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bigger/heavier tires causing shifting issues?


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I'm just wondering if any others are having the same thing happening to them.

 

With 450 miles on the truck I installed the Rough Country 2.5 lift/level kit and the 305/55R20 Goodyear Duratracs, the 285's I initially ordered were on backorder, so went with these. Brought the truck back to the dealership for alignment, and now the truck looks as good as it drives!

I specifically bought a truck with the 6.2/3.42 combo because I tend to like the bigger tire/ taller truck look. Thinking the 450lbs.ft of torque would easily handle whatever tire I would put on.

Apparently I was wrong. Since the install the truck will rarely shift into 6th gear going down the highway with the cruise set anywhere between 60 and 70 mph! Once I manipulate it enough to hit 6th, it stays there and has no problem keeping whatever speed I have the cruise set at.

Also, my mileage has gone down the toilet. Obviously I was expecting a MPG hit, but I'm down around the 10MPG mark right now!

Each tire is about 20lbs heavier!! than the wrangler SRA stockers, but really, it's a 6.2!! I feel like the 5.7 Vortec in my old 96 handled the 285 Duratracs better then this new one with double the power.

 

Any suggestions are appreciated, other than returning anything to stock.

 

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Mine seems to be fine with the 5.3L, 3.42 gears and 35 x 12.50's. No problem running 75-80 in sixth gear and averaging 13.8 mpg over the last 2K miles. If i set the cruise at 65 with the tow haul on it will even go into V-4 mode periodically. Give it a little time to see if it gets better, I would figure the 6.2 will do a good bit better than the 5.3 in that configuration.

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It would have some effect but not to that extent. I figure he is off probably around 5-6% not knowing the exact size of the tires he has on now or what his truck came with. Mine is off 9.2% with the 35's.

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That doesn't sound right, it should do better than that. Mine with 275/65-20's( which are 5 lbs lighter but larger diameter) hardly seemed to notice the tire change. It'll cruise up the mountains at 70 MPH in 6th easily, pulling fairly steep hills, only downshifting to 5th on the steepest. When accelerating to pass, or starting from lower speeds, it's quick to downshift--several gears if you stick your foot in it far enough. But at full highway speed letting the cruise do the work, it's happy to idle along in 6th.

 

Before going for a custom tune you might check with your dealer to make sure you have the latest calibrations. GM is constantly updating these things, especially in the first year.

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They're 305's... 40mm wider than the stock tire. Not much you can do.

Well 30mm wider.....But I get what your saying

 

Have you adjusted your speedometer? The MPG won't be correct if speedometer is not adjusted.

I have not re-calibrated, so I guess I would be doing slightly better than it shows

 

That doesn't sound right, it should do better than that. Mine with 275/65-20's( which are 5 lbs lighter but larger diameter) hardly seemed to notice the tire change. It'll cruise up the mountains at 70 MPH in 6th easily, pulling fairly steep hills, only downshifting to 5th on the steepest. When accelerating to pass, or starting from lower speeds, it's quick to downshift--several gears if you stick your foot in it far enough. But at full highway speed letting the cruise do the work, it's happy to idle along in 6th.

 

Before going for a custom tune you might check with your dealer to make sure you have the latest calibrations. GM is constantly updating these things, especially in the first year.

I can't see myself getting a tune until my powertrain warranty is up, considering all the new parts and technology in this engine series.

I'll check with my service manager tomorrow to see if my truck is "up to date" with software upgrades.

 

Thanks guys!

I'll post my results

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Stock tire is 31.91

New combo is 33.21

 

Your new tire is 1.3" taller than the stock tire, that is throwing your odo of by a decent margin which in turn shows less miles than you have actually driven which then affects your mpg calculations.

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Your shift points are based off of load and speed. If your tires are 10% bigger then you could effectively go up 10% on your gear ratio and it will all be good again without effecting your 5/100k warranty, tranny will shift right again speedometer will be right and you will get your power back, most of it anyway. If you have a good dealer, they will do your gear install and it be covered under warranty. There is no way around The warranty with a tune if GM engineering finds out about it.

 

Another option is Diablo tune, they have told me on more than a couple occasions that their tunes are not traceable because they are able to tune without changing your CVN values... Something to think about.

