Tire/Rim Size Selection

[Jon R]

I am ordering a new truck (GMC 3500HD SLT CC LB SRW Gas) and would appreciate any experience or opinions you can offer on selection of tires/rims from the available options.

The base tire/rim is LT275/70-18. The optional tire/rim is LT275/65-20. (The spare is LT 275/70-18 no matter what.) I remember reading that the 20 inch wheel is 1/2 inch wider. I think it's 9 inches wide and the 18 inch wheel is 8.5 inches wide. The wheels are cosmetically similar, and the option is only $500 more. Using a tire comparison calculator on line, the base 18" wheel package has an overall diameter of 33.2" and the optional package overall diameter is 34.1". That gives an undeflected sidewall of about 7.5 inches on the base wheel and 7.0 inches on the optional 20 inch wheel.

The 20 package probably looks slightly better to many, but I don't care all that much about tire size cosmetics. I want to be able to air down the tires safely on washboard roads both unloaded and with a FWC Grandby (won't be doing extreme off-road stuff), but I want a smooth ride without excessive wandering and sway on the road. I will be doing a lot of highway miles, and will occasionally tow a boat trailer. I'll be using LT load range E tires. I don't think I care about losing 1/2 inch ground clearance with the base package, and in fact the lower truck height may help with getting the truck with Grandby in my 8 foot garage door.

I was only able to test drive trucks with the 20 inch wheel package. It was not a bad ride by any measure, but I did feel the road bumps more than I would like. Of course some of that may unavoidable in a one ton truck. I'm leaning toward the base wheel package thinking it will have a noticeably better ride due to more sidewall, and the additional sidewall is good when airing down. (A bonus would be the spare is the same size.) On the other hand, I don't want to later wish I had bought the 20s for some reason I haven't thought of. Do any of you have any experience comparing such a relatively small difference in tires/rims? Will the ride difference be noticeable on the road, and is the 1/2 inch sidewall difference between 7 and 7.5 inches enough to matter when airing down? Any other opinions to offer?

Thanks

 

[Wandering Sagebrush]

I have 20s on my F350, and wish they were 18s, for the reasons you mention.

 

[Wango]

Three full size trucks here(2 one tons, one 3/4) no twenties for me.
My Camper truck has 18s, run 35 12.5 18s, New work truck will be built the same as the current camper truck.
Am running the same size tire/wheel. Have thought about going to 37's, and down to 33's.
All choices revolve around smaller wheels, more sidewall.

 

[JHanson]

Base 18-inch wheels. There will be no future reason you will wish you had bought the 20s. As you mentioned, airing down will be far more effective with the 18s, and nothing you can do improves the ability of the truck as much as airing down.

 

[Jon R]

Thanks everyone for the quick replies. You’ve made me a lot more confident about going with the 18s.

 

[Kolockum]

I believe you will also have a much broader selection of 18" tires.

 

[W6USA]

And when it comes time to replace the tires, you'll pay $200-$250 more for a set of 20's.

I'm surprised Ford only supplies the 18" spare regardless. Seems like too much of a mis-match in OD.

 

[Wandering Sagebrush]

“I'm surprised Ford only supplies the 18" spare regardless. Seems like too much of a mis-match in OD.”

About 1/2” is what my tire folks told me.

 

[Advmoto18]

20" came on my '16 F-350 Lariat. I will soon be going to 18" Method wheels.

I had 16" on my '03 F-250 and much preferred the ride on 16"s when aired down. Taller sidewall tires always offer a more compliant ride when aired down.

 

[Advmoto18]

“I'm surprised Ford only supplies the 18" spare regardless. Seems like too much of a mis-match in OD.”

About 1/2” is what my tire folks told me.

Steve...just comparing Toyo Open Country A/T IIs, about .68" difference between 18" and 20".

I'm sure I've worn my Michelins down at least 1/2" so I don't think the difference will matter. Of course, I don't want to ride hundreds of miles on a dissimilar spare as it might put needless wear on bearings...perhaps.

Back in the day, the spare was part of the vehicle's tire rotation sequence. No longer.

 

[Outnabout]

My 2017 Ram has 20” but the spare is 18”. Seems like matching the spare to the wheels that came with the rig would be a given, guess not. Better than the donut on my Subaru though.

 

[Jon R]

And when it comes time to replace the tires, you'll pay $200-$250 more for a set of 20's.

I'm surprised Ford only supplies the 18" spare regardless. Seems like too much of a mis-match in OD.

I am ordering a GMC, and the "LT275/70-18 spare regardless of wheels purchased" I described was for the 2021 GM HD trucks, not Ford. I went through the Ford "build and price" web site process and Ford simply says a spare is included. I couldn't find where the size was specified. I don't know whether Ford matches the spare to the installed wheel and tire size.

The OD difference between 275/70-18 and 275/65-20 is about 0.9 inches according to on-line tire comparison calculators, for about a 2.8% difference in non-compensated speedometer reading. My GMC dealer didn't think they recalibrate the speedometer for the different tire options. Not sure they really know, though.

I've decided to get the 18s. Thanks again for all the advice.

 

[Advmoto18]

If budget is not a consideration...

Go with 17" Innov8 forged wheels! These things are sweet and EXPENSIVE! 17" affords an even more compliant ride when aired down compared to 18".