Chevy & GMC Vans

What's required to correct the speedometer on 04 OBD II after adding bigger tires?

4.10 gears of course! haha

I hear it's possible to reprogram the PCM, but it's expensive. My Scangauge has a setting for correcting for tire size on the displayed speed. Other than that, you can find out the % difference in tire height and do it in your head. I have to add 6%.

Make sure you write down the odometer reading when you upsized the tires. That way you can always back calculate your true mileage.

Went ahead and ordered the Jet programmer. That should cover it and a few other things (like my lunch money for the next few months).. )-:}

Bring it for show-and-tell when you get it.

I am not sure if you could correct it at the VSS.Lifted might be a good source for info if he fixed that on SPB.

Also a change because of a larger tire does effect more than the speedo.

I have the JET DST that I used to reprogram the PCM. Cost was $400 I think. It pretty much lets me reprogram just about anything in the PCM including final drive ratio for when I change to 4.11's. The correction also adjusts the shift points.



Which one did you get?

How does it work? Is it a one-time process that modifies something inside the PCM? Or does it need to be plugged in all the time?

Yeah-I forgot Peter had one programmer in his ZZ van.

Mine lets you view and change the data stored in the PCM. The LUTS for fuel and trans, some OBD triggers, etc. You plug it in, download the info, view and modify it then upload the new parameters. It needs a laptop for programming. It's not one of the standalone pushbutton types. There isn't any real time readouts or anything. That is why I also purchased a DrewTech DashDaq XL and a TECH2. Both of those have realtime data that can be displayed in analog and digital meters.

The JET DST allows you to make unlimited adjustments for up to 4 vehicles (similar makes and models). More licenses can be purchased.

LOL a license!? I know a great spot they can stick their little gizmo.

It's better than say a HyperTech that only lets you do ONE vehicle and stays locked to it until you load the tuning program back into it. Someone gave me a $300 unit for free that was locked....I poked around in the electronics for weeks trying to reset the chip with no luck. Now it's just a useless paperweight. If they let you use it on unlimited # of vehicles nobody would buy one, you'd just borrow it. It sucks but I can see the need from a cost of development standpoint. If you come up with something cool you should get paid for it.

Got the new tires on and went for a decent drive, set cruise control at 60, checked GPS and it was exactly on 60.. Hmmm, maybe all I did by adding bigger tires was to correct an innaccurate original. I think I spent $300 for nothing on the tuner/programmer. Well at least it's supposed to firm up the shifts a little, will see about that when it gets here. May make a few things better but it seems the speedometer is closer to correct than it was before I added bigger tires.. )-:}

Lets not pretend that it's not a scam. I can lend my wrench to a buddy, and it will still work on his car. (and a wrench is a lot more useful)

arlon wrote:Got the new tires on and went for a decent drive, set cruise control at 60, checked GPS and it was exactly on 60.. Hmmm, maybe all I did by adding bigger tires was to correct an innaccurate original. I think I spent $300 for nothing on the tuner/programmer. Well at least it's supposed to firm up the shifts a little, will see about that when it gets here. May make a few things better but it seems the speedometer is closer to correct than it was before I added bigger tires.. )-:}

Most vehicles today are intentionally done this way. My truck and my van are both about 1.5 mph over true speed based on GPS. When I went from 32" to 33" tall tires on the truck, the speedo was correct. Our Nitro is closer to true speed although it is still almost 1 mph higher. These speeds are interstate speeds...72 mph.

LiftedAWDAstro wrote:Most vehicles today are intentionally done this way.

Why is that, Lifted? To make us drive slower?