What speedo gear do I need?

[Loco5.0]

I have a C4 3 speed auto w/manual valve body trans in the car w/3.73 gears right now. I am going down to 3.27 gears. Which speedo gear do I need & where can I get it?

 

[TRWXXA]

We can't give you a solid answer with the information you provided.

We need to know hown many teeth are on the speedometer drive gear, machined on the output shaft, and what size tires you are running.

If your speedometer is correct now, tell us how many teeth are on the speedometer driven gear, that's on the end of the speedometer cable. Then we can calculate the number of teeth needed after you change gears.

 

[Loco5.0]

We can't give you a solid answer with the information you provided.

We need to know hown many teeth are on the speedometer drive gear, machined on the output shaft, and what size tires you are running.

If your speedometer is correct now, tell us how many teeth are on the speedometer driven gear, that's on the end of the speedometer cable. Then we can calculate the number of teeth needed after you change gears.

AFAIK it's correct now. I bought the car w/the 3.73s & C4 in it.

I'm running 26" tires. No clue on # of teeth.

 

[TRWXXA]

AFAIK it's correct now. I bought the car w/the 3.73s & C4 in it.

I'm running 26" tires. No clue on # of teeth.

First, determine if your speedo is accurate. Zero out your trip odometer as you pass a highway mile marker. Drive 10 miles and note the odometer indication as you pass the 10th mile marker.

Then pull your speedo cable out of the tailhousing, and count the number of teeth on the gear on the end of it. Also, note the color of the gear.

If your speedo is currently accurate with the 3.73s, then here is the formula for figuring out how many teeth you would need with 3.27s:

# teeth on current gear x 3.27 / 3.73 = # teeth you need

EXAMPLE: 21 teeth on current gear

21 x 3.27 / 3.73 =

68.67 / 3.73 = 18.4

You'd need an 18-tooth gear.

 

[Loco5.0]

I contacted LMR & was told their part# lrs-17271e should work for me, but that gear is listed for an AOD trans (i don't have OD obviously) & nothing for my 26" tires. so i'm not so sure it'd be right for my application.

 

[Loco5.0]

I told LMR C4 trans. The part# they gave me is 19 tooth.

I called c6-c4.com & he told me that if my trans is 7 tooth I need a 18 tooth gear & it it is 8 I need a 20 tooth gear. So now I have 3 different gears here. This is driving me nuts

 

[TRWXXA]

Given that you are running 26" tires, if the C4 has a 7-tooth drive gear, then you'd need an 18-tooth (gray) driven gear for a 3.27 rear end.

If the tranny has an 8-tooth drive gear, then you'd need a 20-tooth (orange) driven gear.

Here is the formula for calculating this:

# of Driven Gear Teeth = 20.168114 / Tire height (in.) x Axle Ratio x # Drive Gear Teeth

20.168114 / 26 x 3.27 x 7 = 17.75

20.168114 / 26 x 3.27 x 8 = 20.29

I'm guessing that the C4 uses the same gears as the AOD, but you should verify this by pulling out your current gear and comparing it to the AOD gears.

If your speedometer is accurate with the 3.73s, then it should currently have either a 20-tooth (orange) gear (7-tooth drive gear), or a 23-tooth (white) gear (8-tooth drive gear).