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20 pounds heavier way out at that radius is a lot of load, even for the 6.2L. And your lift. And your speedo is off. 10mpg seems low, but I only average 16 with a stock 5.3L and I drive about average. Take off 1mpg for the 6.2L, 1 for speedo error, 1 for the lift, and at least 1 mpg for the tires gets you down to 11 - 12 mpg. Then there's your weather and how heavy is your right foot?

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NO way for dealer to do anything in regards to the speedo via a factory option. Even if he was to swap over to the factory 3.73 all of the factory tire options all have the same effective tire height and use the same calibration data for tire height in regards to speed. The gearing will change in the stock calibration in regard to shift points and such. Only way to fix this is either with the hypertech re-calibrator or a tune, point period end of story.

 

That or go back to stock size or a tire that is close to stock over all height.

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I had a 2010 with the 6.2/3.42 combo with 325/65/18 BFG All Terrains. Gas mileage dropped just like it did on yours, and I bought the efi live. First tune was just to correct tire size and it made a big difference in mileage, but since I had the ability to do so, changed shift patterns and some other minor things. It really woke the truck up and I was back in the 16-17 mpg range.

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NO way for dealer to do anything in regards to the speedo via a factory option. Even if he was to swap over to the factory 3.73 all of the factory tire options all have the same effective tire height and use the same calibration data for tire height in regards to speed. The gearing will change in the stock calibration in regard to shift points and such. Only way to fix this is either with the hypertech re-calibrator or a tune, point period end of story.

 

That or go back to stock size or a tire that is close to stock over all height.

Come in now.... If you actually know tuning then you know that's not right. Please don't litter this thread with misinformation because you end up closing a door for this user. My goal was to give him all his options. I also don't want to litter it with a bunch of arguing back and forth so please bring your best theory if you respond, because I will have facts.

 

 

yellowcamaross...

here are your options in a nut shell.

 

A. Change tune with Hypertech or other Box Tuner

i. You will be out $300-$400 and you risk the possibility of having a major engine/trans failure (not tune related) which the first thing GM will do (not the dealer usually) is check to see if a tune has ever been on your vehicle then you have no warranty*.

ii. You will more than likely not be happy with the finished product of the tune but you will get your shifting back an and speedometer will be correct.

 

B. Get a Custom Tune from a "Good" tuner

i. You will be out $300 - $600 and you risk the possibility of having a major engine/trans failure (not tune related) which the first thing GM will do (not the dealer usually) is check to see if a tune

has ever been on your vehicle, then you have no warranty*.

ii. You will be happy with a good custom tune but he can only get so much out of what you have, It may seem like they perform magic but we just free up power that is already there by

customizing the tune to your engine. From what I'm seeing, 20-30hp depending on your supporting mods such as intake, exhaust etc.

 

C. Gear Change

I. You will be out $600-$1500 depending on 2wd/4wd and keep your warranty if you use your dealer to install. They may not warranty the gear set unless you use GM products but nothing else

should be effected. Hell, if you have a really good dealer then you won't have any warranty issues with an outside shop installing them, except for with the differentials themselves.

ii. Contrary to what has been said on previous posts, if you put the right gears in to offset the size of the tire, then your speedometer will be very close to dead on (gears only come in so many

sizes), your shift points will be back to normal, your MPG readout will be close, your MPG will get better and you will notice a very small amount of power loss over stock tire size/gears

because you will gain the power back in the torque multiplier of the new gears.

iii. You can always tune it after gears if you want harder shifts, and to make even more power after your warranty is up or when you just finally get sick of the stock shifting and are ready to take

the chance on loosing the warranty.

 

 

*Note - Loosing your drivetrain warranty is a risk, if you never have an engine or transmission hard part failure then its quite possible that you will be fine if other drive train parts fail. As of right now, Diablo is the only tuner claiming that their tunes cannot be traced so its just a matter of time before everybody else catches on and finds that loophole. I tune, if you were close I would change your speedo calibration and any other basic changes for free, you just pay for the 1 time - 2 credits from HPTuners that it takes to marry to your ECM ($99.98). I would still advise against this method and push the changing gears option if your warranty means anything to you, especially with a first year release new design engine.

 

 

My $.02

Steve

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