Transmission / Overdrive: Speedo gear ratio for 700r with 3.31 rear end

[Bullet]

I know someone has done this. I am trying to determine the correct speedometer gear to put in the 700r to drive the speedometer. I have a 39 tooth in it now and the speedometer does not read until about 40 mph. What have you found that works. This is for a 64 Cruiser, with a TT rear end, 3.31 gears. Tires are 25.5 in tall

Thanks in advance.

Mark

[JoeHall]

According to my notes, I am running 44 & 17 tooth gear and pinion in the wife's 63GT, with 700R and 3.31 rear end. It has 205/75, 15" tires though, which I believe are 27" tall. So you may need an 18T pinion and/or 43T gear for your 25.5" tires. Mine puts the odometer at 98 miles indicated, for 100 miles actual. So it is within 2 percent. I dialed the speedo in for speed adjustment, by tweaking the hairspring behind the needle.

[mbstude]

I dialed the speedo in for speed adjustment, by tweaking the hairspring behind the needle.

Joe, how did you do that? How much adjustment is there?

[JoeHall]

Joe, how did you do that? How much adjustment is there?

I would not know where to begin on a "Cylops eye" speedo, such as the one on your 56, but the Hawks are a piece of cake: Remove the bezel and crystal, then remove the speedo assembly from the housing shell. At that point, the hair spring, and how it works is easy to see. It is adjustable, kinda like the hairspring on a pocket watch. I adjust it, then hang the speedo under the dash so I can see it while driving, then go for a drive. I have used a stop watch and mile-markers, but nowadays, a GPS would be even easier. Keep playing with the hair spring adjustment till is within 1-2 MPH at 60-70 MPH, and then reassemble the speedo and put it back in the dash. I calibrated the 56J speedo in this pic many years and many miles ago. Was just playing with the GPS, and took this pic.

[JoeHall]

Most OEM Stude speedos, at least the Hawks, are extremely optimistic, i.e. an indicated 120 MPH is usually closer to 100-105. Calibrating as above, the 120 MPH Hawk speedos are pretty accurate up to about 70 MPH; the 160 MPH speedos (at least 56Js') will remain accurate up to about 80 MPH. As the above pic shows

[mbstude]

Thanks Joe.. I may just give that a try. Next to a GPS, the speedometer in the '56 Prez reads a consistent 10 mph fast. It could simply have the wrong gear on the end of the cable, but adjusting the speedometer sounds like a fun project.

BTW.. The Cyclops speedometer works the same as any other, except the "guts" are mounted sideways with a dial that rotates instead of a needle. I replaced (and serviced) mine a couple of weeks ago so it's all still fresh in my head.

[Bullet]

Thank you Joe. That's what I will try.

Mark

[mbstude]

Dug out a spare speedometer. Looks simple enough to adjust.. Thanks again for the tip, Joe.

[wittsend]

Hi Mark,
Go to this site and scroll down to the "Speedometer Gear Calculator." http://www.tciauto.com/tc/speedometer-gear-calculator/ Enter the Drive teeth number (the gear on the transmission output shaft), the rear axle ratio and and your tire diameter. This will then tell you what Driven teeth number (gear on the removable speedometer cable housing) you need. If you don't know the Drive gear tooth value you will have to remove the speedometer cable housing and look into the cavity with a flash light. Don't worry about counting teeth, they are color coded. See attached image (or direct link http://67-72chevytrucks.com/vboard/s...d.php?t=322481 ).

As an example I have a Red = 17 tooth Drive gear, 3.07 rear gears and 26" diameter tires. The calculator says 40.44 and I round that down to a 40 tooth (Black) driven gear. It has worked rather accurately.

Do note that there are two speedometer gear housings. One for 34 to 39 tooth Driven gears and often referred to as the "small" housing and one for the 40 tooth and up Driven gears called the "large" housing. Hopefully you can find what you need compatible with whatever Drive gear you have. Otherwise you will have to remove the tail housing to swap out the drive gears. My hope is that you won't have to change the drive gear. I did however have to go to the larger housing for the 40 tooth gear I needed. My recollection is that (new) the housing was between $65 and $100 and the gears about $20. CASO as I am I got the larger housing from Pick Your Part WITH the 40 tooth gear for $5. The downside of that was crawling under a number of 700R4 equipped vehicles to find the large housing... and then the black 40 tooth gear.

Lastly you say you don't get a speedometer reading until 40 MPH (does it jump from 0 to 40?). That would seem to indicate more a problem with the speedometer (or cable) - I'd think.


Update : I played with the calculator. If you have the Blue (18) or Yellow (19) Drive gear you will need to swap the Drive gear as there is no Driven gear available with your axle ratio and tires. If you have a Gray (15) Drive gear or the Red (17) Drive gear the following combinations will work:

1. Gray (15) Drive / Brown 39 Driven (39.2 total value)

2. Red (17) Drive / Dark Gray 44 Driven (44.45 total value). And these are the same results Joe Hall mentioned above.

Note: Total value means the complete math but since gears only come in whole tooth numbers ... .

If you find you have the Gray Drive gear then you already have the right gears (given the Brown 39 toothed gear you already mentioned). If you have the Red Drive gear you will need a large housing and the 44 tooth Driven gear (Dark Gray).

Tom