18" rims front and rear, when the car is saying it is doing 60 mph, you are actually only doing 56 mph. This does not hurt the car other than the motor is going to have to spin more to go a given distance. Granted, does help with launches as you will have an effective lower gear ratio, helping to get the car up to speed.

If you are set on running the front 245/40R18 tires, then if you go with a 20" rim and a 275 tire, you would need to run a 275/25R20 tire (not sure they make that size) to not have issues with the AWD system. This is where the better setup is the 245/50R18's with 275/35R20's as the speedo is spot on, the tire mismatch is essentially 0% (of by a few tenths), and the tire sizes are available. Maybe what I am missing is why you are wanting to run a smaller than normal diameter tire.

 

This is only a temporary set up I am trying to isolate steering wheel shaking at high speeds I think it might be the front wheels/tires


This is how it looks right now 20's on rear and 18's on front

 

I think if you drive the car like that, you are going to get a lot of jerking and in general the car not being happy. There is too much difference between the front and rear that the car is going to think wheels are slipping and it is going to be engaging the brakes, trying to slow things down.. Now, if you put the car up on say 4 jackstands and let the wheels free spin, then you should be good. Just don't go crazy with wild accelerations and knock the car off of the jackstands.

 

I installed the 18" rims all around they are a lot more comfortable and quiet The steering wheel shake is gone at high speeds only problem is my car now looks like a boat on wheels the 18's look too small I think I will get 19's