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Discussion Starter · #1 ·
I'm considering having 2 sets of wheels & tires for our Terrain; a set for winter here near Cleveland & one for summer. I might actually go for purchasing 2 sets of rims - a 17" set of economy alloys to mount 225/65R-17 snow tires & a set of chrome 19" for April through October. I don't think I'll go to 20"; the tires are way too expensive for me & I don't think my wife will be diligent enough in dodging the holes :eek: I should be able to sell the factory 18" chromes & get some of the $$ back.

Anyway, when researching tires sizes I discovered the factory 19" tires have the same profile as the 18"; resulting in a tire/wheel combination that is a full 1" larger. I was surprised - usually, when moving to a larger diameter wheel, the profile will change to keep the overall diameter of the wheel/tire combination as close to the same as possible.

For instance: the 225/65R-17 has a diameter of approximately 28.5"; the sidewall is about 5.8" tall. The 18" tires are 235/55R-18; the sidewall is reduced to about 5.1" tall & the overall diameter is approximately 28.2" - just over 1/4" different than the 17" set up. If you move up to a 20" wheel, the recommended size (aftermarket) is 255/40R-20 resulting in a diameter of 28" (less than stock] or 255/45R-20, which has a diameter of about 29".

However, the factory 19" tire is 235/55R-19; the same profile as the 235/55R-18. You actually gain about 1/2" of ride height going with that combination; & I would think the speedometer & odometer would be off a little.

Does this seem odd to anyone else - or is it just me? Has anyone gone to 19" from 17" or 18" & noticed the difference?
 

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I think that when you go with the 19" offering, they reprogram the speedometer at the factory to account for the difference in overall diameter. I believe that this was discussed at some point in this forum, and can be found with the Search function.
 

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Discussion Starter · #3 ·
I think that when you go with the 19" offering, they reprogram the speedometer at the factory to account for the difference in overall diameter. I believe that this was discussed at some point in this forum, and can be found with the Search function.
Thanks RIT333 - I hadn't seen that. But - I wonder if anybody has checked to determine if there is a recalibration or not? Just curious I guess. I know the speedometer in my Nissan reads ~75MPH when my Garmin tells me I'm going 71MPH and that is with the stock tire size. I tend to believe the Garmin; it was a dead-on match in my Subaru Impreza. I checked that car & the Garmin against one of those "this is how fast you are going" radar signs & all matched up close to perfectly - so that sort of verified the Garmin accuracy for me. I've been meaning to throw the Garmin into the Terrain to check the speedometer accuracy but I keep forgetting to do that as I drive it infrequently.
 

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Kevinch said:
I wonder if anybody has checked to determine if there is a recalibration or not? Just curious I guess.
Not sure what you are asking. My speedometer matches my Garmin - I have the 17" OEM wheels.
 

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Kevinch said:
Thanks RIT333 - I hadn't seen that. But - I wonder if anybody has checked to determine if there is a recalibration or not? Just curious I guess.
Kevinch, what is your OEM rim size? The size from factory.
GM's new Global computer can recalibrate from smaller wheel sizes to larger but not the reverse.
So make your decision before hand.
 

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Discussion Starter · #6 ·
RIT333 said:
Not sure what you are asking. My speedometer matches my Garmin - I have the 17" OEM wheels.
Sorry - I meant I was wondering if anyone has checked to determine for a fact that there is a calibration adjustment made at the factory for different wheel & tire combinations. I know speedometers vary in accuracy; I'm just wondering if GM feels that a few % here or there isn't that big of a deal to worry about. The 17" & 18" tire/wheel combinations are so close in overall height - about 1/4" or 1% difference - that I wouldn't be surprised if the calibration were the same for both. Between the 18" & 19" there is a 3.5% (about 1" difference) which would be more noticeable.

POLRCUB said:
Kevinch, what is your OEM rim size? The size from factory. GM's new Global computer can recalibrate from smaller wheel sizes to larger but not the reverse. So make your decision before hand.
Our vehicle came from the factory with 18" wheels.

