Help deciding between 18" or 20" wheels?

dvdprL · October 13, 2016

Curious to know the pros and cons of wheel size.

I have the factory 265/65/20 tires and wheels. I'm looking to go aftermarket but don't know a ton about the finer details.

If I go down to 18" wheels from stock 20", but I go up in tire size by 1", (32.6 to 33.5 or somewhere in the 33 range) --- would I lose any ride height?

If I understand offset correctly, the more negative the number, the further the wheel sticks out? I like the wheel and tire to sit flush as possible with the body. Should I be at 0 offset, or closer to the factory offset?

If I go down to 18's, will I have to have the car programmed to know it's running a different wheel/tire configuration for speedo and odometer?

Any noticeable difference between 20 and 18" wheels for gas mileage?

Sorry for all the questions!

newdude · October 13, 2016

The tires...

Stock 20"s are 265/60/20. Stock 20" to a stock 18" for example, with stock tire size (265/70/18 for the 18" wheels) there is zero difference in tire overall diameter. Both clock in at around 32.6". Going bigger you won't lose any ride height. Using the stock sizes, you won't lose any ride height. Stock sizes for the 18" and 20", no programming. Going bigger, you will have to have the speedo, etc. recalibrated via an aftermarket source.

The wheels...

The stock 20"s you have now, are 20x8.5, with a 44mm offset (positive offset). Stock 18" are 18x8 with a 44mm offset. The closer to that number you are (44mm), the closer to the body you will be. A 0 offset would stick the tires/wheels past the body.

Mileage...

Probably a wash if you stay stock sizes for tires. The 20" wheels weight more than the 18", but the 18" tires might be a tad more than the 20" tires. Bigger diameter or more aggressive tread for either size? Mileage will go down.

Debra in Cali · October 13, 2016

My son and I both have 2500s. His is a 2012, mine is a 2016. When we were working out our deal with the dealer, my husband wanted 18s instead of 20s due to the smoother ride of the 18s. My son gave him a lecture about the 20s being superior to the 18s in its ability to handle the heavier towing weight as well as the way the brakes are set up for the 20s vs the 18s. That pushed my husband back to the 20s.

While the ride is bouncier after we pick it up from the dealer, we lowered the back tire pressure down to 65 and the fronts down to 55.(Thank you, GM forum for that information.) The dealer of course airs them way up to the max when we took it in, but once we lowered it to a the lesser level the ride was greatly improved. I will be towing a small trailer this weekend, so we might put a bit more air in them for the towing trip, but not even close to what the dealer puts in. Now, when we get our much bigger and heavier 5th wheel and start towing it, then I will be airing up the tires to the max level.

newdude · October 13, 2016

My son and I both have 2500s. His is a 2012, mine is a 2016. When we were working out our deal with the dealer, my husband wanted 18s instead of 20s due to the smoother ride of the 18s. My son gave him a lecture about the 20s being superior to the 18s in its ability to handle the heavier towing weight as well as the way the brakes are set up for the 20s vs the 18s. That pushed my husband back to the 20s.

While the ride is bouncier after we pick it up from the dealer, we lowered the back tire pressure down to 65 and the fronts down to 55.(Thank you, GM forum for that information.) The dealer of course airs them way up to the max when we took it in, but once we lowered it to a the lesser level the ride was greatly improved. I will be towing a small trailer this weekend, so we might put a bit more air in them for the towing trip, but not even close to what the dealer puts in. Now, when we get our much bigger and heavier 5th wheel and start towing it, then I will be airing up the tires to the max level.

Interestingly enough, its the trucks with the 18" wheels that have the highest payload capacities on GM HD trucks. The 20" tires are of a lower load index than the 18" tires they use as well. This is also why the 1 ton trucks only are available with the 18" wheel combinations.

Mark Mortenson · October 14, 2016

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Imcrazy · October 14, 2016

I prefer 18's, 20's leave too little sidewall for my tastes on a truck.

Dirtbiker · October 14, 2016

My 18 x 9 rims with + 20 mm offset , 275 70 R18 tires put the edge of the trend lined up with the edge of the fenders. Any further out and they would throw a lot more mud on the side of the truck. If you go to zero offset you will have about 3/4 of an inch of tire outside the fenders or more if you go with a wider tire.

carkhz316 · October 14, 2016

Interestingly enough, its the trucks with the 18" wheels that have the highest payload capacities on GM HD trucks. The 20" tires are of a lower load index than the 18" tires they use as well. This is also why the 1 ton trucks only are available with the 18" wheel combinations.

What? Since when? I've read my owners manual front to back (yes that's right) and their is no distinction between wheel sizes that differentiates the trucks GVWR. Now, different sized wheels may allow use of certain tire sizes that are rated for more load carrying capacity of the tire itself....sure; BUT, the truck itself stays the same.

newdude · October 14, 2016

What? Since when? I've read my owners manual front to back (yes that's right) and their is no distinction between wheel sizes that differentiates the trucks GVWR. Now, different sized wheels may allow use of certain tire sizes that are rated for more load carrying capacity of the tire itself....sure; BUT, the truck itself stays the same.

GVWR is not different, but the payloads are. Go check the tire placards on the driver's door jamb on some of these trucks. The HD trucks with the 17" and 20"s have 100-300lbs less payload than the same truck with 18" wheels. The factory 18" tires are 124 load index, and the 20" are 121, the same as the 17" tires.

I guess what I am trying to word it as is...the highest published max payload GM has for the order guide, for a Double cab gas 4x4 is 3244lbs. That is going to be a WT trim, with like, zero other options on it. Now, the LT 4x4 and the LT Z71 I just checked at work, are lower than that. The LT 4x4 with the 18" aluminum wheels was 2871lbs payload. The LT Z71 with the 20" wheels came in at 2749lbs payload.

Trucks with the 17" and 20" wheels/tires, due to their lower load indexing will have lower payloads than the same truck equipped with the 18" wheel/tire combinations. Trucks with the payloads closest to base payload GM has spec'd have 18" wheels/tires on them.

carkhz316 · October 15, 2016

Ooooh. Ya, you're right. Now that I looked. It's odd that the owners manual is kind of all encompassing, despite the door placard. I wonder why 18's are the nominal payload capacity.