18 inch wheels vs 20 inch wheels

[artie2908]

I have a 2020 GMC Sierra 2500 SLT Crew Cab Gasser on order.   Just wondering if anybody has an answer.  The gassers only come with one rear end ratio-- 3:73.    You can get 17, 18 or 20 inch tires.  Doesnt that change the effective rear end ratio?  I realize  it's the height of the tire that counts but I would imagine there has to be a difference of at least 2 inches from a 17" to a 20".    so if my truck comes with 20" tires and I replace them with 18" would I be more like a 3:90 ratio rear end?   or would the difference be so small as to be negligible?  thanks

[J-man67]

It depends solely on tire size if not riding on the wheels

Sent from my SM-N975U using Tapatalk

[MaverickZ71]

GM has a knack of having the same overall diameter on their tire/wheel offerings, whether the wheels are 17, 18, or 20 inches.  You are worried about the wheel size, acting like the overall diameter of what's turning is just the wheel, but if the overall diameter of the tire on the wheel is the same size, then the effective rear end ratio is the same, no matter what the wheel size is.  To know for sure, you'd have to calculate the OD on all of the wheel/tire combinations offered.  Also, keep in mind that for aftermarket tires, tires of the same size designation can differ an inch or so in height, just because some of the tire manufacturers like to act like they're big rubber, but in reality they're saving a few $ in manufacturing costs.   

Edited January 13, 2020 by MaverickZ71

[aseibel]

If you are looking to get the maximum torque out of your truck, you can replace the OEM rubber with smaller O.D. tires. But as Maverick said, all of the OEM tire/wheel options should be within a fraction of an inch of each other.

[artie2908]

most of  the online tire sellers, such as Tire Rack, list the overall diameter of the tires as well as revolutions per mile.  There is a fair amount of difference between LT265/60-17 and LT275/65-20.  That's why I asked. 

 

Michelin Defender LTX M/S LT 275/65R20        overall dia 34.1"     revolutions per mile  610

                                                 LT 265/70R18                            32.6                                           637

                                                 LT 265/70R                                31.7                                            657

 

so am I assuming correctly, that it doesnt make that much difference?    that's almost a 2.5 inch difference from 17" to 20"      Not trying to overthink this, just trying to clarify. 

thanks again. 

 

[garagerog]

Artie, reread what Maverick had to say, the larger the diameter of the wheel, the lower the profile or sidewall height of the OEM tires your truck will be equipped with to keep the overall diameter of the wheel/tire combination all roughly the same across the 17,18, and 20 inch wheel offerings.

[aseibel]

30 minutes ago, artie2908 said:

most of  the online tire sellers, such as Tire Rack, list the overall diameter of the tires as well as revolutions per mile.  There is a fair amount of difference between LT265/60-17 and LT275/65-20.  That's why I asked. 

 

Michelin Defender LTX M/S LT 275/65R20        overall dia 34.1"     revolutions per mile  610

                                                 LT 265/70R18                            32.6                                           637

                                                 LT 265/70R                                31.7                                            657

 

so am I assuming correctly, that it doesnt make that much difference?    that's almost a 2.5 inch difference from 17" to 20"      Not trying to overthink this, just trying to clarify. 

thanks again. 

 

Those are drastically different tire sizes you listed. Yes, you would notice a difference in performance between them.

Are you looking at aftermarket tires or trying to compare OEM tires for each of the 3 different rims? The OEM tire options should all be pretty close in diameter, so I'm not sure where you found those tire sizes.

maybe you should read this to understand how tire sizes are written: https://www.goodyearautoservice.com/en-US/tire-basics/tire-size

 

[artie2908]

13 minutes ago, garagerog said:

Artie, reread what Maverick had to say, the larger the diameter of the wheel, the lower the profile or sidewall height of the OEM tires your truck will be equipped with to keep the overall diameter of the wheel/tire combination all roughly the same across the 17,18, and 20 inch wheel offerings.

ok-- think I got it.  The OEM tires diameter will roughly be the same but the profile will differ.

 

[MORSNO]

The diameter is relatively close, however the replacement cost of tires goes up with increased wheel size.  I chose 18" over 20" on my rig for a larger selection of tires and also cheaper per tire.  As 20's become more common, cost might get better, just make sure the tire shop doesn't quote you P and C rated tires which are very common on SUV's, stick with E rated tires.

[BlancoSilverado]

Ok lets make sure this is all correct,

 

Americas tire list 4 different tire options for a 2020 2500HD (stock)

Size                                          overall diameter                            rev/mile (not a great comparison)

245/75-17                                     31.46"                                        661

265/70-17                                     32.64"                                        638

275/70-18                                     33.16"                                        608

275/65-20                                     34.08                                         591

All are E rated tires no one should quote you non-E rated tires. if some one does call for a manager(they need to get retrained). 

 

If you want more torque, go with the 17" (smaller overall tire size).

If you want better HWY mpg then go with the 20"(larger tire, KEEP VEHICLE STOCK RIDE HEIGHT) 

 

Your vehicle(speedometer) will be programmed to the correct tire size (not rim).

 

The tire size is broken down like this;

 

275/70-18

the 275 means that the tires width is 275mm. The wider you have the better traction you have (if all other variables stay the same).

the 70 is an aspect ratio, the sidewall height is 70% of 275mm (192.5mm) so from the ground to the rim should be 192.5mm. 

the -18 is the rim size. 

[artie2908]

thanks for the detailed explanation!  It's crystal clear  now. 

[SkidooSteve12]

artie2908 had a very valid question, just worded it incorrectly saying tire instead of wheel size. You guys should have checked what actually comes on the trucks instead of saying the tire diameters are the same with different wheel sizes, because that is not correct. There is a 2.5" difference between the tire diameter sizes available on these 2020 2500HD trucks stock. I have the 20" wheels with 34.1" diameter tires and I know that they are the reason I can't get as good of MPG as other guys with the smaller diameter tires and wheels. All stuff I learned after buying the truck, so oh well. But let's give out correct information when answering questions.

 

Screen shots from the GM specification web page and tire comparison web page.

 

 

 

 

[artie2908]

Thanks, Steve.  That tire size comparison chart is great.  answers alot of questions I had. 

[SkidooSteve12]

You are very welcome. Here is the website I used. https://tiresize.com/calculator/

 

[Skidaho]

One concern to consider are the use of tire chains. If you plan using chains you might consider 17” of 18”. With the shorter sidewalks on a 20”, chains will could damage the wheels and 20” chains are not as common.