I recently went to tear out the gauge cluster, and noticed that the speedometer needle was a bit... Off. In other words, it was pointing straight down, then would just sit there and rest on the steering column arch in the cluster instead of going back to zero. So I put everything back together and went for a quick drive around the block (And then some). It appears that the speed reading is off by a little more the faster I go (The higher the speed, the bigger the mis-read). Can anyone point me in the right direction of a speedometer calibration or calibrator tool that is at least affordable?

I feel it's worth mentioning that the rear end is from a Z28 (Previous owner did the swap), and I read somewhere that the gear ratio might be causing the issue with the wrong speed being shown, though other posts are saying that the speedo is transmission-driven, and the rear end would have nothing to do with it. HELP! Being a couple MPH off really doesn't make a huge difference to me, but it's a big annoyance that I want to address ASAP

FYI: I used Android Speedometer to record my actual speed compared to what the speedo was reading. Speedo would read 70 when my actual speed was around 65

 

UPDATE: I found after a bit of research that my stock gear ratio is 3.08 (According to https://camaroforums.com/forum/93-02...e-codes-54483/ <-- that link). Now I just need to figure out the gear ratio for the rear end... Which is apparently wrong one way or another, and there is no RPO code stamped on it anywhere that I can see

EDIT: It appears that (Through some serious Google-Fu) the GR for my rear axle is 3.42, according to this link:
http://www.fbody.com/anything/1017252

Is there anyone that can confirm this?

 

Jack the car up and turn the rear wheel one turn the the drive shaft should turn 3.42 times. It would be a little more acurate to turn the wheel 10 times and see if you get 34.2 rotations.

 

Unfortunately, getting the rear end high enough for that is a challenge until I can get the new jack, which is why I'm trolling for help. If I don't get any answers soon, I'll give your thesis a try and let you know how it goes

 

go to the 4th Gen Powertrain part of the forums. I tried looking for the exact link but I couldn't find it. there is a post that shows the actual location of a set of numbers on the axle and shows what gears they are through these numbers. I saw it about 2-3 weeks ago. good luck

 

I think you are wasting your time. If you find that numbers all that tells you is what gears where in the axle when it was new. Since this rear has been replaced is there any way to know if those gears on still in the rear? Gear swaps are one of the most common upgrades done to these cars. So even if you found the information you are looking for you still need to verify it before you lay down the money to get the car reprogrammed.
If you have a way to measure the actual speed you could reverse engineer the gear ratio. If it is an Automatic put the car in 3rd gear and drive it as close to 2500 RPMs as you can. Take a speed reading.

Example:
Tire Diameter = D in inchs
Car actual speed = S in MPH
Rear ratio = R
(D*3.1416/12 *2500/ R).01136364 =S
Since you know D and S you can solve for R.

So with 26.8 inch diameter tires you should be going with the car in drive gear (3rd gear automatic 4th gear T5) at 2500 RPMs:
58.3 MPH with a 3.42 rear
61.7 MPH with a 3.23 rear
64.7 MPH with a 3.08 rear

 

I'll have to pick up the pieces of my brain later. On my way home I'll take some speed readings (At work now, have to wait) in third gear and get back to this thread with an update. Thanks for your help, everyone. As I have said in the past, I'm a bit of a car newbie and I'm learning as I go

 

So it looks as if I'm doing around 55 when the Speedo says 60, 60 at 65, and 64 at 70. Leads me to believe I was right about the gear ratio, and I'm a bit iffy on my findings.

I FINALLY found a site (PCMforless) that offers a Speedo calibration tool for LT1 engines, and I'm not too sure on how to go about it. It's going to cost around $100 after shipping, and the installation and calibration procedures seem a bit complicated. Has anyone used this site before, or knows anyone that has dealt with them?

Direct link: Performance Parts

Dakota Digital Speedometer Recalibration