which speedo gear should i use?

Falcon79

Active Member

Apr 6, 2009

214

8

29

Dallas, TX

Dec 9, 2009

#1

• Dec 9, 2009

• #1

I am switching my 68 coupe 6 cylinder car over to a v8. I bought a c4 to go with my 302. Should i use the speedo gear that came with the new transmission? or should i use the one from the 6 cylinder transmission? I know very little about this so if someone can give me an iea what to do that would be great!

Thanks

 

latamud

Founding Member

Oct 22, 2002

791

2

19

Tampa, FL

Dec 9, 2009

#2

• Dec 9, 2009

• #2

TCI - TECHNICAL INFORMATION: Speedometer Gears

Things that will affect what speedo gear to use:
rear tire diameter
gear ratio
drive gear

 

Falcon79

Active Member

Apr 6, 2009

214

8

29

Dallas, TX

Dec 10, 2009

#3

• Dec 10, 2009

• #3

rear tire diameter: running 17x8 wheels from a 2003 GT
gear ratio: no sure but should be in the 2.79-3.20 range
drive gear: no idea

 

2

2+2GT

10 Year Member

Apr 25, 2009

3,333

10

79

Southeastern Pennsylvania

Dec 10, 2009

#4

• Dec 10, 2009

• #4

You HAVE to know how many teeth the drive gear has before you can proceed. With the speedo cable removed, put a dab of white lithium grease on the gear. Turn the shaft, counting the teeth, until you get back to the dab of grease.

 

D

D.Hearne

New Member

Sep 29, 2000

11,730

6

0

south louisiana

Dec 10, 2009

#5

• Dec 10, 2009

• #5

If the speedometer was dead on before the engine swap, it'll be dead on afterwards. Almost all Ford transmissions used the same number of teeth on the drive gear in the transmission.....7 teeth. The only ones that didn't had rear gear ratios in the 3.90-4.30 range. If you changed the tire diameter, then that is the only thing that will throw off the speedo reading. Going to a shorter tire requires more teeth on the driven gear (one on the end of the cable) Taller tires, requires fewer teeth on this gear.

 

D

D.Hearne

New Member

Sep 29, 2000

11,730

6

0

south louisiana

Dec 10, 2009

#6

• Dec 10, 2009

• #6

All C-4 transmissions have 7 drive teeth cut into the output shaft.