Speedometer reads 8mph slow?

The speedometer in my 92 Roadmaster reads way fast. When it shows 70 I'm really going 62. What could be the problem and how can I correct this? Tire size is correct.I have the 3.23 posi rear.

Did your car come factory with the 3.23 rear?

Otherwise, Fred K. will chime in and knows more about the speedometer gears you can find/buy to make it read as close to accurate as possible.

Yeah right on the window sticker it has 3.23 gears and posi.

One thing to check first.
Run the car though a speed test section or pace a known good car for ten miles.
Figure out how close the odometer is first .
If the oddo is correct then the issue is in the speedo head.
If the oddo is out just like the speedo then you might have a ratio issue.
Speedo ratio issues are always a percentage or ratio.
That is to say if it is out 8 at 50 it will be 4 at 25 and 16 at 100.
If it is out 8 no matter speed, that typically points to speedo head

I used my GPS. Definitely a ratio issue. 8mph off at 60 and roughly 4 off at 30

I've owned 13 wagons and about half of them were TBI cars and your car is normal. They all were like that. They were designed that way. How do I know? On three of them I swapped in a tbi 9c1 certified cluster and guess what? It was accurate within 1 mph vs. GPS and my stopwatch on mile markers. Pop the original back in and it was off about 7 to 10 mph depending on the car.

I'm not 100% sure but a tbi 9c1 cluster I believe should pop right into the buick/olds dash as those also had two connectors like the 9c1. If you are interested, I have an extra one I'd sell ya. Can take pics etc...


the LT 1 cars on the other hand, were pretty close.

What size tires are you running?

Sprocket, what is the 9c1 cluster exactly?

And Jay, I have 225/75 R15's

It is a cluster from a 9C1 sedan. The Caprice police car.

Is that the digi speedometer?

Yes.

And it's not a direct swap.

Looks like you have to repin some connectors and add 2 wires. No big deal. But how do I reset the odometer.....

My 92 olds is the same way. The PCM and the GPS show accurate speed but the speedo is off about the same as yours. I'm blaming the cluster

It's so irritating!!

You can change the driven gear on the VSS. They come with a 42 tooth gear, and you can put a 43 tooth on in place of it. When you pull the VSS, you will get about 2 quarts of trans fluid out of the hole, so put a pan under it. That will make it about 3-4 MPH slower than the indicated. There is a 44 tooth gear available, but they are difficult to find. I will check to see if I can find one (or more).

You can also get a pulse modifier, and put it in series with the VSS output. You must make a patch cable to attach it, and be able to manipulate it while driving. They are about $80. I used a VSS buffer to make a case for the input and output patch cable, and an extra cable end (pigtail) to attach it.

Fred, the 44 tooth gear sounds perfect! If you know of one that would be fantastic.

I found one on ebay: https://www.ebay.com/itm/700R4-Transmission-Electronic-Speedometer-Driven-Gear-44-Tooth-GM-GMC-Chevrolet/172567494992?hash=item282dd30150:g:W9oAAOSwiIxaRVQHUSPSFirstClass!33774!US!-1&vxp=mtr

This is an aftermarket gear, and not marked with the # of teeth.

I bought it!
Can I access the gear through the VSS hole?

Yes, but about 2 quarts of trans fluid will pour out when you remove the VSS. Put a good size pan under it. If you use a paint filter, you can reuse the trans fluid, or put new in to fill it if the old stuff looks crappy. I would just put new fluid in it. New fluid is cheap, and it never hurts to put new fluid in a trans.

Noted. I'll pick up new fluid from the store. Any specific recommendations?

Get good fluid, it is not much more than the cheap stuff.

Will do. I don't like cheaping out on car stuff especially anything transmission related.

Cliff notes.

speedo reads 70 MPH  when car is doing 62 .

this a a 1- 1.129   factor.

It is important to see if the  odometer is out this far before figuring a ratio change.

Lets "say" the speedo head is fine and this really is all a ratio issue .

44/ 42 = 1.047 not the 1.129 it would take to fix an 8 MPH  error at 62 MPH

My math shows that a driven gear swap from 42 to 44 would move the indicated speed from 70 to a little less than 67.

Least, that is my uninformed thinking.

Oh, and how much oil do you loose when you just jack up one side of the car ??

That oil loss is when jacking up one side of the car. It may only be about 1.5 quarts, so check your level as you fill it.
The gear change is about as close as you are going to get. An increase in tire size to 235/75/15 will increase the actual speed about another 2-3 mph, and be closer.