Forum Discussion

Timeking's avatar
Timeking
Explorer
Oct 29, 2016

worries over 20 inch rims

I found a 2015 Silverado within my price range which has all the options I want, at the price I want, still under Chevy warranty …. BUT It has 20 inch rims and tires. The truck is rated to tow 9400, and if the rating was developed using base 17 inch tires then that drops the tow capacity down to 7990! Why did GM put 20 inch rims on a tow truck? Also bigger tires means increased leverage on the brake pad (axle leverage to the road) which increases stopping distance – a real (not) plus coming down the Rocky Mountain passes towing heavy weight.

I got on the chat directly to Chevy, but they didn’t know (apparently) how they rate the Silverado as far as what tire size was used. The factory doesn’t know? Neither does the local dealership.

I also think that since GM upped the tire diameter, they also had to recalibrate the speedometer, which means if I bought the truck and switched out the rims to whatever size was used to rate the truck to tow 9400 lbs, I’d still have to re-program the PCM so the speedometer would be accurate. By my calculations, dropping the rims back down to 17, would have the speedometer saying I was going 60 mph when on the road I would be going 70.5 mph. ****.

Is any of the above accurate thinking?
Tow Vehicles
  • Timeking's avatar
    Timeking
    Explorer
    Oct 29, 2016
    Grit dog - thanks for that info on craigslist. Didn't think about that.

    It turns out that the killer on this deal is that some guy in TX bought it and 2 months later was rear-ended and smashed into a barrier front first. I guess that explains why it was so cheap.

    gotta keep my guard up!
  • Grit_dog's avatar
    Grit_dog
    Navigator
    Oct 29, 2016
    This thread is as messed up as a soup sandwich!
    Ok, Here's the deal, it's not the OE rims limiting the towing capacity, and there's a ton of threads on tow ratings.
    No, the rims aren't weaker, and no they won't bend on every pothole you hit. About the only downside is tires generally cost more, now that 17s are bargain basement prices like 16s.
    Whether you prefer the look.....subjective. To pass up buying a truck that is otherwise what you want based on rims? Silly.
    Btw if you want 17s, there's always a grip of OE takeoff tires/rims and plenty of 17s for sale on Craigslist that were switched out for fancier wheels.
  • coolmom42's avatar
    coolmom42
    Explorer II
    Oct 29, 2016
    Your 20" rims will come with lower-profile tires. That compensates for the difference in rim size. If there is any difference in total rim + tire diameter, the speedometer will be calibrated correctly.

    If you look up the tow rating & axle ratings for the specific trim level of your truck, the 20" rims will be accounted for in that.

    This assumes they were installed at the factory, and not some aftermarket item that the previous owner added. Again, the dealer should be able to run the VIN and find out if they were a factory option.
  • BenK's avatar
    BenK
    Explorer
    Oct 29, 2016
    If OEM 20's...then they should be rated for that truck...if after market by one
    of the previous owner(s)...check them out, as many are 'car' wheels and NOT
    rated for that truck's F-R GAWR nor GVWR...

    Larger dia vs normal dia has ups and down sides...check out the comments on other
    threads
  • ktmrfs's avatar
    ktmrfs
    Explorer II
    Oct 29, 2016
    My 2015.5 2500HD GMC denali has 20inch rims, tires have the same or higher load rating as the 18" rims or 17" rim tires.

    Advantages I've seen vs. my 2004 is the 20" with lower profile make the truck more responsive and less wander. Downside is there aren't as many choices for tires when it's time to replace them.

    and BTW the tire diameter is exactly the same for 20's, 18's and 17's. In fact chevy uses a 18" as the spare regardless of if you have 17 ,18 or 20's as the factory rim.

    And why did they bigger?? For one they increased the brake rotor diameter, larger brakes.
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Oct 29, 2016
    Timeking wrote:
    Why did GM put 20 inch rims on a tow truck?
    .
    .
    .
    I also think that since GM upped the tire diameter, they also had to recalibrate the speedometer, ...

    If you are buying new, see if the dealer will swap wheels and tires with a different truck that has the size you desire. They should also re-calibrate the speedo for you.


    It is not just GM ! Ford and I am sure Dodge is doing the same thing. The 2016-17 Ford Edge can be purchased with an optional turbo V6 engine. I am sure there is enough power there to tow 5,000 lbs (maybe with additional cooling), but they limit it to 3,500 because that engine can only be ordered with the Sport package which include 20" rims with ridiculously low profile tires.
  • MFL's avatar
    MFL
    Nomad II
    Oct 29, 2016
    On previous Ford, having the optional larger tires/rims, did lower my tow rating. It had 3.73, but actual tow rating of 3.55 gearing.

    Frank is correct, when comparing 18" tires/wheels with higher profile tires, compared to 20" with lower profile. There is not a noticeable difference. However in my experience, it makes a difference when comparing either to the standard 17s, which will likely have an overall lower gear ratio, due to smaller diameter. Yes, if going from either large diameter tire 18 or 20, to standard 17, your speedometer would need correcting.

    Jerry
  • camp-n-family's avatar
    camp-n-family
    Explorer
    Oct 29, 2016
    I don't think you understand tire sizing. The rim size doesn't necessarily change the overall diameter. The factory 20's will be rated to carry the max weights that the truck is rated for, just as the 17's. The speedo will be close (or already factory recalibrated)and there will be little difference in braking etc as the overall diameter is similar. If you upsize the tires where the overall diameter is larger then you will affect the speedo, ratings...

    The Silverado has several tire size options; 255/70r17, 265/65r18, 275/55r20 and even 22s

    The first number (255)is the tire width in millimeters. The 2nd (70) is the height of the sidewall in percentage of width. As in the sidewall is 70 percent of 255 tall. The last number being the rim size.

    In the 3 sizes available you'll see that as the width and rim size increases, the percent of sidewall decreases. This keeps the overall diameter the same or close.

    See here for tire size comparison
  • Slownsy's avatar
    Slownsy
    Explorer
    Oct 29, 2016
    No as a general rule the larger rims has tyres with lower side wals, giving them about the same outside diameter, but they can fit larger brakes in rims.
    Frank.

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