Speedometer Gear Questions...

[MrSony]

If I change the driven gear (small one), do I have to change the drive gear (on tailshaft) as well? TCI's chart isn't really that clear. I have no idea what my drive or driven gear tooth count is, I guess the only way to find out is to look.

Anyone know what gears I'd need for a 3.42 rear, 26.10 (235/60/15) tall tire?

Thanks for any input.

Edit: since I started typing this, I took an internet break and dropped my trans for the converter swap. I have a red driven gear, which I think is 37 teeth... or 21, at least according to TCI. I don't know if TCI goes by GM factory colors or their own.

 

[Bonnewagon]

The drive gear may or may not have to be changed depending on by how much the driven gear changes. IIRC the driven gear housing should have a couple of numbers on it and they are the range of gears that fit that particular housing. I have an old Buick factory manual and it has charts for the gears for the ratios but only goes to 3:23. This may help you better. http://www.wallaceracing.com/speedo-calculator.php I find that by buying a handful of driven gears I just swap them around until I get the right speedo reading. I have a friend drive ahead of me and when he hits 60 mph he puts his flashers on. I adjust from there. EDIT: tell me what your original set up was and I may be able to tell you what the drive gear is. I need to know original rear gear ratio, and original tire size.

 

[MrSony]

The drive gear may or may not have to be changed depending on by how much the driven gear changes. IIRC the driven gear housing should have a couple of numbers on it and they are the range of gears that fit that particular housing. I have an old Buick factory manual and it has charts for the gears for the ratios but only goes to 3:23. This may help you better. http://www.wallaceracing.com/speedo-calculator.php I find that by buying a handful of driven gears I just swap them around until I get the right speedo reading. I have a friend drive ahead of me and when he hits 60 mph he puts his flashers on. I adjust from there. EDIT: tell me what your original set up was and I may be able to tell you what the drive gear is. I need to know original rear gear ratio, and original tire size.

Original was 2.41x195/75/14.

 

[Bonnewagon]

My chart does not show a red driven gear with that combination. It could have been changed somewhere. What it does show is a purple drive gear (10 teeth) with a blue driven gear (20 teeth) ; a green drive gear (9 teeth) with a brown driven gear (18 teeth) ; a red drive gear (17 teeth) with a LT green driven gear (34 teeth) ; a blue drive gear (18 teeth) with an orange driven gear (34 teeth). can you peek inside the gear housing hole and see what color the drive gear is? Knowing that will make it easy to pick the proper driven gear for the 3:42 gears. I do see a red driven gear with 37 teeth used with a red drive gear with 17 teeth used with a 2:41 rear and 225-70-15 tire in "B" body cars.

 

[MrSony]

I took the extenstion housing off, the gear appears to be an brown one with 20 teeth, which coincided with the TCI chart.

 

[MrSony]

Also, this transmission is a TH350 from a firebird, or so I'm told. Has a brown 20 tooth drive gear and red... something tooth driven gear. I can't get the housing apart.

 

[Turbo Zach]

I used one of these sets, but can't remember witch. My speedo is dead on.

 

[Ribbedroof]

I have always used the following for the small type driven gear:

To slow down the indicated speed by ~ 5 MPH, add one tooth on the driven gear

To increase the indicated speed by ~5 MPH, subtract one tooth on the driven gear.

Has always gotten me really close. I have never had anything with the large driven gear that needed to be changed, so I have nothing to add for those types.

Edited to add that the above is applicable at approx 60 MPH

 

[MrSony]

I have always used the following for the small type driven gear:

To slow down the indicated speed by ~ 5 MPH, add one tooth on the driven gear

To increase the indicated speed by ~5 MPH, subtract one tooth on the driven gear.

Has always gotten me really close. I have never had anything with the large driven gear that needed to be changed, so I have nothing to add for those types.

I do know at 85mph, I'm doing 60mph. At 5mph, it's 3mph.

 

[Bonnewagon]

Well with a brown 20 tooth drive gear the Wallace calculator indicates that you would need a 46 tooth driven gear. My book doesn't list one but there is a LT blue 45 tooth gear. Look at the calculator and plug in various gears that are commercially available and see what you can come up with. http://www.wallaceracing.com/speedo-calculator.php