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 1992 Buick Roadmaster rear gear swap.

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letrbuk

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PostSubject: 1992 Buick Roadmaster rear gear swap.   Tue Mar 28, 2017 2:08 pm

I have a 1992 Buick Roadmaster wagon currently it has 2.73 gears and I am looking at swapping to 3.42's. My abs doesn't work now so I'm not worried about getting the right reluctor ring. My question is will the car have to be reprogrammed since it's a 1992? I know I'll have to change the speedo gear to keep the gauge correct. I'm not sure what controls TC lockup on my car. Any help would be appreciated.

Thanks, Kevin
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Fred Kiehl

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PostSubject: Re: 1992 Buick Roadmaster rear gear swap.   Tue Mar 28, 2017 10:06 pm

The TC lockup is controlled partially by the computer and by the throttle position. You will not have to change anything for the TC lockup to work.

You may need to change the VSS drive gear because the largest driven gear you can get is a 44 tooth. Driven gears are a little difficult to find. There are some on ebay.

3.23s use an 18 tooth drive, and 42 tooth driven gear. The 2.73s use an 18 tooth drive gear, and a 36 tooth drive gear. My 92 has 29 inch tires and 2.73s with stock gears, and reads 2 MPH fast. With 28 inch tires it read about 6 MPH fast, so the ratio is not all that close. I think the stock gears for the 2.73 ratio is wrong, and a 37 tooth driven gear would work better with the 28 inch tires.

You have a VSS for the 39 and under tooth driven gears. If you go to 40+ driven gear, you need to use a VSS for the 40+ tooth gears.  You need a 40+ driven gear to go to a 3.42, unless you use a smaller drive gear. A chart I refer to states that you should use a 42 tooth driven gear with a 17 tooth drive gear, but I think you can use an 18 tooth drive gear with a 44 tooth driven gear as well. The ratios are 2.470, and 2.444 respectively. An 18 tooth drive gear and a 43 tooth driven gear gives a ratio of 2.388. These are all with 28 inch tires. If you use smaller tires, you need larger driven gears.

The formula says you should use a 44.296 driven gear with an 18 tooth drive gear and 28 inch tires. Since the speedos usually run a little fast, it should be pretty close. I have seen the 44 tooth gears on ebay. They are the gears without the shaft on them. There is a hole with a flat on the inside of the gear. The easiest way to get this to work is with a 40+ VSS, and the 44 tooth gear. Other than that, you need a speed modifier, but it may upset the VSS module under your dash. You can get the VSS from a junkyard trans. You can tell the 40+ unit from the 39- unit because the 40+ gear housing is centered in the unit, and the 39- is offset a little. They are also marked on the housing, so you can check them after removal.

I have a different computer system in my car, and when I disconnected the computer from the VSS module under the dash, the signal for the speedo went off by 5 mph fast at 40. I chose to get a modifier instead of a gear because I am going to change ratios in the not too distant future, and I do not want to hunt for gears. As long as you keep your original type of computer, you can change gears to get your gauge correct. It will also affect your cruise control, and the computer gets a feed from the VSS module at 1/2 of the rate of the speedo and CC.

My 91 had a 15 tooth driven gear, and a 35 tooth driven gear. That ratio is 2.333, and that worked with my 3.23 gears, and 27 inch tires (same as 18-42).
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letrbuk

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PostSubject: Re: 1992 Buick Roadmaster rear gear swap.   Wed Mar 29, 2017 4:27 pm

If I don't change the speedo gear will it affect anything other then the speedo being wrong? My car doesn't have cruise control.
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Fred Kiehl

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PostSubject: Re: 1992 Buick Roadmaster rear gear swap.   Wed Mar 29, 2017 7:16 pm

For those who do have CC it will affect the CC. Since you do not have it, and are in the minority don't worry about it. It still has an output to the computer and the speedometer. I am not sure what the computer uses the info for, but it may have something to do with the EGR and air injection systems. The computer does use the signal to determine if the car is moving, but as to what it affects, I do not know.
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atlantadan



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PostSubject: Re: 1992 Buick Roadmaster rear gear swap.   Thu Mar 30, 2017 10:07 am

Is this something all gear swaps have to worry about or is it specific to certain years? I'm taking mine to the shop for a gear change later this week.

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Fred Kiehl

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PostSubject: Re: 1992 Buick Roadmaster rear gear swap.   Thu Mar 30, 2017 11:53 am

All 91-93 B-bodies have gear driven VSSs. If you are going to change ratios, you will have a speedometer error unless you either regear the VSS, or get a pulse width modifier.

If you have a 94-96, you have a VSS with a tone ring, and pickup. You need to have the computer reprogrammed to the correct tire size and gear ratio.
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