POLRCUB, you are saying that a dealer with the GM Global computer can re-calibrate the speedometer if you upgrade to 19" rims? (Not that I would buy the GMC accessory rims through the dealer; I think these are nice looking but at about $500 per rim a little too pricey:)



If the dealer can accurately calibrate the speedometer for the 19" tire/rim combination, then going a 225/65R-17 for the winter would be a 2.4% difference in height & maybe not too much of an error in the speedometer to live with during the winter months.

To summarize why I asked: I would like to decrease our rims to 17" for the winter; knowing that 17" is a factory offering and assuming that it is a more narrow, taller sidewall tire that will be approximately the same overall diameter as the 18" setup. That assumption is correct, as the 17" is a 65 series tire & the 18" is a 55 series tire, & the overall height between the 2 varies just over 1/4". The more narrow profile would be a little better in snow; the taller sidewall (5.8" compared to 5.1") offering a little more cushion over winter roads.

I would also like to increase our rims to 19" for the summer, again knowing it is a factory & GMC Accessory offering & assuming it is a wider, shorter sidewall tire that will be approximately the same overall diameter of the 18" setup. That assumption is NOT correct, as the 19" tire is the same 55-series, 5.1" sidewall as the 18" tire and therefore results in an increase of 1" overall height when compared to the 18" factory offering. I found that surprising, as most of my experience has been the vehicle manufacturers usually offer optional wheel & tire sizes that result in a more consistent overall height.

As an aside: Interestingly enough - at least to me - is that the factory 18" combination is the lowest profile offered by GMC for the Terrain. Also, going with a 20" rim & a 255/40R-20 tire will result in a lowered profile when compared to any of the factory setups - but it is very close to the 18" combination. The tough part of 20" to me is the cost of the tires. From what I've seen, they are way more expensive than 19" tires. I think my wife would have something to say - especially when I tell her to watch those narrow 40-series sidewalls.... :thumbdown:
 

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Kevinch said:
Sorry - I meant I was wondering if anyone has checked to determine for a fact that there is a calibration adjustment made at the factory for different wheel & tire combinations.

Our vehicle came from the factory with 18" wheels.

POLRCUB, you are saying that a dealer with the GM Global computer can re-calibrate the speedometer if you upgrade to 19" rims?

If the dealer can accurately calibrate the speedometer for the 19" tire/rim combination, then going a 225/65R-17 for the winter would be a 2.4% difference in height & maybe not too much of an error in the speedometer to live with during the winter months.

To summarize why I asked: I would like to decrease our rims to 17" for the winter;

I would also like to increase our rims to 19" for the summer,
I understand what your trying to do; Yes GM Global Tech can re-calibrate speedometer but ONLY going from an original OEM size to larger, once you do this it cannot be reversed. So make sure of your tire sizing and rim size before hand. Also, double check with you Dealer Head Technician if re-calibrating from 18"s to 19"s is indeed possible - I have been through this with my Tech and he called in to Tech Support to make sure, but it is possible. Reason I say to double check is that the 17 and 18 inch factory ordered vehicles have a different final drive ratio than the 19 inch vehicles. And we all know how 'consistent' GM is when giving answers.

On that note, here is my two cents, a 2.4% differential in tire size is brutal - don't go there (too close to the 3% tolerance).
Keep it simple, use your OEM 18"s for winter use with winter tires but only if you decide to go with a 19" rim for summer, then re-calibrate the speedo for 19" rims and just buy a 235/60R18 tire and you'll make up nicely between the 18"s and 19" and your speedo will be correct.

If you decide not to go with a 19" rim for summer than simply use your OEM 18"s for summer (can buy a different 18" rim by aftermarket if you want a different look) then you can use the OEM 18"s for winter - if your set on the 17"s for winter then simply buy a 235/60R17 tires for the 17" winter rims. Either way you don't have to mess with re-calibration on this one as the differential is only .1 mi/hr

I'm with the OEM 19"s from factory so for winter I run an aftermarket wheel (rim) but with a taller tire so that my speedo is in check as GM can't downsize with calibration.

Hope that makes sense
 

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****, Polrcub knows her stuff, and very well said !
 

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Discussion Starter · #9 ·
POLRCUB - thank you for a very detailed response!

