New Truck -- Need Tire Help

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by medicmatt, Dec 27, 2011.

  1. medicmatt

    medicmatt Member 100 Posts

    So I just got back into a truck yesterday, purchasing a 2008 Silverado ext cab 4x4. Much to my surprise, it has the 20" tires on it. My old one had the standard 265/70/17. This has the 275/55/20. So other than the obvious that it's a larger tire (and I understand what the numbers mean), what really does it mean? The tires are more of a road tire than an off-road/AT tire. How much worse is my fuel mileage, does it drop my take off ability, anything else that you can think of?


  2. srr2

    srr2 New Member

    Size 265/70-17 tires range from 657-660 r/mi. Size 275/55-20 from 650-655 r/mi. So it's the wheel that's larger, not the tire. Anyway, that's pretty close, within approximately 1%, so your speedometer error could be about .5mph high with the lower profile tires, depending on the specifics. As far as "take off ability" goes.... son, you're in the wrong vehicle if you're worried about that. Get yourself something that weighs under 3 tons.

    You are giving up ~1.5" of rubber between the road and the rim with the lower profile. May not be such a big deal in NC, but in PA with the lackadaisical road maintenance we have here, the potholes would have a much easier time costing an owner big $$ for rim repair/replacement than with the higher profile. I recommend that you avoid running into debris and holes in the road.

    Typically larger diameter wheels weigh more than smaller, again depending. But since this has the unsprung weight of approximately a baby elephant in the first place, a couple of pounds one way or another in tires/wheels isn't going to make a bit of difference.

    Get any illusion that this is a "performance" vehicle out of your mind. It isn't. Period. You're not going to turn it into a 911 Carrera 4 GTS no matter WHAT parts you add or substitute.

    Bottom line, unless there's an issue with appearance, you can safely give the tire/wheel issue no additional thought.
  3. PearlWhiteLTZ

    PearlWhiteLTZ Rockstar

    The stock 20" tires with the low profile can make hitting some square edged potholes pretty jarring in comparison to the 17" taller profile tires
    . The goodyears are great tires on dry warm days in areas with perfectly paved roads..But I dont live in a place like that, So i slapped a set of 275/60/20 ProComp AT's on my truck. The biggest PITA with 20" wheels is the cost of tires..The stock replacement ones aren't bad, but if you want to get into an AT, big dollars. I dont know what you mean exactly by "take off ability"...But if you are thinking difference. I've driven my truck (20's) and a friends truck (17s) back to back, and they feel exactly the same..

    CKNSLS Rockstar 100 Posts

    So- this is an interesting discussion. I too have the 20" wheels. It's a 2011 Crew Cab 5.3 with the 9,600 tow rating. I too have notice the cost of the 20"tires while browsing on line. SO-could I switch out the 20" wheels and tires for the factory 17s and have no ill effects, other than the speedometer being off 1%? Would the 17s have any effect on the towing experience for better or worse?
  5. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    He'll be fine on the roads in NC. However, the truck is a 4x4 ... and with those wheels/tires should not be off-roaded. A big valley/rut could easily cause him to crease a rim on low profile tires, as there's too little tire to act as a cushion in such a case.

    The lower the profile of the tire the stiffer the ride tends to be. Thus, if you moved from 55's to 70's and kept overall diameter the same, you'd add more tire wall between you and the road and the ride feel would improve, slightly.

    As for the towing experience, that depends on the load rating of the tire. Most people air up their tires to tow ... to compensate for the extra weight. Low profile tires are often D-rated while commercial tires and/or tires intended for towing are usually E-rated. The difference is tangible in that D's can often only handle 50psi or so while E's have more plys to them and can usually take 80psi. (Be sure to check load rating of the tires before buying, as I was speaking very generally, there.) More plys equates to a stiffer tire, so you're back to a stiffer ride on E's ... but you can air them up to handle much heavier tows.

  6. 747

    747 Member 2 Years 100 Posts

    If you switch from the factory 20" wheels and tires to factory 17" you will have no ill effects whatsoever. Its possible that you will even save some unsprung weight, which is obviously beneficial. The ride is a lot softer on 17" or 18" wheels. If you stay within the factory size tires then I wouldnt worry about the spedometer being off, it will be close enough to accurate. I dont think GM calibrates each vehicle for the specific set of tires (please correct me if Im wrong).
  7. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    The computer defintely knows what the overall height of the tires is supposed to be and uses this for speedo calc. It can only know this if told, so I believe it's been programmed.

    I've learned from tuner adjustments on mine that GM vehicles are, in fact, picky about specific sizes and it's the ABS module that's cranky about it. With 305/70R17's on my truck I was seeing ~11% speedo error compared to the stock 17inch tire size. 305/70R17's are 34.3" tall; setting to 34.5" via the tuner caused occasional ABS/Parking idiot lights to come on when the key was placed in the ignition and turned on. I found 34.0" to be fine (no warning lights) ... but it means I have about 1mph speedo error at 45mph and higher. Very livable.

    Anyway, that's anecdotal experience that tire size is programmed into the vehicle. Colour me very surprised if a GM person says differently, as it'd be shocking for them to deliberately permit speedo error; just seems to be a liability concern around that...
    Last edited: Dec 28, 2011
  8. medicmatt

    medicmatt Member 100 Posts

    Thanks everyone. Trust me, I don't expect this to be a performance ride, as this is my 3rd Silverado and I know better. What I was referring to was my acceleration, because I had read someone else on another forum complaining that they couldn't "get up and go" as quickly.

    As for my off-roading, I don't do a lot of it. Most of what I do is confined to deer and duck hunting, but even then I don't go hard off-road in it. I usually drive a portion of the way in and then park it and walk the rest of the way. I've always tried to avoid the big ruts and holes while off-road anyway so I'll just make more of an effort to be careful.

    As for the speedometer being off, I don't think that should be a big issue, since these are the stock 20's that come from GM, on the stock GM rims. So I assume (maybe I shouldn't) that they would have that properly calibrated.

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