I want to change from 15 x 12 rims with 35 inch tires to something more modern with more rim and less tire. When riding on the street, they bounce like basketballs all over the place. I want to go with a 20 inch rim on my 1990 fullsize Jimmy Sierra Classic. What back spacing or offset fits my vehicle? What is the bolt pattern?

 

Bolt pattern is 6 x 5.5". Back spacing can anywhere between 3" and 5" depending on the chosen tire width. A very standard size is 15x10 rim with 4.5" bs.

What tires are you running now? I would blame the bouncing ride on the 6" lift springs before I looked at the sidewall of the 35" tire.

 

BFGs 35x12.50R15 Side wall has alot of flex. I don't like the feeling when I take corners either. I can't get a tire with good side wall rigidity in that size.

 

I see the other vehicles in your signature are very different than a K5 with 6" lift

Do you have a sway bar still hooked up with that lift?

either way the bolt pattern is what it is and the back spacing should be at that 4-4.5" if you're using a 10" wide rim.

 

The previous owner did the lift, or had it done. It appears that it was done correctly without any shortcuts. All lines we elongated, properly bracketed. The speedometer reads correctly. It does still have a front swaybar. I have never owned a truck with this much lift. I did have a 77 K5 before. I had an extra leaf in the rear and was running on 31 inch tires. It handled well for a truck. This truck is decent at higher speeds, but not so good at low speeds. I will be doing mostly street driving with it so I do not need the capability to air down my tires and have a wide footprint. I think the bigger rims/less tire combo will give me a better street drive and modernize the look of the truck a little. The big balloon tires give it a 80s look. I don't mind 80s technology, as you can see from the vehicle in my signature, but I always end up resto-modding them all(including the bike).

 

According to a tech at Discounttiredirect, The rims I'm interested in need to have a -10mm offset. This translates to 4.5 inch backspacing.

 

Yea, rim style and tire size play a big part in how these older square bodies look and feel. Keep in mind that the 6" spring is pretty stiff and the tires do their part in cushioning the ride. It's a trade off for sure.