Speedo gear hold down bolt size?

[Nathan in MI]

I finally got the right (or at least close enough) speedometer gear for my car, but now that I'm at school, I can't be right next to my tools while I'm working on the car, so I need to know in advance, what's teh right size wrench for the little hold down for teh speedometer gear? I changed it once before but I can't remember what size it was, now.

Thanks!

Nathan

[Tiggie]

It's a small metric if I remember right. Dunno exact size. I'm not much help...

[gadget73]

I think its either 8mm or 10mm. Just take the 1/4" drive ratchet set and you're good.

[gabegt90]

Yeah I think it was an 8mm also might want to put some rtv around the plug when putting the speedo back in the shop manual said it might leak if the o ring is bad

[Nathan in MI]

Well, I made up for knowledge with quantity. Just ran over to the Heavy Equipment center and grabbed a slug of different sizes, knowing that one of 'em would be right. I'm guessing it's actually about 11 mm since 10 was just a wee bit too small, but I didn't have an 11 with me. Turns out, though, that 7/16 is pretty much a perfect fit for it.

Anyway, thanks for the info, guys.

Oh, and I would have RTVed it before I stuck it back in (I did last time), but I didn't have any, so I just stuck it in there and now I'm hoping it doesn't leak. Stupid car has so many leaks on it already I'm not sure I'd realize it if it did leak, though. lol

[Nathan in MI]

Here's a small quandary, though--can someone explain to me why it is that with an '89 transmission, 3.55s, and stock diameter tires, I need a 21 tooth gear to make the speedo read right rather than a 20 tooth gear?

[gadget73]

cuz it are the way it are.

http://www.sccoa.com/faq/speedgr.html

usually you're gonna get a 21 or a 19, depending on the driven gear in the tranny. Sometimes if the speedo cable is going bad it can make the thing read a bit funny, or so I'm told.

[85crownHPP]

i found a purple gear at the junkyard if anyone needs it...

[Nathan in MI]

Well now I'm even more confused than I was. Thain, that site you posted says '89-'93 AODs use a 7 tooth drive gear, and '94-'95 AODs use an 8 tooth drive gear. With a 7 tooth, which according to that info mine should have, 3.55 rear gears, and stock diameter (26.5") tires, that calculator says I need a 19 tooth gear.

This site says that pre-'90 AODs use a 7 tooth drive gear, and '90+ AODs use an 8 tooth drive gear. With a 7 tooth gear and 3.55s, I'm supposed to need 20 teeth.

In reality, I needed 21 teeth.

Nathan says...

WTF, mate?

[85crownHPP]

Drive Gear Teeth
NOTE: SC manual transmissions (M5R2) '89-'95 and automatic tranmissions (AOD) '89-'93 use an 8 tooth drive gear. SC automatic transmissions (4R70W) '94-'95 use a 7 tooth drive gear.

Thaine's site says the AOD use 8 tooth and the 4r70w use 7 tooth mang

[Tiggie]

I don't ask questions on speedo gears anymore. Somehow mine is slow as balls (about 8MPH slow) after the tranny swap. With 3.08's and an 18T gear, it should be right. I think my new trans has a 7T drive gear, so that could be my problem.

You got it right now and that's all that matters.

[Nathan in MI]

You got it right now and that's all that matters.

Yeah, I know. Still bugs me, though. lol

[gadget73]

i dunno, its usually orange or purple. not sure when they made the switch but 88+ trannys seem to use a purple, older ones use an orange. also, here's something else to make your head spin. My last trans was a reman from a 1986 LSC. No idea what the trans was originally in, but the car came with a 3.27 rear and a purple gear. I used that same purple in my car with the 3.55 and the speedo was right on. Now I have an 86 LTC transmission, and it uses the orange.

[Nathan in MI]

I think the only good conclusion we can draw here is that the best way to figure out what gear you need is to swap rear end gears, keep the original speedo gear, check your error, figure out what the percentage is, and then use that to determine the number of teeth you need.

For example: When I switched from 2.73s to 3.55s, the speedometer was around 23.5% off. I had a 17 tooth driven gear, so 23.5% of 17 is 4; 4+17=21. That's the way I should have done it to begin with.

But here's another odd little layer--2.73 to 3.55 is a 30% change in ratio, so why would it require only a 23.5% change in speedo gear?

[1987cp]

Here's a small quandary, though--can someone explain to me why it is that with an '89 transmission, 3.55s, and stock diameter tires, I need a 21 tooth gear to make the speedo read right rather than a 20 tooth gear?

Did I mention that I had to change my speedo gear on the Colony Park even while running the factory-stock rear gears? I really don't think my 28" tires are *that* much taller than the 27" tires I used to have.

Right now I'm looking around for info on C4 speedo gears. As luck would have it, the C4 uses a different style speedo gear than the AOD (I checked before getting the different gear for the Colony Park). The green gear that came out of the 2.26-geared Country Squire has 16 teeth (green), so I presume that for the 3.08 rear I need to find one with (3.08/2.26)*16 = 21.8 ---> 22 teeth (tires are very close to stock height). Now all I need to know is who or where has one .......