How do I adjust or what do I change to re-cal my speedo to function properly if I change my 2.73 rear end over to a 3.27 posi

 

bump

 

There is some kind of gear on the transmission speedometer drive that needs to be change. You have to change to either a bigger or smaller gear. Can't remember which way you have to go when you switch to a lower rear end. I read a tech article somewhere once that told which gear to put on depending on which rear end you have. This tech article tells you how to change it.

https://www.thirdgen.org/newdesign/t...eedogear.shtml

 

What size tire will you be running with the 3.27's? So that we can calculate what gears you need for your speedometer pickup.

Lou

 

I am using stock 245/50/16's I just pulled the cover off the differntial and the gear inside has stamped on it GM14035665 13:42. These are 3:42 gears right? I guess this is the set-up numbers need BigBabyLou. Thanks guys
Tony

 

If the stamped numbers say 13:42, then the rear end ratio is 42/13 = 3.23.

If you currently have 2.73 gears, it is most likely that you are running the 17/38 speedometer gears. With a switch to 3.23's, you'd need to replace the 38-tooth driven gear with a 44-tooth driven gear (#9780470). That's a very easy swap, takes about 5 mins.

However, you need to first inspect your current speedometer gears (especially the drive gear) so that we can be sure that the 17/38 assumption is correct.

Another assumption on my side is that you have the stock mechanical speedometer, no upgrades to VSS.

Hope this helps.
Lou

 

Lou,
How do I check the drive gear in the speedo? I realize to remove black plastic cover, clear lense oner gauges, and then just unscrew the bolts holding the speedo in? It is stock original cluster. I can pull this anytime today so pop me an email back so I can check it. The 44 tooth gear is that still available thru GM? Thanks
Tony

 

No no no, the drive gear and driven gear are in the the rear of the trans. If you follow the speedo cable into the trans, it will screw into the driven gear. One bolt to remove that. The drive gear is inside the tailshaft housing, and requires a little more work to get out.

 

The speedometer gears are actually in the tailhousing of the transmission, not in the actual speedometer (sorry! ).
It is best to go about it this way:
- block the front wheels, put the car in Neutral, jack up the rear and set it on jackstands
- put a drain pan under the tailhousing
- remove/unscrew the speedometer cable from the gear housing on the tailshaft
- remove the gear housing
- some fluid will drain out
- remove the driven gear and count teeth
- take a look inside the hole at the drive gear, note the color
- mark one tooth on the drive gear
- rotate the driveshaft 360° and count teeth on the drive gear
- replace the driven gear and gear housing
- attach speedometer cable
- lower the car
- refill the transmission fluid

Lou

 

Hah, Lo-Tec beat me to it.
Forgot one little detail: the speedometer cable pickup on the transmission is on the driver's side. Might be helpful to know so that you can decide which way to park the car in the garage to get to it best.

 

One more thing. I myself know very well how helpful it can be to know what the heck I'm looking for. So here are some pics to give you an idea.

The gear housing you'll remove first:



The driven gear you will pull out:



The hole with the drive gear inside:



Hope this helps!
Lou

 

You guys are the bomb. That will be a project I will tackle when I have my car up on the ligt Wednesday after next. I figure it will take me allday to do the rear and run all the brake lines and jazz. I will do it on that day. I will post here middle of next week wht it is and also pm you Lou if you dont mind.
Tony

 

I swapped from a 2.77 to 3.27 gears just a few weeks ago. for all intents and purposes, it's the same speedo gear swap.

I have a slight problem with Lou's suggestion to swap in a 44 tooth driven gear and use the 17t drive gear that you currently have. Lou, I could be wrong, and if so, please let me know.

First off, you need a new speedometer gear housing for driven gears 40 teeth and above. That's not a problem, but rather a hassle.....

Here's the problem. My stock 2.77 gears had a 17 tooth drive gear and a 38 tooth driven gear. I also did the calculation and came up with a 17/44 combination for a 3.27 gear ratio. The problem is that the 44 tooth gear obviously has a larger diameter than the OE 38 tooth gear. Well you can't have a bigger gear in place of a smaller gear and have it work. Well, I thought maybe it could, and within 50 miles I mangled the 44 tooth gear.

