speedo gear swapout

[k belknap]

'81 Vette...3 speed w/shift kit. I'm needing to swap out the speedo gear in order for my speedometer to read correctly. Right now it's reading 95 MPH when I'm going @70 at 3500 RPM. Several ?'s:

1) Any guesses as to what gear ratio I have? 2) What all is entailed in changing out the speedo gear in the tranny (can it be done w/the transmission still in the car)?

3) I've been told you can determine the gear ratio by jacking the back of the car up, spinning the back tires one revolution and counting the number of turns the driveshaft makes. Is this true and accurate?

Thanks,

K

[Jack H.]

Re: speedo gear swapout

You're 'close' but you've got it backwards. How many driveshaft revs are needed to make rear wheels accomplish a rev....

Also, prior threads (see archieves) indicated the need to match BOTH speedo cable gears (drive and driven). The driven gear is easy, you pull the speedo cable, remove old and insert new. The drive gear is a BIGGER problem since it's INSIDE the tranny tailhousing and this means drop the tranny and dissasemble to get at it.

Drive gear has to match driven gear and each are color/composition coded with a limited range of mating options. Sooo, check your rear end ratio (actual installed ring/pinion vs. what might be stamped on the housing or indicated by other documentation you have). Then, inspect the drive/driven gear combo in use and check this against the Chevy Parts Book to see if they agree with your current rear axle config.

Also, don't forget all this presumed factory stock wheel/tire dimensions on the car....

[Jack H.]

Re: speedo gear swapout

You're 'close' but you've got it backwards. How many driveshaft revs are needed to make rear wheels accomplish a rev....

Also, prior threads (see archieves) indicated the need to match BOTH speedo cable gears (drive and driven). The driven gear is easy, you pull the speedo cable, remove old and insert new. The drive gear is a BIGGER problem since it's INSIDE the tranny tailhousing and this means drop the tranny and dissasemble to get at it.

Drive gear has to match driven gear and each are color/composition coded with a limited range of mating options. Sooo, check your rear end ratio (actual installed ring/pinion vs. what might be stamped on the housing or indicated by other documentation you have). Then, inspect the drive/driven gear combo in use and check this against the Chevy Parts Book to see if they agree with your current rear axle config.

Also, don't forget all this presumed factory stock wheel/tire dimensions on the car....

[Wayne M.]

Re: final drive ratio (determine)

K.B.-- Jack has outlined the steps to change drive/driven gears, and previous archival posts on this subject. Allow me to guess that you have a 3.70 or 4.11 final drive ratio, depending on tire diameter. You're right about field checking the ratio by spinning the rear tires (jack both sides off the ground), but I would go further and suggest 10 turns on the tire (using valve stem as benchmark) while a helper counts the driveshaft turns (a strip of masking tape helps). This way you count 37 turns or 41 and a bit, and no calculator is required. You cannot (with any accuracy) determine the ratio with one turn of the tire.

[Wayne M.]

Re: final drive ratio (determine)

K.B.-- Jack has outlined the steps to change drive/driven gears, and previous archival posts on this subject. Allow me to guess that you have a 3.70 or 4.11 final drive ratio, depending on tire diameter. You're right about field checking the ratio by spinning the rear tires (jack both sides off the ground), but I would go further and suggest 10 turns on the tire (using valve stem as benchmark) while a helper counts the driveshaft turns (a strip of masking tape helps). This way you count 37 turns or 41 and a bit, and no calculator is required. You cannot (with any accuracy) determine the ratio with one turn of the tire.

[Chuck S.]

Re: chassis paint

Bruce,

When I started the restoration of my 70, I bought a gallon of 1050104. I have beadblasted, primed (PPG DP-90 epoxy primer) and topcoated every black painted bracket, shield, reinforcement, strap, door, etc. on the car. I still have about 2/3 of gallon remaining of my second? gallon, with a few odd and end parts to go.

At the factory, GM applied this paint by the quickest, most efficient method: by dipping. If you examine your black painted parts carefully, you will see "run" marks where the excess paint drained off the parts. If you bead blast your parts, you will begin to see areas along the "bottom" of the part where the last of the draining paint accumulated and surface tension held it there to dry, making it much thicker right at the edge. Whenever possible, you should observe and record the "hanging point" based on the run patterns on the parts.

Priming the parts with epoxy primer prior to top-coating with 1050104 will give the part more "build" and a very slightly different appearance that can be detected if compared side-by-side with an similar unprimed part. In my opinion, any slight variance from the original paint procedures is worth the durability of the epoxy primer.

If you are a PURIST, then ideally you want to dip the parts. You can dip the smaller parts in the gallon can; but most of the parts should be sprayed because the scale required to dip larger parts is prohibitively expensive for the hobbyist. When you spray the parts, if you hang them from the "hanging points", and apply the 1050104 quickly and very wet, you will obtain runs and an appearance of "dip" painting. Rolling is not a good option to obtain the look of "typical factory production". Thinning of 1050104 is not required; it was designed for dipping and is therefore already thinned.

Frames and control arms (both front and rear) and a few small parts on the car were painted with paints different from 1050104. For shark cars, the frame should have a higher level of gloss than 1050104, and the control arms should be slightly more glossy than the frame. I understand that mid-year frame gloss is lower than that of the later cars, and almost exactly the gloss level of PPG DP-90 epoxy primer. The best way to match the frame finish is to observe the original frame paint in a relatively sheltered area of the frame, and experiment using acrylic enamel (PPG DMR-490) with a flattener to duplicate the gloss level. Make the control arms slightly more glossy than the frame. Working in small batches, this process is tedious and exacting, and frustrating in that the results may not repeat exactly in larger batches.

Chuck Sangerhausen

[Terry M.]

1970 Chassis black out - Not *NM* *TL*