Drive Ratio from: 2.95 to 3.05 ?

[93buicklesabrelmtd]

I know the 3.05 is the better ratio. I know it came with the Grand Touring Pkg. I have a 93 Limited which the tranny is about to poop, it lived it share of lives. Will the 3.05 ratio work for me? I have the electronic traction control and all that stuff.
The CV axles should be the same and everything right>?
I can get both 2.95 & 3.05 for the same price.

Anyone knowlegeable?
thanks

[maxwedge]

There will be not noticeable difference in mileage or performance for a 4% gear change.

[93buicklesabrelmtd]

so i could use either correct?

[maxwedge]

Slight speedo error, that's it.

[HotZ28]

If you currently have the 2.95 and choose to go with the 3.05 ratio, the tranny should have the right VSS in it. If the tire size is the same as the donor car, the speedo should be correct.

[maxwedge]

I believe the pcm is programmed for the gear ratio and tire size to make the speedo accurate, pretty sure that is not the job of the vss, it just sends the signal the pcm which is then interperted, based on programming, to drive the speedo to match gear ratio and tire size.

[HotZ28]

Actually, I thought that the VSS output pulse was based on the VSS drive and driven gear in the transaxle. If all things are the same between the donor Trans and the original, regardless of Tran’s final ratio, both should have the same VSS signal output to the PCM. The VSS pulse is based on the speed at which it turns; therefore, the pulse output is based on the ratio of the drive and driven gear. The tansaxle VSS ratios are different. The PCM speed tables are fixed and depend on matching counts from the VSS pulse to interrupt to a speed output.

On my "hotrods" using VSS and after changing final gear ratios, we install an adjustable “pulse modifier" between the VSS and the PCM to control the input pulse to the PCM therefore adjusting the speed shown on the speedometer. If a drive and driven gear ratio was available for the particular final drive ratio, we would just change the gears on the VSS. The factory had a limited number of VSS drive ratios, hence the invention of the pulse modifier.

[maxwedge]

Originally Posted by HotZ28

Actually, I thought that the VSS output pulse was based on the VSS drive and driven gear in the transaxle. If all things are the same between the donor Trans and the original, regardless of Tran’s final ratio, both should have the same VSS signal output to the PCM. The VSS pulse is based on the speed at which it turns; therefore, the pulse output is based on the ratio of the drive and driven gear. The tansaxle VSS ratios are different. The PCM speed tables are fixed and depend on matching counts from the VSS pulse to interrupt to a speed output.

On my "hotrods" using VSS and after changing final gear ratios, we install an adjustable “pulse modifier" between the VSS and the PCM to control the input pulse to the PCM therefore adjusting the speed shown on the speedometer. If a drive and driven gear ratio was available for the particular final drive ratio, we would just change the gears on the VSS. The factory had a limited number of VSS drive ratios, hence the invention of the pulse modifier.

Interesting issue for some research, I know on late model GM trucks we have to reflash or reprogram for larger tires, I replaced a vss on on fwd gm car a couple of years ago, and no issue with gear ratios was ever brought up, so you say there are different vss's for different final drive ratios just like the old speedo driven gears. I thought the pcm was programmed for that cars final dr. ratio and tire size and interpeted the info as programmed and drove the speedo based on that. Good info, Hotz28.

[HotZ28]

When you reflash the PCM with a new set of parameters to interpret the VSS signal, it would have the same effect as changing ratios on the gears, however before OB11 that was not possible.

The newer cars with OBD11 PCM's do accept flash programming; however the tables are still fixed and compare the pulse from the VSS using a new table. Basically all you are doing is downloading a new set of values or tables to the PCM.

The pulse output from the VSS is strictly controlled by the ratio of the drive gears driving the VSS. It can vary from 10,000 pulses per sec to 17,000 or more.

Think of the VSS as being nothing more than an AC pulse generator, the slower it turns, the less pluses per sec and the faster turns the more pluses per sec. The PCM then counts the pulse and compares the pulses to the matching table to interpret speed.

If you find any of the above is not correct, please advise. I have posted strictly from past knowledge & memory here and did not use any reference material or manuals.

Maxwedge, again thanks for your input, it is very interesting, thought provoking and knowledgeable. I know we have gone way beyond what would be considered routine maintenance here and the average Joe would never understand what we are talking about, however, it helps me recall my understanding of how the system works.

[maxwedge]

Ok, cool I was referring to OBD11 vehicles, never fooled with anything prior other than mechanical set ups, so 95 down uses different vss units, makes sense prior to reflash capabilities. Good discussion!!

[HotZ28]

I just need to clarify one thing. The VSS for OB1 & OB11 are still basically the same unit; they are both driven by a set of gears. I assume that the OB1 VSS gears would interchange with the OB11 VSS, so you could still alter the ratio on the new units by changing ratios of the VSS gears and using the existing tables in the PCM. The PCM reflash capability just reduced cost and added flexibility to making speed changes. I have no idea how many pulse tables have been developed for tire & gear changes, however I do know it is easier for a programmer to change a few digits on tables than to change the ratios on the VSS units.

[93buicklesabrelmtd]

Im just going to get my original ratio. Doing want to run into any problems what so ever.