Here's the solution: Run a 15 tooth drive and a 39 tooth driven. This is also a correct ratio for a 3.27, just like the 17/44 ration, but this combo WILL FIT. I wasn't sure, so i called the tranny guy, here's what he showed me, which in hindsight is painfully obvious:

OE 2.77 gears 17t + 38t = 55 teeth
3.27 gears 17t + 44t = 61 teeth
3.27 gears 15t + 39t = 54 teeth

He said that you want the total teeth count to stay in the mid 50's for a 700r4.

...plus this way you don't need a new gear housing since you're under 40 teeth on the driven gear.

Lou, does seem accurate? I don't mean to come across like I'm showing you up, I'm just telling what I've seen and heard during my quest for the proper gears. If I'm wrong, please correct me.

 

According to the general, 89 with 245/50/16's came factory with a 17/45 combo with 3.27 gears. The part # for the correct gear is 25522497, which comes with the sleeve. However, doing the math with what you've installed, your new combo of 15/39 is less than 2% off, so I would say you are fine.

 

Thanks for pointing that out, Abubaca, I appreciate the feedback. And no, it doesn't upset me ... afterall, it's better to discuss all options and choose the best one than give wrong advice.

So anyway, I had no idea that the 17/44 combo would shred the gears. That's where your life experience comes in handy because the smart GM book says to use 17/44 gears for a 3.23 rear end.

I was also looking at the 15/39 version but didn't want to suggest it because it would require swapping the drive gear, i.e. pulling the tailshaft housing and paying more for another gear. I was trying to suggest the easier way.

Last thing, you are perfectly right about the new bigger gear housing. I totally forgot about that, my apologies. Thanks for reminding me.

So all in all, both ways (17/44 or 15/39) would probably cost the same (44 gear + new housing vs. 15 and 39 gears) but the latter requires a bit more work. Though, if it works better (doesn't shred gears), it'd be most likely the best way to go.

Lou

 

That may be the case, I can't speak on what was originally installed. In my experience, that combo didn't work too well. Regardless of my experience, doesn't it seem logical that you couldn't simply swap ONE gear and have them still mesh properly???

...plus that same logic was confirmed by the transmission guy. Now I can't speak as to his expertise, but his advice, + my shredded 44t gear, + my mechanical reasoning all tells me a 17/44 combo ain't gonna work. If that's what GM did, more power to them, but it didn't work for me.

....I've got a new 40+ housing with 50 miles on it! I'll let it go for $5 bucks plus shipping.

 

I would think you should have been able to swap the one gear and be OK. The only thing I could think of might be a mis-matched housing/gear combo. From 84-92, there are only two different drive gears used from the factory, a 15 tooth and a 17 tooth. There is only one part # for each. However, for the same tooth count driven gear, there are multiple different housing/gear part #'s for different years. For example my '84 takes a 15/45 combo. The part # for the 45 tooth gear is 9775187 and the housing is 25512339. The 45 tooth part # I posted for your 89 is totally different, and includes the housing. Now, what could be difference between them. Trans is the same, trans extens is the same, both hook up to the same speedo cable. You would think GM would use the same part number, but noooo. They seem to be a$$backwards when it comes to certain things, and this is a good example.

Glad you got yours working correctly.

 

I am going through this problem as we speek. if you use a 44 tooth grar you have to change the housing that the gear is attached to. With the correct housing you will not shred the 44 tooth gear. I originaly thought if I just switched the housing sensor I would be fine, but with a 38 tooth gear and the correct gear housing for a 44 tooth gear your speedo will not work. I now have the correct gear orderd, and when I get it I will put it on the new vss housing and not shred gears. I hope this helps!!!! Just my

Mark

 

the speedo gear swap only takes like 5 mins? i paid a transmission shop like 150 bucks to fix my car when my speedometer wasnt working... and i will have to swap it out again soon b/c im changing the rear gear. where can i get the speedo gears from?

 

You can try these places:
700R4, 700R4 Speedometer Gears, 4L60, 700R4 Speedometer Calibrations, 700R4 !
TCI AutomotiveŽ - Speedometer Gears

Or a dealership. But don't be surprised if the price doesn't seem right. They've been known to jack it up unbelievably high because of the "supply-demand" rule. I had to pay $25+ for my T-5 gear, although the auto gears are only $3 a piece.

Hope this helps.
Lou