My daughter bought a 2004 Silverado a month ago. She noticed that the speedometer wasn't reading accurately. When it was at 60, she was actually doing about 54.

Looking at the RPO codes, it appears that the truck came from the factory with a QDG package (235/50 R18), as opposed to the 245/75R16s it has on it. Looking at the computations, that fits the speedometer differential pretty closely.

Given the electronics involved, not only does the speedometer not read right...but the transmission gating is wrong, etc.

So...I have two options. Either upsize the tires or have the electronics recalibrated (we happen to have an electronics firm here with an international reputation).

Thoughts? Pros and Cons?

Thanks.

 

How long do you plan on keeping your Chevy? Me, I'd put the original tire size on it, and sell off the old ones. The last thing I'd want is for the automatic transmission to become damaged. Thankfully, my old 1998 K-1500 Silverado has a 5 speed OD stick-shift transmission, but my tires are still the original size.

 

You're not hurting anything by driving on undersized tires. I did the same thing on my '01 Burb to give me an effectively higher axle ratio because I was towing heavy trailers. And also because it was much easier to find 245/75/16 E load tires.
You can confirm the OEM tire size by checking the loading/weight stickers in the driver's door frame.
Me, I'm just cheap. Unless you can get the electronics programmed for $50 or so, I'd leave it and just tell her she'll get good practice at math to make sure she doesn't speed.
Then, when the tires wear out, you can decide to upsize, if you want to lay out the additional cost of new wheels.