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Effects Of Slightly Bigger Tires

bgrif005

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bgrif005 Enthusiast

bgrif005

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I would like to level out and go to 285’s on my truck, but in short I’m looking for possible effects and mitigations.

 

Will the transmission shift poorly with the increased diameter tire? Should the computer be reprogrammed so it knows it has 33” tires instead of 32”? how much power & mpg loss?

Stevens03 Senior Enthusiast

Stevens03

Posted
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In my experience, yes you should get the truck reprogrammed for the appropriate tire size. Shift points will be a little off and the speed-o will be a little off. Can you run 285's without change?, yes.. and the differences will be minor worth updating the PCM for.

 

You can count on loosing a solid 1-2 mpg average too.

bgrif005 Enthusiast

bgrif005

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In my experience, yes you should get the truck reprogrammed for the appropriate tire size. Shift points will be a little off and the speed-o will be a little off. Can you run 285's without change?, yes.. and the differences will be minor worth updating the PCM for.

 

You can count on loosing a solid 1-2 mpg average too.

 

 

would something like a Blackbear Tune get me back where I need to be? I was thinking of doiing this regardless of the bigger tires anyway.

texasblueline Enthusiast

texasblueline

Posted
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I added 285's to my truck recently. The speedo was off slightly when it came from the factory (it was optimistic, it'd read 60 when I was actually doing 58-59 verified by GPS/radar), so the 285's actually made it pretty accurate so I didn't have to worry much about recalibrating for them. Almost like the truck was meant for 285's, at least that's what I told my wife to help justify it. :D But if you're doing a BB tune anyways, have him recalibrate for the larger size while he's at it.

 

As far as the weight and the effect on gas mileage and power, I was looking at GM's ordering site to try to pin it down. My stock GMC 17" cast aluminum wheels showed to weigh about 16.1 lbs less per wheel than the base WT painted steel wheels. And when I replaced the stock Goodyear Wrangler AT/S 265's with BFGoodrich All/Terrain 285's I was increasing the tire weight by 16 lbs per tire. So, the lighter wheels and heavier tires pretty much cancel themselves out, and therefore my unsprung weight should currently be about the same as a stock wheel/tire combo on the work trucks. So I figure the weight aspect won't be too detrimental. If you've still got the stock steel wheels you could make up for the heavier tires by going with lighter wheels like the GMC SLE/SLT 17" cast aluminum ones if that's a concern.

 

Now the more aggressive tread pattern on my BFGs probably does add to the rolling resistance, so I will probably take a slight hit on MPG's there. That and I removed my lower air dam.

bgrif005 Enthusiast

bgrif005

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The tune would probably help.

 

 

By looking at your user name, I take it you like Led Zeppelin? I do.

 

Yes, I am a Zeppelin fan!

Matt_ Senior Enthusiast

Matt_

Posted
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I didnt adjust for my 33s either and dont have any issues in the way of Abs lights or anything. when my speedo says 65 my gps says 68, so its not really a concern IMO and the shift points dont feel off. Just keep in mind that there will be a notable difference in calculated gas mileage though.

 

After the mileage adjustment i lost about 2 mpg... but my tires are heavy as hell, much more aggressive than the OEM tires, and measure almost 13" wide- none of which helps :cool:

 

...but like previously stated, if you were already planning on getting a tune then none of this will matter since you can recalibrate the ECM for the new tire size.

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