I recently bought a 83 HO and it's stock as far as I can tell. I noticed when I drove it that the speedometer seemed off. I got out google maps and sure enough it's way off. When the speedometer says 70 the GPS says 55 and at 45 GPS my speedometer says 55. Tires are only mildly wider not taller than stock. My cruise control does not work and looking around on the web it seems that this could actually be a cruise control issue rather than a gear and cable problem. I wouldn't care too much if it was the other way around but I'm definitely putting miles on that aren't real. Not sure where to start with this....any ideas?

 

I don’t see the connection between cruise control and the speedo being so far off, but I don’t know the failure modes common to that system.

If the transmission has been replaced, the speedo gear may be incorrect for an H/O which I think has 3.73 rear gears.

Correcting the speedo that is not matched to the rear gears could be done by changing the speedo gears in the transmission or installing an adapter inline with the speedo cable.

 

Welcome to Classic Olds. No matter what your question, there are experts here who are glad to help.

Make a post in the Newbie section and include pictures of your car. We all love pictures.

You've got a common problem with older cars. We went over this recently. Catch up at https://classicoldsmobile.com/forums...e-gear-167929/

 

The first thing I would check is to see if the trans is original or not. My money is on a replacement trans. Since the 3.73 gears in the rear axle are unique to the H/O, odds are that the replacement trans had the wrong speedo gears installed in it.

 

For example, if the transmission came from a car with 3.08 ratio, 45 mph by GPS would show 54.5 mph on your speedometer.

Joe's bet may a winner.

 

Appreciate all the input. I crawled under the car and verified that it is the original transmission the TH200-4R with the OZ tag indicating those that were put in higher performance cars i.e. Grand nat'l, monte SS etc. It shifts like it should and the lightning rods perform as designed. Interestingly, I looked at the rear end and this is not original to the 83. It has a 8.5 10 bolt posi rear end which only came with the 84 HO not the 83 which had the 7.5. I'll have to see if it is still a 3.73 ratio...if not I'm thinking that could be the issue???

 

The only things that determine speedo calibration are the axle ratio, tire diameter, and speedo gears. Also be aware that your trans may still not be original. GNs came with 3.42 gears.

 

That's unlikely because the current gear would need to be in the 4.50 range.