Drivetrain Speedo is fast

[The Shadow...]

Hello, I recently got a 89 convertible with a 331 track heat intake, afr heads, 70mm tb, c4 reverse valve body. I'm not sure of the rear gears without looking or trying to rotate tires and counting drives haft revolutions. The speedometer is about 15mph fast. Any ideas what gears I may have, or should I pop a cover.

 

[General karthief]

What is the hole in the floor by the tail shaft housing?
There is some math that could get close but need to know more. The easiest way is to do the drive shaft count.

 

[The Shadow...]

I believe it's for the b&m shifter, I only got the car in September after my father passed away. I was always a gm guy. So I'm learning Ford now.

 

[jrichker]

@The Shadow...

Find out what rear axle gearing you have.

This takes 2 people to do it right...Put the car's rear end on jackstands high enough to crawl under the car. Put a paint mark on the driveshaft and on the bottom of the tire sidewall. Have a helper rotate the wheel 1 turn while you count the driveshaft turns. The driveshaft will make many turns when the wheel makes 1 turn.

2 /3/4 turns = 2.73
3 turns = 3.08
3 1/4 turns = 3.27
3 1/2 turns = 3.55 (best overall for street/strip/highway for a 5 speed car)
3 3/4 turns = 3.73
4+ turns = 4.10
4 1/4 turns = 4.30

See https://lmr.com/products/79-93-Mustang-Transmission-Speedometer-Correction for speedometer connection gears.

 

[Mustang5L5]

You’ll want to pull the VSS sensor out and see what gear is currently on there. Also you can figure out the rear end gesring by marking and counting rotations.

Also, the wire that normally plugs into the VSS sensor is not present. You’ll want to locate it and plug it in. Without that signal to the ECU, the engine can sometimes stall when coming to a stop

 

[Olivethefet]

In the picture of the transmission there is a plug hanging to the left in the foreground. Could that be the VSS? It looks like some wires have been cut. At least the green wire with the white stripe.

 

[Mustang5L5]

VSS plug is flatter.

I think that's the reverse light plug, or the NGS plug. Either way...all that should be addressed

 

[The Shadow...]

what's the ngs plug? color of the vss plug?