need new speedometer gear

[85md]

I need to replace my speedometer gear. I am confused about which one to use. Tranny is a T-5 5 speed manual. Car is 1985 Mustang GT. All stock. I have never changed the rear gears. I am not sure about the stock rear ratio and how many teeth to choose. LRS has many choices. Please help.
Thanks,
Paul

[86capriASC]

What is in it now? Have you pulled the old one out yet? If so what color is it?

[JACook]

+1. If you know it needs a new gear, you should know what color the existing gear is.
The gear is also marked on the top with the number of teeth, or you can count them.

If you don't know what the existing gear looks like, that begs the question how do you
know you need a new one?

[85md]

just found it. actually the tranny and engine just got overhauled. I have the speedo gear here. 25 years old and worn. It is like a dull purple or maroon color. Does 16R sound right? what is the stock rear gear ratio? Looking at the chart on LRS, they indicate no 16 tooth gear for any GT. Mine is all stock, and I thought it has 2.73 gears. Thoughts?

http://www.latemodelrestoration.com/...lication-Guide

[FoxChassis]

A 16T driven gear would be about right for an '85 T-5 (7T drive gear), a 2.73:1 rear gearset, and factory tires (~815 RPM).

Speedometer Driven Gear = Speedometer Drive Gear x Rear Gear x Tire RPM / 1000

[Str8sixfan]

Alright, need a little help. I had done some of this research back in January, but now I can't figure out how to double check my info to make sure my notes I have written down make sense.

I have a C4, 3.27 gears and a 225/50R17 tire (about 25.86" dia). Came up with needing a 19 tooth driven gear. Does this compute to you guys?

[FoxChassis]

What is the tooth count on the speedometer drive gear in the C4?

[toddf]

lmr has a tooth calculator try that it will let you know which one you need

[Str8sixfan]

I saw that, for whatever reason thought they start at 83, and only list T5s and AODs...my 78 Fairmont is a C4. I'm of the belief the gear is the same for C4s and AODs (looking at CJ Pony Parts website), but I need to find a better way to confirm that than just a year range listing on a part.

[KevinVarnes]

Can you pull your current speedo driven gear and count that and work backwards from the stock tire size and rear end gears and figure out how many teeth the drive gear is? I think C4's could some with several different drive gears.

[Str8sixfan]

Current is Green - 16 tooth. Stock gears were 2.47s. Stock tire size was DR 78-14 (basically a 195/75R14)...but the speedometer was off when that was the setup. When I thought I was going 55, I was really going 65

https://youtu.be/eNudDFqNEIk

[erratic50]

A couple things ...

#1 — looking at the bear in the air white stripes on the highway the cops use to time cars on the roads and write tickets where are you at? Going 60 mph past 2 does it take 1 minute or more or less? Does the car count 1 mile or more or less? That can give clues on what’s going on

#2 - are you running an original speedometer or a gauges pulled from something else. I ask this because some jackass sold me a 140 mph “mustang” speedometer. After looking up the part number it actually came from a Ranger. His loss — its a $500 gauge to the right buyer who wants one, but useless to me really.