LoneCynic

I purchased my Camaro a couple years back, and I noticed very soon after that the speedometer reading was off. It showed me going excessive speeds and other cars were just flying around me. So I hooked up a GPS unit in the car that shows the current route speed limit and the current speed of the car. As it turns out, the speedometer is off by a full ten miles per hour. So basically, if it says I am going 65, I am actually only going 55mph. I've gotten used to this, and know I can go "ten over" the speed limit to actually be going the speed limit. However, I would like to fix this if it is not too difficult.

Now, all the info about the car, and what I know from talking to the original owner (I am the second owner). The car is a 1984 Z28 with the L69 High Output 305 paired with the T5 Manual Transmission. The original owner ordered the car through the dealership in the fall of 1983. He ordered the Limited Slip rear end option, but when the car was built, they had run out of the parts temporarily at the factory, or they hadn't come in yet if it was a new for 84 option or something along those lines. The dealership got the limited slip gearing for the rear end delivered to them when the parts were available, and they performed the swap in their service center for him. He had paperwork and such that he provided me at the time of sale to confirm all of this. He also gave me a box with the open rear end gearing that was swapped out of the car. Furthermore, he took a disliking to the "twin needle" MPH/KPH speedo that came in all 1984 cars. So he had a "Z28 Speedo" swapped in at the dealer from what I assume was an '85 model speedometer gauge part number. I have another box with the original twin needle speedometer. This car was a good buy because the gentleman I purchased it from had all original parts included with the car for anything he had changed, as well as documented service history and the big GM service manual for 1984 Camaros.

Those two things I mentioned, as well as an aftermarket CD stereo, and front brake upgrade, are the only changes he ever made to the car during his ownership. It is otherwise completely stock. Any ideas on what could be causing the speedo to read ten mph off? I assume it has something to do with one of the two modifications I mentioned in detail. They were both performed at a dealer service center, relatively close to the time of original purchase in 1984 and 1985. A time when you would hope they knew how to correctly install such things and make the proper adjustments.

I'd rather not tear the dash apart if I don't have to, as it is near pristine condition. 36 year old car, and I imagine that plastic would be extra brittle at this point if an attempt to disassemble were made. Has anyone made similar changes to their Z28 and encountered anything like this? I am open to suggestions on where to look or something to try to get it to read correctly. It is something I can live with on a car this old, and don't have to have fixed if it is going to mean tearing the dash apart. I don't daily drive it, and mostly just take it out on nice days for car cruises/car shows, or occasionally to get groceries just to run it some and keep the battery charged. If it is something that I can attempt to fix otherwise though, I'm willing to give it a try. Thanks in advance for any help or suggestions. I know this was a lot to read, but I wanted to provide as much info as possible so I could help you help me.

sofakingdom

iTrader: (1)

The problem is probably not the dash. Not that it's hard to take apart or whatever, but that hardly ever goes bad, especially not in such a way as that. And, the difference between the 2 speedos isn't the cause of it either.

More likely it has the wrong speedo gears in the transmission, such that they don't match the ones in the rear axle.

First thing to do is determine the rear gear ratio. Pop the cover off (the fluid needs to be changed anyway) and count the teeth on the pinion and the ring. From there it's easy to find out what speedo gears you need. The car should have 3.73 gears but who knows, without looking you can never be sure.

LoneCynic

Sofakingdom, thank you for your reply. I probably won't pull the differential cover, because I did a full fluid change on the drivetrain in the the past six months. I changed the engine oil, the transmission fluid, and the rear differential oil. Had I thought about it then, I would have counted the teeth on it then. Being that I have the original gears in a box that came with the car, I could count the teeth on those and likely figure out which speedo gear is in the transmission, correct? Then would we be able to make an educated guess on what speedo gear I would need to acquire? Correct me if I am wrong, as I am still learning a lot about the car, but wasn't there only one limited slip set of rear gears option for 1984 Z28?

Kincer

If your speedo is off I'm betting the gear ratio in the rear end is slightly different than the original gears that you have in the box, so if you want it right you're gonna have to pull the cover to see exactly what ratio is in there. If it was me and it didn't bother you I'd just leave it as is, I need to fix mine though because it's just too hard to remember because mine is worse. I had 2.73s and now have 3.42s, for me when mine reads 60 I'm actually going approx. 46mph, around 70 is 55mph and 77 is 60 mph, to hard to remember at a quick glance.

sofakingdom

iTrader: (1)

Maybe, maybe not.

There's no reason that the stealership would have "had to" change the gears when they put in a posi carrier. In fact that's a whole lotta extra work. I doubt that they did although who knows.

One way or another, to get it right, you're going to need to know what gear is in your rear axle, though.

Given that a complete change of rear axle grease, plus posi additive, is around $20 - 25 and only takes a few minutes, I'm not sure what the big deal over it is. In fact if you're real careful and clean everything up real good before hand, and are careful about how you drain the fluid, you could probably even re-use it if you wanted to.

LoneCynic

Checking the rear isn't that big of a deal, but if changing the speedo gear is, then I might be better off just living with it for now. I have that old GPS unit that I just leave on the info screen to show my actual speed. I'll just keep the info in mind so that the next time I'm doing major work on something else anyway, I'll look into fixing that. Thanks for the help guys.

Jaysz28

iTrader: (30)

Does the paperwork not mention what gearing was installed?

you could always determine the rear axle ratio based on the rpm the car is turning at 70 mph while in 4th gear (ie 1:1 ratio). It gives you a general idea what you are working with.

here are the gear options for the stock 10 bolt.

4100 = 4:10
3700 = 3.73
3400 = 3.42
3200 = 3.23
3000 = 3.08
2700 = 2.73

rt66er

iTrader: (2)

You could also just raise the back wheels off the ground, make sure it's out of gear, spin a wheel and count the drive shaft revolutions. If it spins about 3-3/4 times for one wheel revolution, then it's gonna be 3.73. If it spins about 3-1/4 revolutions, then it's gonna be a 3.23, etc. Would be easier with a friend to help.