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Need help with an old GMC

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New User

Dec 16, 2015, 9:30 AM

Post #1 of 5 (884 views)
Need help with an old GMC Sign In

Year 2000
Make: GMC Sierra 1500
Mileage: over 200,000

This was my son's college truck and when he bought a new vehicle the truck became mine. When my son had the truck he added larger wheels and for the most part it has not been a problem until recently. The speed odometer runs behind fives mph (going 75mph on odometer will be 70mph on radar) and when you get between 60-75 mph the entire truck starts shaking. Once you get to 75mph it levels off and drives smoother.
Here's the question: is it the wheels/tires or after 200,000+ miles should I have the shocks replaced?
Please help - I do not know anything about vehicles.
Coach Williams

(This post was edited by ballgame on Dec 16, 2015, 9:32 AM)

kev2 profile image

Dec 16, 2015, 9:54 AM

Post #2 of 5 (877 views)
Re: Need help with an old GMC Sign In

just got this SO lets
Have the system checked for codes- yes even if light is off. ITS FREE easy fast- most auto parts stores do it FREE!
Different size tires can be a problem, is the yellow brake ABS light on? Also when you start vehicle you should see a BULB check of the lamp.
The shaking coming then smoothing out is suggesting a balance issue- sometimes you have steering wheel shake - other times its more a rear end whole vehicle feeling.
Now 200k if original shocks there due. Unlikely the cause of shake.

BUT lets work the shaking and wheel issues first...See if the code check adds any issues.
Hopefully this is NOT 4wd.
tell us what wheels were STOCK and what is there now- driver door has a decal for stock wheel size.

(This post was edited by kev2 on Dec 16, 2015, 9:58 AM)

New User

Dec 16, 2015, 11:40 AM

Post #3 of 5 (868 views)
Re: Need help with an old GMC Sign In

kev2 - the STOCK decal states FRT & BK: P255/70R16 rim: 16x6.5J - the tires on the truck now are 225/R16 and I think the rim is a 20 (?)
truck model # C15703

(This post was edited by ballgame on Dec 16, 2015, 11:42 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Dec 16, 2015, 12:05 PM

Post #4 of 5 (862 views)
Re: Need help with an old GMC Sign In

That's a total change for the truck and should have been if it could when changed aligned for the new size. Probably couldn't or didn't try so tires all worn oddly a reason now you'll never win. At times and this was front end and all parts checked? It has to be greased a lot or tons of items would just be plain worn to dangerous but generally this has nothing to do with vibrations, balance or shaking nor do shocks as said.
Any will make existing balance and shaking worse not the cause first.
Smart kid - gave the truck to you. Now I suggest finding correct wheels and tires for it if you plan to keep it. IMO enough miles unless cared for to the letter.
Do you really want those at all - tires and wheels? That doesn't help at all to go outside of OE suggestions if not thought out some and aligned for it. Yes - tire size totally changes alignment including those that trash tires.
The speed and when balance appears and can go away is somewhat common but doesn't address the possible total incompatibility of those for the truck.
Miles - lots less than that total front and rear end items to do with steering and suspension should all be checked out. All worn items are a bit costly if 4X4 that much more.
IDK - Alignment check should include all wearing suspension parts so if you can check only first now to know if it's even safe do so and suggest correct wheels and tires possible to get all at once with OK tires then get the alignment and parts fixed unless some found not safe upon initial inspection.
If wheels are in good shape see if a junkyard would make a credit toward a swap of nice even matched wheels and hope for tires already to go as well - a chance of that.
All that if you are going to keep this at all. It could easily get real costly and the miles kills the value even if put back to just safe it's personal to me but wouldn't look at a vehicle with those miles,


kev2 profile image

Dec 16, 2015, 12:06 PM

Post #5 of 5 (860 views)
Re: Need help with an old GMC Sign In

on the tire you have on rear, you are missing a number, 225 - xx- R16. That xx is a ratio we need to look at for dimensions. Your requirement is 255-70-R16.
Baring a wild ass ratio number something crazy - I still would say start with a wheel balance..
The R16 tells us the rim is a 16" diameter so they are NOT 20's.

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