Speedo recalibration

[Bernie]

I'm curious... Just how do you handle a speedo recalibration? I've a '99 GMC C300 dually that the speedo seems to consistently read high on. (ie: it'll read 65 or so when my GPS and/or those "Your Speed Is" signs by the side of the road will say I'm doing 62 or so)

 

[Zembonez]

If it's a GMT400 chassis, does it have a speedo cable? I'm not sure about HD trucks. I believe yours is too new for that but if it's a cable, GM offered different speedo drive gears at the trans. If it's electronic, then the calibration would be done within the programming of the computer. MIne's off by about 3 miles per hour at 70 (because of non stock tire size) but I haven't ever done anything about it.

 

[Bernie]

1999 GMC 6.5L diesel on a C3500 dually chassis (not HD) with the 4L80E tranny. All electronic.

 

[Bernie]

What I'd really like to do is put a tailpiece overdrive on it, to get the revs down a bit at highway speeds, but not lose towing capacity. With the 4.10 rear my RPM's are up around 2,100 @ 65 MPH, which kills fuel economy on a diesel.

 

[Zembonez]

True. I'd bet there is plenty of torque available down around 1700 RPM. Do they make such a beast?

 

[hotrodpc]

Bern, 3 miles off at 65 is dang close for a speedo. I do beleive the chip to your speedo is going to be behind the glove box under the dash. My dad did a gear change in his 94 C3500 Cab and Chassis, and the guy there said, there is a chip that had to be burned with the gear chane to make the speedo accurate. Now his is a NV4500 trans and gas motor, but still same body style as yours.

 

[Bernie]

True. I'd bet there is plenty of torque available down around 1700 RPM. Do they make such a beast?

Yep... costs about $3,000 for my setup, which is why I don't have one yet.

A place called Gear Vendors makes them.
http://www.gearvendors.com/

 

[Zembonez]

That is pricey... but quite cool!

 

[Bernie]

Bern, 3 miles off at 65 is dang close for a speedo. I do beleive the chip to your speedo is going to be behind the glove box under the dash. My dad did a gear change in his 94 C3500 Cab and Chassis, and the guy there said, there is a chip that had to be burned with the gear chane to make the speedo accurate. Now his is a NV4500 trans and gas motor, but still same body style as yours.

"Supposedly" mine can be reset with a TECH 2, but I hate going to the dealer for anything.

And mine is off by 3-4 mph across the entire range, not just at 65. Heck, sitting still it says I'm doing -4 mph.

 

[Bernie]

Whoops... I just realized I posted that link in a sponsors area, and that probably was not very cool. My bad... sincere apologies!