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jipper

Bigger tires...

Question

jipper

I have a 04 GMC sierra, It came with 16 inch rims ( 265/75/16 )... I put 17 inch rims on it.( 285/75/17 ).. Do I have to get the computer for the truck re-calibrated or can I leave it alone??. And if I leave it alone will it screw up anything...

Thanks

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Valv

You should have it recalibrated to compensate, your speedometer speed will be considerably less than actual speed.

This will influence onboard computer calculations also.

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Dragonsm

Yes, you should get it recalibrated. (whether it be at a dealer or some aftermarket chips/programmers can also recalibrate for this)

Just to give you an idea....if your speedometer says 70, with the new tires, you will be doing closer to 74.8 mph.

60 = 64.1

30 = 32

Your new tire setup has just over 2 inches more in diameter.

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Powerstroke

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ChuckN

My Ford F150 had 16" tires and I went to 17" rims 265's tires. My speedometer is off as noted, but it can be changed to read the correct mileage. Not a big deal.

There is absolutely no difference in power and gas mileage with this swap.

But, I don't know about the Chevy and never heard of any "re-programming"..

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MuskieJunkie

It is more than just a speedometer issue. With newer trucks the transmission shifts points are tied to the speed. If this is the case with your '04 I would say you do need to get it recalibrated. Remember the new tires change the gearing your truck was originally set up for.

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hanson

I'm running 285s on my GMC Sierra and I know my speedo is off, but its not off as bad as whats been mentioned. When my speedometer is at 75 mph, my GPS speed is usually around 76.5-76.8 or so, about 1-1/2-2 mph off.

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Sonicrunch

Quote:

Your new tire setup has just over 2 inches more in diameter.


As those tires wear, he will get closer and closer to the original diameter.

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Macgyver55

So far most of the conversation has eluded to speedometer error, transmission shifting points and power loss. All of these are real factors, but no one has addressed the most important aspect of changing tire size. Its the one you may not realize until you need them. Your Brakes!

"On ABS-equipped vehicles, all vehicle manufacturers recommend using the same size and aspect ratio tire as the original. ABS systems monitor the rotational speed of the tires through individual wheel speed sensors. Changing to an oversize tire with a taller diameter than stock would cause the tires to rotate at a slightly slower speed relative to vehicle speed than the stock tires. Though the difference isn’t that much, it may be enough to upset the calibration of the ABS system and have an adverse effect on its ability to detect and prevent skids."

It will also have an effect on brake wear due to the increased rotational forces of stopping a larger tire. This would not be much different than carrying a load around with you, but it definitely will have an effect on brake life and stopping distances as compared to stock sizes. The less you vary from stock the less difference you will see.

Although the effects are small they are real and should be considered. Calibration is a simple operation and literally takes only minutes.

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311Hemi

Quote:

It is more than just a speedometer issue. With newer trucks the transmission shifts points are tied to the speed. If this is the case with your '04 I would say you do need to get it recalibrated. Remember the new tires change the gearing your truck was originally set up for.


I agree on the shift points in relation to the speedo....but you may find that it is not that big of difference and don't care to have it reset. This is not a very big jump in tire size so you probably wont notice much difference in mileage.

One other thing to keep in mind.....I believe this is also tied into the ABS system if you have ABS brakes....so regards to safety I would probably have it reprogrammed. Any dealer should be able to do it for $50-$120....unless you know a mechanic.

I had to do this when I went from 245's (very small) to 315's (35" tires) on my truck. It was a substantial difference.

EDIT: Macgyver55 you beat me to it!!! tongue.gif

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Powerstroke

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Macgyver55

Quote:

Shift points are not related to speeds. Shift points are related to your engine RPMS. No vehicle will shift at the exact same speed everytime unless you can somehow apply the exact same throttle input under the exact same load all the time.


The first statement is incorrect. Transmissions are controlled by a combination of inputs processed into a resulting shift point, but vehicle speed IS definitely one of the necessary inputs.

The computer uses sensors on the engine and transmission to detect such things as throttle position, vehicle speed, engine speed, engine load, brake pedal position, etc. to control exact shift points as well as how soft or firm the shift should be. Computerized transmissions even learn your driving style and constantly adapt to it so that every shift is timed precisely when you would need it.

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Jeremy airjer W

I'd have to agree with Macgyver55 on this one.

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theoilman

AMEN! The ABS and brakes is a real safety issue. You definitely should have the computer reset so the anti-skid works properly. (A much larger tire size change will even generate computer errors and set engine codes, though this change probably isn't big enough to do this.)

The transmission will 'relearn' and readjust shift points within a week of driving. This is not a big enough change to worry about overall power or need to change differential ratios (ring and pinion change).

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311Hemi

Quote:

A much larger tire size change will even generate computer errors and set engine codes, though this change probably isn't big enough to do this.


Not always....as I know on 2003 Dodge's it doesn't. When I changed from 245 tires (smallest available) to 35" tires everything worked just fine....as did others that I know of. Unless that size isn't enough still....

I agree on the rest though!!! tongue.gif

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theoilman

I have seen too many that did set codes, though the oems change so many things every year, it could only be trial and error to know which ones do and which don't.

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