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Discussion Starter · #1 ·
Ok so I need a good explanation, I am having problems with my speedometer being off by as much as 15 mph at 60mph. Only gets worse as speed increases. Everyone keeps telling me that my tire size is off and the stock size for my 05 3500 srw is LT245/75R16. I believe and was told my truck is stock. It has LT265/75R16 on it. Attached is a picture of the sticker inside my door. It clearly says 265. Also I remember reading somewhere that GM 1-ton SRW definitely came factory equipped with 265's. Also, it is not the stepper motor problem. Already had that fixed. Is it a speed sensor? Is this rare for this truck to come with the 265's? What am I missing here?
 

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Have you used a gps to tell your speed exactly to compare to your speedometer or are you basing it off driving next to another vehicle? Just checking.
 

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my 3500 srw came new with 265s and speedo is dead on even

245s were on the 2500's.


what does your speedo read if gps says 60?
 

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If its the speed sensor you should get a code. I use to get speed sensor codes on mine when it had an edge for whatever reason
 

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Discussion Starter · #5 ·
I used a gps. The whole reason I looked into it in the first place was because I got a speeding ticket.
 

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pop the wheel speed sensors off and clean them. and if it were tires going from a 245 to 265 will not change the speed that much so forget that. only other thing is gearing that would change speedo, like going from 4.10 to 3.73. if the wheel speed sensors dont fix your problem take it to the dealer or tuner and have them see what its tuned for and they can change it with a click of a mouse.
 

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Probably had a lift or big tires before it was sold / traded in and and they were taken off.

Just get efi or a cts or something to change the speedo. Easy fix
 

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Yes, your right the 3500 single rear wheel trucks did come w/ 265/75R16s. Somewhere in the programming/tuning of your truck, the tire size has been changed. You definitely need to reprogram the truck w/ a tuner or have a dealership fully reflash stock tuning into the truck. This should definitely help. As stated, previous owner probably had different size tires on and recalibrated at that time. Reprogramming truck would be my first option.
 

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Sounds like your going faster than your speedo reads so tires must be bigger than ecm is calibrated for , possible they did calibration for a dually (smaller tires O.E.)??
 

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Sounds like your going faster than your speedo reads so tires must be bigger than ecm is calibrated for , possible they did calibration for a dually (smaller tires O.E.)??
That's the way I was taking it. Sounds like it's calibrated for tiny tires. Mine is spot on with 245's and I'm only off 12 at 65 with my 315's. Someone correct me if I'm wrong but the duallys did have 'smaller' tires; I think 215/85's which really has about the same diamater as a 245/75, just skinny. So even the dually calibration don't make sense. At any rate his spedo should be off by less than 3 at those speeds.
 
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