POLRCUB said:
Reason I say to double check is that the 17 and 18 inch factory ordered vehicles have a different final drive ratio than the 19 inch vehicles. And we all know how 'consistent' GM is when giving answers.
Really? Do you know what the ratios are? I'd be interested to know.

On that note, here is my two cents, a 2.4% differential in tire size is brutal - don't go there (too close to the 3% tolerance).
I agree that it is pushing the limit, definitely not desirable.

Keep it simple, use your OEM 18"s for winter use with winter tires but only if you decide to go with a 19" rim for summer, then re-calibrate the speedo for 19" rims and just buy a 235/60R18 tire and you'll make up nicely between the 18"s and 19" and your speedo will be correct.

If you decide not to go with a 19" rim for summer than simply use your OEM 18"s for summer (can buy a different 18" rim by aftermarket if you want a different look) then you can use the OEM 18"s for winter - if your set on the 17"s for winter then simply buy a 235/60R17 tires for the 17" winter rims. Either way you don't have to mess with re-calibration on this one as the differential is only .1 mi/hr
Agreed - & thank you for taking the time to figure it all out! - but my goals would be:
  • To go to the larger 19" rim & tire combination for the appearance, & the additional ride height.
  • Go to the more narrow 225 series & taller sidewall in a smaller wheel size for winter use.
I'm with the OEM 19"s from factory so for winter I run an aftermarket wheel (rim) but with a taller tire so that my speedo is in check as GM can't downsize with calibration.

Hope that makes sense
It absolutely makes sense. What is your winter set up - the 235/60R/18?

I did more research after reading your post. It appears that if I go with the GM tire size of 235/55R/19 (& have the speedometer calibrated); the closest I can come in a 225 series tire would be a 225/60R/18. That is a 1.9% difference - still more than I would like but a lot better than 2.4%. I'm really curious about that final drive ratio difference though - how much of a difference does that make?
 

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Kevinch said:
POLRCUB - thank you for a very detailed response!
Really? Do you know what the ratios are? I'd be interested to know.
I agree that it is pushing the limit, definitely not desirable.

Agreed - & thank you for taking the time to figure it all out! - but my goals would be:
  • To go to the larger 19" rim & tire combination for the appearance, & the additional ride height.
  • Go to the more narrow 225 series & taller sidewall in a smaller wheel size for winter use.
It absolutely makes sense. What is your winter set up - the 235/60R/18?

I did more research after reading your post. It appears that if I go with the GM tire size of 235/55R/19 (& have the speedometer calibrated); the closest I can come in a 225 series tire would be a 225/60R/18. That is a 1.9% difference - still more than I would like but a lot better than 2.4%. I'm really curious about that final drive ratio difference though - how much of a difference does that make?
Yes, the 60 series for winter.
With the V6 the axle ratio is 2.77 the I4's are 3.23,, so you may experience some powertrain hp loss because of the larger wheels (the I4 may seem more sluggish)

IMO, the 235 tire will give you ample traction in the snow, I understand your reasoning for a narrower tire but unless your doing some serious 'snowplowing' I would go with the 235's and get the speedo back in check.

Cheers and Happy Motoring
 

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Newbie question - sorry

My 2013 Terrain Denali has 235/55-19" wheels and I want winter 17 or 18" wheels/tires.

Will the 225/65-17 tirerack recommends be brutal and too close to the 3% limit mentioned above?
 

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Marathonman said:
Newbie question - sorry

My 2013 Terrain Denali has 235/55-19" wheels and I want winter 17 or 18" wheels/tires.

Will the 225/65-17 tirerack recommends be brutal and too close to the 3% limit mentioned above?
Buddy, ignore my returned PM to you!! I didn't realize that you were sizing up winter wheels.

I just read your post here lol, as you didn't mention sizing up winters in your PM,,,,

So yes, TireRack's recommendation puts you over 2.2%, which means that when your actual road speed is 100km/hr your speedo will indicate 102km/hr,,,

You can run those if you choose to but I would go with something closer to OEM diameter.
Can try an 18 inch set up as well if that gives you more tires to choose from.

Cheers!!
 
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