BILLY 63 swc

I had a 4:11 rearend and installed a 3:36 for better driveabilty. Problem is the Transmission has a 20 tooth gear for the 4:11 and I need to go to a 18 tooth gear for the 3:36 for accurate MPH reading. The book says the metal drive gear in the transmission has to be changed if you go to the 3:36 from the 4:11 to match up with the 18 tooth plastic gear. Is there any other way to do this to get the right MPH or Just live with the 10 MPH to slow a reading untill I have to remove the transmission at a later date for whatever repairs that it may need at that time and install the larger metal drive gear then.
Thanks
Billy

Plasticman

Another option is to use a speedo gear box adapter. It is a small gear box that couples directly where the cable goes now, and then the cable connects to the adapter output.

They come in many different ratios, and should be available at an "old time" speedo shop for about $100. I got one for Evergreen (fellow Vette forum guy who resides on an island off Alaska) a couple of years ago. I am in the process of moving from IL to FL this coming week, so please don't ask for me to get you one!

Plasticman

EVERGREEN

Yeah John got me a "ratio box" a couple of years ago when I went from a 3.36 to a 4.56 (sort of just the opposite of yours) and the speedo reads pretty close (within about 3mph of the cops radar gun). Its just a 90* box that fits on the output of the speedometer gear on the tranny then the cable goes on the end of it. Don't remember the ratio change inside the box but perhaps someone on here could chime in with the math? The best part is the fact that it eliminates that awful bend in the cable!
EVERGREEN

BILLY 63 swc

With the limited room avalible behind all the shifting rods on a 63 T-10 transmission I would think a gear drive unit would not fit to good. The late 65 and later have the output speedometer drive on the right side of the transmission and would be no problem. Have you installed a gear drive unit on a left side output and had room to do so on the 63-65 early cars? Thanks
Billy

magicv8

WTF?

My parts manual shows the following gears:
ratio__tire size___drive gr_____driven gr_teeth_color
2.73__8.15-G70__3895494____3987918_18____brown
3.08__7.75-15___3924098____3987918_18____brown
3.31__8.45-G70__3924098____3987919_19____natural
3.36__8.15-G70__3924098____3987920_20____blue
3.55__8.15-G70__3924098____3860344_21____red
3.70__8.15-G70__3924097____3860345_22____green
3.70__8.45-_____3924098____3860344_21____red
3.73__8.15-G70__3924097____3860345_22____green
3.73__F70______ 3924097____3860348_25____orange
4.10__8.15-G70__3924097____3860347_24____yellow
4.11__F70______ 3924097____3860348_25____orange
4.56__8.15-G70__3924097____3860345_22____green* uses 3890575 adapter

The 18 tooth gear is for 3.08 ratio diffs. What wheels and tires do you have?

EVERGREEN

Don't know what your spacing is on the 63 as mine is a 61 with a T-10. Had to drop the crossmember to get enough room to work the wrenches, also the reverse arm and linkage. Once the box is mounted it is just like the cable, the box is out away from the tranny and the linkage just barely clears, at least on an old dog like mine! Another good thing about the box is if I ever change back to the 3.36 all I have to do is remove the box and hook the cable up to the tranny!
Have the name, address, and phone # of the place that John got mine from if you need it.
EVERGREEN

BILLY 63 swc

Magic-Nice info,thanks. I got my #s wrong in the first post. Must have had a brain fart. I removed a 25 tooth and want to install a 20 tooth. I have Goodyear Eagle GT-HR 215/65R/15. OD is 26". Would the 20 tooth be correct?
Evergreen- Thanks for the info also. Yes I would like the #s to get hold of someone to supply the part I need. Thanks
Billy

EVERGREEN

Speedometer & Clock Co.,Inc.
2337 S. Michigan Ave
Chicago, IL 60616
Phone # 312-842-6378
My ratio box #1565203
0.750 ratio for 3.36 to 4.56 rear gears
EVERGREEN

newbe60

Going to a HEI tach drive distributor for a 1960 and would like to replace the generator with a 1 wire 100 amps alternator .. so i understand if i connect the tach to the HEI tach drive distributor it will read 1/2 what it is suppose to.. i think i need a box or somthing that will 2x the reading i ? Do they make such an item or do I have to take out the entire dash and recalibrate the tach unit?

JohnZ

If you're switching your tach drive from the original generator-driven cable to a distributor-driven cable without changing the tach, you need a ratio box that doubles the cable speed; otherwise you need to have the guts of the tach changed from "Reads cable speed" to "Reads 2X cable speed".

Before you buy a 1-wire alternator, you might want to read this:


http://www.madelectrical.com/electri...evymain1.shtml

JOliver

I don't know if there are any boxes out there to do the 1:1 or 2:1 conversion but if you take the Tach out you can fix the problem by simply recalibrating it...This is probably the most inexpensive way to go.

Jason

magicv8

billy - "I have Goodyear Eagle GT-HR 215/65R/15. OD is 26". Would the 20 tooth be correct?"

27standard/26new tire height is 1.0385 which means your speedo cable will turn 3.85% faster. To slow it down (assuming your speedo was correct with a 25tooth gear and 7.75x15 tires) you would need 3.85% more teeth on the driven gear - or a 26 tooth gear to accomodate the tire change.

Working from that point - switching from 4.11 to 3.36 ratio diff gives a ratio 4.11/3.36 requires 22.3% fewer driven gear teeth to speed up the cable via only driven gear change - or - 21.26 teeth on the driven gear.

The 3924097 drive gear does not have a 21 tooth driven gear. If you install the available 22 tooth gear, your originally accurate speedometer will read 3.4% too low.

I would change to the green gear before tearing out the trans and changing the drive gear. The tire heights are always suspect, and a reality check with the new rear and the small tires is in order.

My memory tells me the number of teeth on the drive gear 3924098 is not the same as 3924097. The driven gear teeth will have to be recalculated to accomodate that change, if you make it.

IMO, if you have a 3.8% speedo error after tests, the easiest change is to get 25.125 inch high tires on the car.

BILLY 63 swc

Magic
Wow what math and good info. I can live with 3 or 4 MPH to low a reading before I would remove the transmission. I will change to the green gear and give it a run. Is this a great forum or what. Great info and great people.
Thanks
Billy

magicv8

Do not worry about your difficulties in mathematics; I can assure you that mine are still greater" - Albert Einstein