Slick 60's

I have recently installed a 1966 bronco instrument cluster into my 1966 f100. Question i have is do i need to change the speedometer gear? I have a 4-speed transmission. When I look at LMC truck they show that the bronco came with a 3-speed trans but I have a 4. So which gear should i use? any help would be appreciated.

Why not run what you have and see where you are? Might not have to change it..
Good luck

I have. Speedometer needle jumps all over the place no mater what speed

OK, so without being there, this would be my next step. Pull cable from trans and compare the gear to others you might have, checking for bad teeth etc, then pull cable from sheath and make sure its not damaged, sometimes all they need is some lube,

Ok so let's say that the gear is messed up.
Replace it with f100 4speed gear or bronco 3speed gear?

Id suggest sticking with the amount of teeth on the one you had (4 speed). The internal part of the cluster should be the same and it would be the gear that you want to replace that changes the internal speed of the speedometer.

Count how many teeth are on the one you have, and replace with that.They are also colour coded so you dont have to count the teeth if you dont want to!
These gears are still available from ford and are only a few bucks if the speedo is still out after you change the gear.

Report back after you've changed it out

I would think the 3 speed and 4 speed are both 1:1 high gear which means it doesn't affect tooth count. there may be mechanical differences in a 3 speed gear vs a 4 speed gear but I don't think so.

You usually worry about the rearend gear when picking speedometer gears. It's about the relation of the final drive and the head unit, not how many gears are in the transmission.

Just curious, did the speedometer work correctly before the head unit swap? If so, that tells me there might be a problem with the head unit, not the tranny gear.

JMO,
SPark

the old cluster speedometer bounced as well but not as much as this one does.

of course my confusion is the LMC truck sells two different gears. Looking at their catalog there is a physical difference.

The speedo gear is based on the rear axle ratio, not any stock transmission. All stock transmissions (except for the O/D ones) have the same final drive ratio. The rear end ratio can vary though it seams the 3.70 is pretty common.

Unfortunately, there are several different speedo drive gear arrangements. Most of the three speeds and automatics have the cable input on the driver's side, some 4 speeds are opposite, requiring a reversed drive and driven gear. More than likely, your root problem is a dry or damaged cable.
Paul

If your speedo is bouncing try pulling the cable out of the sheath and grease it. It worked on my first car a 1979 Grand Prix. That thing bounced a range of over 60 km/H!

I'd say that the needle bounce is likely from a dry speedometer cable.

The drive cable can be removed from the speedometer cable housing and inspected. If it is in good condition and not fraying apart, then use some dry graphite powder to 'lube' up the cable housing.

Slowly reinsert the cable into the cable housing and apply the graphite as you reinsert the cable.

Ok so I installed new gear. I greased just the tip of the cable into instrument cluster. No change. Cable was replaced with a new cable in 2012. So any suggestions would be appreciated.

Hate to say it but four people have suggested pulling the cable from the sheath and lube it.......not just the tip.
Keep trying it will get there.

On a side note.
I believe any original slick speedometer from a truck or bronco will fit in place of your problem unit, while retaining all the bronco side gauges.

It's just a matter of disassembly and replacing problem gauges in the cluster.

Where it may be possible to do so,the back of the bronco unit is actually punched different for the gauges.
I just "fixed my lwb speedo cable. The cable/sheath was all gunked up with oil/grease so I sprayed the sheath out several times with carb and brake cleaner, blowing it out with air and soaked the inner cable as well. Graphite went in when all was dry and no more jumping

You don't have to change the speedometer gear when you change speedometer. The Bronco speedometer will work just the same as your Slick speedometer. It does sound like you have some issues with all the bouncing and so forth. I can look up what speedometer gear you need. I will have to have you rear end ratio and the diameter of your tires.