Neato Speedo

01 hope you can help me with this question. I had a 700-R4 trans (from a '91 Chevy pickup) installed in my '68 Camaro. The trans has wires coming out for the speedometer hookup. I am wondering if I could convert this to accept the cable for the Camaro speedometer, and if so, how do I go about it?

Joe Marino

Via email

A Rather than the standard cable-driven speedometer gear used on 700-R4s in 1982 to 1986, the later version of the 700-R4 (and its titular morph into the 4L60 from 1990-1992) used a Vehicle Speed Sensor (VSS) to more accurately sense rpm and convert to driveline speed. On some of these later 700-R4s the VSS is actually still gear driven like the earlier cable, but most versions use an internal reluctor gear pressed onto the output shaft and a magnetic pickup. We couldn't find a reliable vehicle-specific breakdown for the two different VSS versions, but for your purposes it won't matter, since fortunately either can be easily retrofitted to a cable drive with a little parts swapping.

The gear driven VSS, as is often found in third-gen F-bodies, is the simplest to work with. The drive gears are the same for mechanical or electronic speedometers with three options: gray 15-tooth, red 17-tooth, and blue 18-tooth. If your transmission has one of those three, retrofitting is as simple as removing the VSS and installing a speedo cable with the proper driven gear in its place.

Since your trans is from a '91 Chevy pickup, it's more likely that it has the more common reluctor setup. While the VSS unit can be removed simply by unbolting the retaining tab and sliding it out, the reluctor itself is located on the transmission's output shaft, which means the tallhousing will need to be pulled to remove it. The reluctor is press fit, so a gear puller will be required. GM retained the same output shaft throughout the 700-R4 life, so there will be two holes in the shaft for the retaining clip for the drive gear. Standard tailhousings will use the front hole for proper gear position; the rear is for Corvettes.

As for the parts, TCI Automotive carries everything necessary for the swap, and Bowtie Overdrives has conversion kit PN CS4045700. PA TC Transmissions has a unique kit (PN SW41) that converts a 700-R4/4L60 to a standard GM manual speedometer and VSS in the same tailhousing.

Since this is a custom application, the next item on the agenda is determining what gear combination you'll need to get the correct ratio to accurately drive the speedometer. The size of the drive gear will be determined mostly by the relationship of your tire diameter to your gear ratio, since bigger gears or small tires will necessitate a smaller drive gear to compensate. TCI has a convenient calculator on their website that will do the math for you, but we'll use the standard formula for an example:

No. of drive teeth * axle ratio * tire revs per mile 11001 = No. of driven teeth required

To determine the tire revolutions per mile we use:

20168 / tire diameter (in) = tire revolution per mile

Let's throw a few hypothetical numbers in there. TCI typically installs a red 17-tooth gear in their Street Fighter 700-R4, so we'll use that as an example combined with a 3.42 axle ratio and a 26.5-inch tall tire:

From there we plug in that number:

700-R4 Speedometer Gears Drive Gears:

15 teeth, gray

17 teeth, red

18 teeth, blue or brown (truck)

19 teeth, yellow or dark aqua (truck)

20 teeth, brown or silver (truck)

21 teeth, red/chartreuse (truck)

22 teeth (truck), gold

23 teeth (truck), maroon

24 teeth (truck), dark blue 34 teeth, light green

(Data courtesy TCI Automotive)

Driven Gears:

36 teeth, white

37 teeth, red

38 teeth, blue

39 teeth, brown

40 teeth, black

41 teeth, yellow

42 teeth, green

43 teeth, purple

44 teeth, dark gray

45 teeth, light blue

Cutaway of a 700-R4 tailshaft showing the speedo drive and driven gears. Photo courtesy PATC transmission.

holley.com | Techline: 27D-781-9741

The Cable X conversion is the easiest way to adapt just about any mechanical speedo to a transmission with an electronic speedo output. Photo courtesy Abbot Enterprises.

That's the tried-and-true traditional method of converting from VSS to cable, but if you're not keen on the idea of swapping out the necessary parts, the easy way out comes via a signal converter. The Cable X from Abbott Enterprises is a universal adapter that converts the electronic speedometer signal into a mechanical output that correctly drives cable input speedometers. Installation only requires routing 12-volt power to the box, as well as ground, and a single pickup wire from the trans. You can find it at the company's website, www.Atrol.com. ijji

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