Yo No Juice, (but soon to be known as having a "Juiced-Up" 86

!)
"92+ use a psom, whereas 91 and back use a mechanical gear and cable.
ADDED NOTE:
Gear ratio will matter if it's a 91 or older..."
"If it's a 91 or prior, then you just change the gear at the end of the speedo cable.%
There's a formula for it, but it's much better to base it off of your current gear.
This is what I did on mine. I pulled the speedometer gear out and counted the teeth and saw that it was an 18 tooth.
I then went for a drive (the faster the better) and got up to 80mph on the
speedometer. I then had a GPS open on my phone (plenty of free apps) that showed that my
actual speed was about 75.
Then:
New Gear Teeth = (Speedometer Speed / Actual Speed) * Old Gear Teeth
New Gear Teeth = (80 / 75) * 18
New Gear Teeth = 19.2 (19 tooth).
This is the simplest way to figure it out because you don't to figure out how many teeth the ring gear inside the transfer case has, you don't need to figure out the revolutions per mile of your tires, and you don't need to take your gearing into account. Plus, tires aren't exactly the advertised size. So a 32" tire (which calculates to 630 RPM) might actually be around 31.5" or 31.25" (645 RPM) in diameter once the weight of the vehicle is on it. I think, with the formula, my speedo gear actually "calculated" to a 20 tooth gear. It would have been off."
"be aware that all AOD and AOD-E/
4R70W transmissions can have either a seven or eight tooth speedometer drive gear machined into the output shaft. Generally speaking, vehicles with 2.73-1 axle ratios may have an eight tooth output shaft. If unsure, it would be wise to remove the extension housing and count the helical teeth. Depending upon tire size used, an eight tooth output shaft may make speedometer correction difficult for axle ratios lower (numerically higher) than 3.27-1. Unfortunately, output shafts cannot be changed without disassembling the transmission.
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"The 4R70W is a transmission with the following features:
- Wide ratio gears
- Four speeds
- Rear wheel drive
- Automatic
- Electronic shift
- Torque converter clutch control
- Line pressure controls
The transmission uses a Ravigneaux-style double-pinion compound gearset to produce four forward speeds and reverse. Two bands, two one-way roller clutches and four friction clutches are used to hold or drive various planetary gearset members.
4R70W Transmission
All vehicles are equipped with a vehicle certification label, affixed to the LH (driver's) side door lock post.
Refer to the code in the space marked TR on the vehicle certification label for proper transmission identification."