New speedo gear for new tranny

[jd21476]

I swapped out my tranny from a T350 to a 700R4 and now my speedo is reading fast. It says I'm doing 80-90 when in reality I am probably running about 65-70.

Anyone know where to buy a new speedo gear and possibly which on I will need?

I have a '73 K20 4x4 with a 350 and 700R4

[77c10]

You will need to know your tire diameter and rear end ratio. Google "700r4 speedometer gear calculator" and you will find numerous websites that have charts.

[tlawre87]

http://www.transmissioncenter.net/sp...__va.htm#700r4

That will allow you to purchase the gear.

This will allow you to calculate:
http://www.tciauto.com/tc/speedometer-gear-calculator/

[jd21476]

So to get the drive teeth I'm going to have to open my rear diff, correct?

[77c10]

Originally Posted by jd21476

So to get the drive teeth I'm going to have to open my rear diff, correct?

To be 100% certain, yes...

[tlawre87]

If you have the 4.10 you should, anyone chime in if Im wrong because this is not a strong area for me, technically have 41 teeth on the ring and 10 on the pinion. Check the SPID for the ratio or jack it up and count the driveshaft revolutions to the wheel turn.

[geezer#99]

Easier yet is figure out the percentage off your speedo is and buy and install a ratio adaptor. There's some on the post tlawre87 gave you in the link.

[jimbonice]

Check out the faq's section; there's a link to a speedo gear calculator. When I went from a turbo 400 to a 700r4 in a 4.11 geared dually, I had to change both gears. No big deal, but the tail shaft has to come off for the inner gear.

[mrolds88]

Originally Posted by jd21476

So to get the drive teeth I'm going to have to open my rear diff, correct?

Not really. On a flat spot, jack up one rear wheel (after blocking at least the opposite wheel on the front. Put the truck in neutral. If the truck doesn't have a posi or limited slip, you will be able to turn the rear wheel that is in the air. If this is the case, make a mark on the tire and one on the pinion. Now turn the wheel and count how many times it turns to get the pinion to turn twice. That will give you the gear ratio. If it takes a little over 4 times to turn the pinion twice, it's 4.10's. Now if it has a posi, you will have to lift both rear wheels and turn a tire. But this time you will turn the tire as many times as needed to turn the pinion once.