Ok I replace the speedo cable and found out th spedo off by about 10 mph.. ( So if im doing 70mph the speedo reads 80mph) The last owner put 225/50 H15 tires and wide wheels.. this is a stock 1988 2.8 V6 with a 700R4 tranny .. does anyone know what combo of gearing will set this right? ( The speedo cable gear )

 

What rear gears do you have? That is a necessary piece of information to calculate the gear combination you need.

One of the stock tire sizes is 215/65R15. That is a 9% taller tire than what you have now. That would explain why your speedometer reads about 10% higher.

With a 700r4, there are plenty of gears to get you very close to the correct speed reading.

Just post your rear axle gearing.
Lou

 

the car all stock.. ( if that helps any ) Where would I look to find out what the rear gears are?

 

To be absolutely sure, you can open the diff and look for the numbers stamped on the ring gear.

However, you can use the "turn-the-tires-and-count-driveshaft-revolutions" method. You can find a lot of info on how to do it properly in this section of the forums.

Once you have the rear end gearing number, we can calculate your speedometer gear combo.

Lou

 

So none of the RPO # or VIN will tell me.. bummer.. Its real hard to get a jack under it..

 

Not sure if RPO codes would reliably tell you what gears you got. Call a dealership, they might know.

Otherwise, it's not hard at all to get the rear end in the air. I can do it in 15 seconds (including wheelchocks in the front).

 

OK .. By what the RPO say.. its running a AXLE REAR, 3.42 RATIO.. Thats RPO code GU6 matched the RPO sticker to the VIN .. there 3 and 1 not for this car..

 

yes RPO codes will tell you what rear end gear ratio you have. They are located either inside the center console or underneath the console lid.

GU2- 2.73
GU4- 3.08
GU5- 3.23
GU6-3.42


ALSO G80-LIMITED SLIP, G92 IS LIMITED SLIP/ PERFORMANCE AXLE

 

I now have a master list of all GM RPO codes if anyones needs them..

 

Wow, didn't know that RPO's could tell you precisely what gears you have. Also, not many of us have readable RPO stickers anymore. But I'm sure I can find a GU code there somewhere on mine.

Thanks, good job.

 

ok.. now on to my problem.. How do I figurer out what speedo gear I need?

 

no problem but does anyone know where i can get a new speedo gear for my rear end gear swap. i have a 700r4 trans with 2.73 gears stock, i'm upgrading to a lmd slip 3.42 or 3.73 gears. know anywhere i can get a new speedo drive gear?

 

yeah thats what i need to know to haha, i guess were're in the same boat

 

Another way to find the rear gear ratio(at least what came in the rear originally).Looking at the axle from the front(driveshaft/input side)located on the passenger side axle tube,roughly in the middle,you'll find a set of letters and numbers stamped into the tube.This will be the axle code and date code the axle was built.

 

I have a GM book(printed in 89) that covers fcars from82- 89 that has a speedo gear (drive-driven) chart that tells which ones to use (color and teeth count) for which tire, trans and rear gear ..
I need to scan it and submit it for FAQ tech

 

thats woyuld be cool.. Pls post here when U upload it

 

Unfortunatelly, this proves to be unreliable. Especially up north, where the axles are all eaten away by rust. I couldn't find any stamping on mine either and I'm down south.

Tony, if you can post all the lookup tables, that'd be great. I'm sure everybody would appreciate that.

Mark, to make the gear change easier, you need to find out what drive gear you have in the tailhousing, so that we can calculate that into the equation and get you the correct driven gear. Sorry bud, you will have to get the car up in the air. But it will save you a lot of time and money wasted.

Where to get gears?
You can find many places online (like http://www.transmissioncenter.net/sp...n_______va.htm) or call your dealership, they might have the parts for the same price, who knows?

Hope this helps.
Lou

 

Ill try to find a way to get them posted within the next week or so

 

can't delet my own post

 

Just a thought here.. but what if I put 225/60-15 on the tires instead of the 225/50-15 that the last owner put on it? would that be enough to correct the speedo?

 

Yes, that would help you get back into the common range of cca 2% deviation on the speedometer. That is very liveable with.

 

Thats good to hear.. maybe it will look better with the 225/60-15 on it than the 225/50-15.. It looks like wheels with rubber bands on it right now anyway