93 Buick, Correcting an optimistic speedometer

Howdy folks,

Looking to see if perhaps I can have some light shed on the problem, or should I just leave it alone it's not worth the fix.

I've got a 93 BRW with the 3.23 rear. Tires are 235/60R16 all around. Spec shows it at 27" tires (-ish).

The transmission looks like it was a junkyard swap before I got it, and it's running fine except it had the gearing as if I had a 2.70 something rear gear. Essentially if the speedo showed 95-100, I was actually going 75-80 (according to the Speedometer app on my phone, a GPS speedometer)

Using the TCI calculator, it showed I needed a 43.something driven gear for the VSS. I ordered the 44 tooth, as well as the white housing VSS since I was going from a 36 tooth driven to the 44.

Swap went simply enough, no trouble at all. The thing is, I'm still off 5Mph at 75. Maybe 4 at 60Mph. All this is tested against the GPS speedo app.

While looking at this, I hooked up the computer, and noticed the RPM displayed on the tachometer is at idle 150-200rpm high from what the computer is seeing. Perhaps I have optimistic gauges?

I suppose what I'm asking, is there any way to dial in the gauge cluster? Or should I just go to a taller tire the next time I buy a new set and put this little episode behind me? I'm a bit OCD when it comes to making old cars run like new. (Not that old, but people keep asking why I drive that old "Thing")

Thanks!

The 36 tooth driven gear would be on a car with the 2.73 rear.

You can put a smaller drive gear on the tail shaft of transmission, and adjust the size of the driven gear. 18:42 is the same as 15:35. You should have had a green driven gear (42 tooth) to start with, if it had the driven gear for the 3.23 in it. There are pulse rate modifiers available for about $80, and you can tune it to your hearts content. You can get 15, and 17 tooth drive gears. A 17 tooth may fix your issue.

My tach reads a little high, and I do not worry about it. I do not know why they even put a tach in a car with an automatic, especially one where you can not manually shift it accurately.

The standard tire height with the 3.23 gears is 28.3 inches, so that would account for the discrepancy in the speedometer. I use a 43 tooth gear with the diameter tire that you have (27.1 inch), and my 91 OCC runs almost right on the money. I just did some work on the car, and I may have to manipulate the VSS ratio to compensate. I am also going to a 3.42 rear, and will have to change it again.

My 92 OCC has the 2.73 gears, and has the original trans with the factory VSS gears. It reads about 3 MPH fast with 29 inch tires. When I had the 28 inch tires on it, it read 6 MPH fast.

Ah, so sounds like i may be just within whatever GM considered "spec" The VSS box looks interesting, but I'd rather buy another gear than get into messing with the factory wiring.

As far as the tach goes, I like having a tach! While my speedometer was wildly innacurate (took me a bit to find the 42 tooth gear i tried, as well as the different VSS module) I knew that approx:2400RPM was about 75. Made it really easy to do my commute in hours without traffic.

The 4500 redline is depressing though!

Thanks again Fred!

If I end up at Wagonfest Daytona this year I'll have to get you at least a couple beers to compensate for the knowledge!

If you would have contacted me, I have the sender for the 40+ tooth gears. The square wave modifier connects between the VSS and the signal splitter under the dash. You intercept the VSS wires, and put the unit between the VSS and the splitter. It is not a big deal, and you can run any size tires you want with any gearset you want by changing the settings of the DIP switches.

Next time I'll reach out to the forum first. I've gotten too used to the big auction site being where I find stuff

If you are attending the WF FL, and you need something, let me know, and I will bring it with me.