Changing speedometer gears in an M22?

[bobs409]

Anyone know how hard it is to change the speedometer drive gear in an M22? (the one inside the trans, not the driven gear that the cable attaches to)

I did the calculations at: http://www.stl-vettes.com/65Vette/GearCalc.htm and according to that, when I switch to a 3.55 rear gear, I'm going to have to change the gear inside the trans.

It came with a 1.84 large 8 tooth gear but I'll have to switch to a small gear to make this work.


I've done them in TH350's but never a 4 speed.

 

[chevymusclecars]

Bob

It is not difficult to make the change but it does require that you remove the transmission, drive out the pin for the reverse lever, remove the side cover and then the bolts for the tailhousing. When it slides back the pin that the reverse gear rides on will pull out and you will need to watch the washer that controls end play. Once all that is done you can use a bearing puller to remove the speedometer gear and reverse everything to reassemble. If the large gear is good though 3:70 why is it necessary to change the gear? There is a gear reducer available when going to the gears lower than 3:70 which is what I used in the past?

Bill

 

[bobs409]

Using that calculator, it appears the large drive gear is only good up to 3.31 rear gear. Maybe there is an error in that calc?

 

[64ss409]

When I tried it, it looks like that drive gear should work on anything from 3.31 to 3.73.

 

[bobs409]

Try putting in 24.75 for the tire diameter. That's what mine are. It then says it won't work with the large gear.

 

[62impala409]

Bob, Having trouble digesting your tire dia. Common tires dia. on these cars is 28-29". You are not running the LOWRIDER TIRES that they put on those jumping bean cars are you? Leo

 

[MRHP]

24.75 tire? What size is it? That is a tiny tire.

 

[SSpev]

So Bob, are you running a 195/70-14? I guess it could be a 215/70-13. Maybe an F size?
Sounds like an awfully small tire for such a beast.

If you have tires sized this way, do a calculation.
Diameter = [(wide x %ratio x 2)/25.4 ] +rim
[(195 x .70 x 2 ) /25.4 ] + 14 = 24.75

 

[bobs409]

They aren't tiny tires, they are 215-70R14's I checked again and they are 24 3/4 tall.

A chart on the web lists this size as being 25.85 tall but mine sure aren't.


Here's what they look like on the car:

 

[bobs409]

Coker website says the SS/396 cars came with 7.74-14 tires. (bias ply) Not sure how tall those would be but probably ugly tall.


Bob

 

[SSpev]

Did you measure the tire on the car? Is the tread worn down? You should measure with the tire flat on the floor against a wall and lay a long 2 by next to the tangent of the tire, opposite the wall with both ends of the board equal distance from the wall. This would be the diameter. it won't be exact. Tire pressure, tread wear, defection(aged), can effect the measurement. I wouldn't rely on a measurement.

Hear is a good tire size chart.
http://clubs.hemmings.com/clubsites/chevylist/tech/tire_size_conversion.htm

 

[BubbletopMan]

calculating tire size

Bob, when I have used that speedo gear calculator before and I didn't have the height of the tire readily available, I have used the tire size calculator on Mark Williams Enterprises' website. It's under the technical section in the sub-heading "calculator". There's several different calculators in that section, just look for the tire size calculator. Plug in your 215/70R14 number and it will compute tire height, width, and rollout. The times I've used this calculator combined with the speedo gear calculator, I've gotten within a mph or two, which you won't notice anyway. I'd do it myself, but I'm working off of my Blackberry today and I don't have a Flash Player on here. Use the numbers generated from that calculator and see if u can avoid cracking open your RockCrusher! Jeremy

 

[bobs409]

Thomas, I did measure on the car, that must be where I'm making a mistake. Tires are like new but I can see how the weight of the car and/or low air pressure could throw this off.

I'll take one tire off later and measure the way you suggested. I bet your right!


Jeremy, I did as you mentioned and that site is also saying a 215/70R14 should be 25.8 tall I then checked this on the speedometer gear calculator and if that's the right size, I can get away with a silver 22 tooth!


I'll let you guys know later what I find...


Thanks,


Bob

 

[chevymusclecars]

Bob

Here is the website I have always used to determine the gear for my cars. I know it is pontiac but the GTO also used a muncie.

http://www.teufert.net/speedo/speedo.htm

Bill

 

[bobs409]

Ok, I took a tire off and put it against the wall and using a carpenters square, I got a measurement of 25 5/8" Since these tires have 10,000 miles on them, I guess 25.8 would be correct for a new tire.

So that makes me very happy in that I don't have to tear my trans apart to change the gear but now a new problem.... today I decided to swap the driven gear to a blue one as the one I had in there was reading too slow.

What I found is the teeth on this one are chewed up. The first one I had in there looked the same way. :eek: Both were fine when I installed them.

Something must be wrong somewhere. So I have a brand new blue one in there now. I'll be taking it for a drive tomorrow so will have to see if the speedometer works properly.

I looked into the hole with a bright light and the teeth look ok on the drive gear.

Any ideas on why it's chewing up gears? I know I have been installing them correctly, they really only go one way. ????

 

[chevymusclecars]

Bob

Isent the parts pages to your email so you should be able to tell from that if you have the correct combination.

Bill

 

[62impala409]

Bob, You might be installing auto trans driven gears on a manual trans. According to the chart in the previous post the auto DRIVE gear is 18 tooth and the manual trans DRIVE gear is only 8 tooth. As a result the auto driven gears have approx. 40 teeth verses 18 teeth for the manual trans driven gear. Leo

 

[bobs409]

Thanks for sending those Bill. I'm trying to figure out what I need from there but it's confusing. I only see the drive gears shown, no driven gears.

Leo, your probably right! I didn't know there were different driven gears for a 4 speed & auto. These gears I had are from a TH350.

I'm getting my butt kicked on this one.


Ok, I'm just going to ask and avoid more headaches. What gears do I need? I'll let the experts figure it out. LOL

Here's the specs:

M22W with a 27 spline output shaft. The drive gear in there is a 1.84 large according to my reciept. I can only assume it's part #3708144

What I need is the correct driven gear (with part number hopefully) to go with this for a 3.55 rear with 25.85 tall tires.

So who's up to the challenge?

 

[64ss409]

Maybe this chart will help.
http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm

 

[bobs409]

Ok, I'm not seeing a different driven gear being used for a Muncie 4 speed over a powerglide or TH350. Even the chart Ron just mentioned shows them all in the same list.

I'm getting more confused as I go.

Here is a pic from a website that deals with Muncies. These plastic gears are the type I've been trying. So far, I tried the brown one and the purple colored one next to it. Both got chewed up. I have a blue one in there now but it will most likely get beat up too.

According to all the charts I've looked at, I should be able to use a 17-22 tooth plastic gear for this 1.84 8 tooth driven gear. (which all 3 colors fall into)


HELP!!!!!