Technical Replacing a Speedometer gear

I am in the process of replacing the gear on my 51 Dodge truck. It has a 700R Chevrolet transmission. My question is will I need to drain some of the transmission fluid before I pull the gear? Thanks Joe

 

No just put a drain pan under it won’t losses hardly any if it’s at proper level. Extra insurance jack the rear end up some.

 

Carpok thank you

 

No problem just went thru all that change the drive on the output shaft and the driven gear. Several times to get the right combo. There’s two separate housings for the driven gears depending on the tooth count.

 

If you haven't done so and your speedometer works but is off by ____ mph take a run down the road using a GPS or GPS speedometer app on your phone and figure out the exact difference and percentage of difference. Then you stand a far better chance of getting the correct drive and driven gears the first time.
I'd imagine that you have already done that and have it figured out though.
I've used this site a number of times to figure out drive and driven gears on various transmissions over the years. Speedometer Drive Gear, (transmissioncenter.net)

 

I have done that. I may have the speedometer with the most difference in speeds than ever. I used my 51 Dodge truck speedometer to a 700R and when it reads 72 I am going 55. I cannot tell what it reads when I am going 65 because it is pegged!

 

I am only assuming that I will need to evaluate the gear once I take out the existing. With such a big difference I am not sure I will be able to synchronize the speedometer to the correct gear.

 

You might be OK. By mix and matching the various TH700 drive/driven gears, you can get a ratio from between 0.1778 and 1.0556 (88% range).

Cheers,
Harv

 

Parts to fix your speedo are on Amazon. Very good prices.

 

@Mr48chev was spot on, I had some problems with the 700R4in my Stude (using the Hawk SW speedo) and found that the drive gear and driven gear were both wrong that I had. Had I just used the "Speedometer Drive Gear" suggestion (which I did....after jacking around a bit) I'd have been done a little sooner. I lost a little transmission fluid each time I took the tail shaft housing so be safe and put a pan under it.