K3500 Speedo reads too slow.

[stutaeng]

Hi guys. So my '00 K3500 CCLB speedometer needs to read 77 MPH in order to be going 70 MPH on phone's GPS. My tires are LT265/75/R16 over the stock LT245/75/R16 and the RPO code shows 4.10s.

I was driving the other day with my brother and he pointed that out, but didn't think much of it. Well, I think that's why everyone is passing me even though I'm "speeding!" LOL

I thought everything was stock on the truck, but now I'm wondering if the PO swapped the rear end gears? That would be possible, right? Or the transmission got swapped with another transmission from a vehicle with different ratios? Or is that stored on the computer?

Darn, now I have to check...

 

[stutaeng]

Ok guys, I just went for a drive with my phone GPS and my scanner hooked up.

2 things I discovered are that my speedometer is way too fast and I have 4.56 gears instead of 4.10 that is marked on the glovebox.

At 2,000 RPM my GPS is reading 58 MPH and my scanner was saying 57 MPH, while my speedometer was saying 64 MPH.

At about 2,400 RPM my GPS said 73 MPH, while my scanner read 70 MPH and my speedometer read 78 MPH!

For 4.10s: 2,200 RPM should give 71.8 MPH, so I don't think I have those. I've been thinking that's what I have all time since I bought the truck 2 years ago, LOL. Can I trust tachometer?

I'll jack it up and count driveshaft turns to confirm. I can live with 4.56. Who knows if they got the front regeared as well?

But what do I do about speedometer being way off?

 

[someotherguy]

Somewhat unlikely someone spent the bucks $$$ to re-gear the truck, and it almost surely didn't come with 4.56's; more than likely the very common 4.10's for a 3500. I'd verify first though before worrying about which path to take. (By the way, what gear ratio is indicated by your RPO codes?)

As far as tire size being the issue, your speedometer is backwards in that regard. Going from a stock 245/75/16 (30.5") to 265/75/16 (31.5") should cause your speedometer to read slower than you're actually going.. not faster.

Tachometers are *usually* correct.. but don't assume. It's entirely possible someone pulled all the needles off your gauges to put a custom gauge face on, and hosed all the original placement of them. My 454SS cluster was like that when I got it, although I had to guess at what happened because the face had been removed and it was back to original, except every single gauge was wrong.

For 96-up you need a handheld type tuner (or custom tune) to correct the speedometer for gear or tire height changes.

Richard

 

[stutaeng]

Thank you.

Yes, the speedo is backwards of what it should be. Based on slightly larger tire size, there is something like a 3% difference. Speedometer should read MORE than actual. That's why I was puzzled.

I just went and re-cheked and the glovebox shows GT5. I will jack up the rear to confirm. I'll check tachometer also with the handheld scanner live data. Forgot to check that..

Since the truck's computer was close to the GPS, it seems like I need to fix the instrument cluster speedometer? It's not aftermarket and looks original. The needle does fall to "0" at a standstill. Or perhaps a remanufactured cluster?