Speedo, Gears, Ratio Help!

[phraza]

Hi all,

I've done several searches and can't pin point what I need. Sorry I know it's been covered heaps of times. Basically my speedometer is displaying an incorrect speed due to larger tires.

My car is a 64 Chevy C10 Short Bed. It's got 29.1" tyres (255/70/15).

Tranny is Saginaw 4 speed on the floor. With a 350 V8 (had a standard straight 6 originally)

I'm guessing it has the standard gear ratio of 3.73.

Can someone please let me know what I need to buy to get the speedo reading accurate? What gears in the tranny to I need to adjust/buy. I'm down in New Zealand so it's a bit hard getting parts and I need to make sure they are the correct ones before buying them

Thanks so much!

Fraser

[phraza]

Also, someone can let me know what to look at on the gearbox I can supply pics. Thanks!

[Palf70Step]

You'll need 4 pieces of information to get it right

Tire diameter
rear end gear ratio
Number of teeth on the transmission speedometer gear
Number of teeth on the speedometer gear

Then you can use this to find the right combo of gears for the tranny and speedo cable
http://www.tciauto.com/tc/speedometer-gear-calculator

Most Saginaw came with an 8 or 7 tooth gear in the tranny, although that can vary.

How far off do you feel you are?

You can jack up the truck and rotate the rear wheels to get an idea of it's gears.

Pull the speedometer cable and bullet out of tranny and count the teeth on that gear. You might be able to guess what the tranny gear has based on that.

Remember, the variable you are off will change at speed. The faster you go the more offf you will be. The variable is in percentages. I have to get unlazy and pull my speedometer cable to change the gear out. Mine reads about 20% off with the tire and gear changes I did.

[phraza]

Thanks Bill, that is awesome.

It's late Friday night here, but the plan is to attack it tomorrow. I'll jack it up and take some pics etc and post back

Cheers!

[geezer#99]

Also look here.
https://transmissioncenter.net/?swoo...edometer-gears

You might want to look into a ratio adaptor also. Saves pulling the tail housing.

[phraza]

Hi guys,

My speedo is reading about 10-12 mph slower than it should (drove around with a GPS to get accurate reading).

Had a look at the transmission also. Pulled out the speedo cable to have a look at the speedo gear.

See below the pics of the red gear. Now, if I get a new plastic speedo gear with different teeth, will that correct my speed? (Sorry if that's a silly question). If yes (woohoo), which one do I get?

Counted the teeth on this one and it has 21 teeth.

Thanks heaps in advanced.

[Palf70Step]

Yes that is how you can adjust it....is ur speedo slow or fast?

If I remember right you'll need to go to about an 18 tooth if your speedo is slow. If it is fast maybe a 23or 24 tooth.

Normally with a Saginaw/muncie 3 speed, the 21 tooth red gear is used with a 3.73 rear gear, putting the tranny (drive gear) at an 8. Since you have a car 4 speed I am guessing maybe your tranny gear is off (probably 7 tooth). If that is the case and your speedometer is currently 10-12 slow, then the 18 tooth gear (brown I think) should correct it. If your speedo is reading fast, then I am guessing you have a 9 tooth drive gear and a 23/24 willget it back again.

[phraza]

Originally Posted by Palf70Step

Yes that is how you can adjust it....is ur speedo slow or fast?

If I remember right you'll need to go to about an 18 tooth if your speedo is slow. If it is fast maybe a 23or 24 tooth.

Normally with a Saginaw/muncie 3 speed, the 21 tooth red gear is used with a 3.73 rear gear, putting the tranny (drive gear) at an 8. Since you have a car 4 speed I am guessing maybe your tranny gear is off (probably 7 tooth). If that is the case and your speedometer is currently 10-12 slow, then the 18 tooth gear (brown I think) should correct it. If your speedo is reading fast, then I am guessing you have a 9 tooth drive gear and a 23/24 willget it back again.

You sir are a legend. Thank you so much! Yea its reading 10-12 mph too slow so i'll go for the 18 tooth.

I'm coming up to the US next Friday for a week, so I'll get that part while I'm up there and install it when I get back (will post on here when all done).

Thanks again!

[Palf70Step]

You may want to get the "bullet assembly" and O-ring for the end of the speedo cable too. They could leak if it doesn't seal against the new gear well.

[phraza]

Thanks bill will do!

[geezer#99]

You do know you'll likely have a fun time finding a gear.
Not a common stocked part.

[Palf70Step]

He is probably right, depending on where you can shop. Finding on line is easier.

http://www.thegearbox.org/catalog/it...82/3697857.htm

[oliver63]

What part of the US are you coming to? I have some gears I will give you.

Originally Posted by phraza

You sir are a legend. Thank you so much! Yea its reading 10-12 mph too slow so i'll go for the 18 tooth.

I'm coming up to the US next Friday for a week, so I'll get that part while I'm up there and install it when I get back (will post on here when all done).

Thanks again!

[vince1]

Is New Zealand in kilometers or miles? In Canada we read in kilometers and I found after switching from 3.73 to 3.07 ratio that all I have to do is double the miles to convert to kilometers.

[phraza]

Thanks guys, I found it on Summit yesterday all good, and all ordered. Getting it sent to where I'm staying in Anaheim

[phraza]

Originally Posted by vince1

Is New Zealand in kilometers or miles? In Canada we read in kilometers and I found after switching from 3.73 to 3.07 ratio that all I have to do is double the miles to convert to kilometers.

Yea NZ is in KMs, but think i'll just keep it in Miles

[phraza]

Sorry should of said, from Summit I got the bullet gear housing and the plastic gear. All TCI products. Probs paid a few dollars more but summit seems one of the easier websites to order stuff for me.

In Cali for a week, first time in the States so looking forward to spotting a few sweet rides while i'm there haha

[phraza]

Just got back from the US and installed my new plastic brown speedo gear and bullet housing.

My speedo is still out, looks like it's closer, but it's still out about 10mph.

The needle jumps slightly, would it be the speedo cable that's causing the incorrect speed reading?

Cheers,

[Palf70Step]

Cable would cause the jumping, not sure if drag would throw the MPH off though.

[phraza]

Originally Posted by Palf70Step

Cable would cause the jumping, not sure if drag would throw the MPH off though.

Ok cool thanks, might get a new cable, and look in to getting a ration adaptor

Cheers,

[phraza]

Just spoke to a speedo/gear specialist guy here in NZ, and he said to bring in the actual speedo dash and he will calibrate it.

He told me to spin the rear tire 10 times and count how many times the speedo cable goes around (and also let him know the tire radius).

Think I'll give this a crack in the next few weeks and see how I go

Cheers,