Slick 60's

1964 F100 292/COM 3.10 gear L78-15 (29.3 in) tires, 17 tooth speedo gear.

According to my gps, indicated speed is 50, actual speed is 67.

I thought that I had an incorrect speedo gear, but since I have a 17T gear and 16T is the smallest, its not the gear.

I saw a post by Garbz that states the problem may be the needle hanging up.

Can anyone elaborate on how to fix this?

This may be early morning "fuzzy thinking", but if I am not dead wrong, the 16t should put you fairly close.
Paul

I don't think the gear change will make it. Its only about a 6% difference and you are seeing closer to a 30% variation. I think these are designed to work by the cable turning a magnet that rotates inside a cylinder. The cylinder is attached to the needle and a spring. The turning magnet works against the spring to give you your reading. If there is more drag on the cylinder/needle, your speedo will read low. A little oil and some manipulation may free it up. There are also shops that can correct these.

ok I know this is really low tech and bad.. but my Pops truck always ran WAY WAY OFF on the high end! so I took apart the speedometer cover, and I intended to BEND the needle to the left some so it would be closer at highway speeds.. and if I remember correctly I didn't have to because the shaft thru the center of the speedo needle was a little loose and had slipped I think after all the times it had came down and hit the pin on the bottom? Either way I turned it on the shaft and put a little superglue on it. Oh my...its been a few years but thats the way I remember it

I don't think these trucks ever came with a 3:10 rear end, so you probably need to change the drive gear in the transmission.


Johnny

My research indicates that the rear end or at least the gears came out of a 57-58 Ford. The tag on the diff has 3.10 stamped on it and nothing else. It does not look like a homemade tag.

Im going to get back out with the gps and see if it is off at 20-30-40-50 mph.

I guess I also need to jack it up and see if I can confirm that it is a 3.10 gear. The tag may be incorrect.

Just for info purposes............. Ford dealers carry a 14 tooth gear.

Fanatic wrote:Just for info purposes............. Ford dealers carry a 14 tooth gear.

Do you have a p/n by chance? Everyone has told me that a 16T is the smallest/

On a related note what is the problem with a speedometer that the reading just jumps all the place? Been driving my 66 a lot more lately and its bugging me a lot.

Speedo jumping all over the place is usually a bad cable, occasionally it is the speedo head, but my money would be on the cable 95%.

Sometimes just lubing up the cable will cure it. If not, time for a new cable.

I don't know if you have a tachometer or not, but if you do, if the axle ratio is 3.10, and the tires are 29.3" tall, it should be turning ~2133 RPM @ 60 MPH (assuming 1:1 through transmission).

**and when dealing with used carriers, I wouldn't put too much faith in what's on the tag, I would count the teeth on ring & pinion, and do my own math......

EDIT:**Just googled it, and on Richmond gear webpage, it shows 31 tooth ring gear & 10 tooth pinion

EDIT^2: Summit has a universal replacement cable core for just under $6.00

http://www.summitracing.com/parts/atp-yc101

More " off the top of my head thinking" ; since the rear ratio is not original, one may need to change the drive gear to attain the needed speedo ratio. If memory serves me right, at least the COM has replaceable drive gear. It is still a little effort, but nowhere near as much as needing to source and install a different output shaft. I am not aware of a 14t that is compatible with the COM. The 3.00 nine inch rears behind the V8 COM utilized a 16T, again if memory serves me correctly. A speedo shop may prove your most economical choice if in fact a mis-match exists.
Paul

I went out today and checked the indicated speed against the gps again.

At 40 Im really doing 54, 45 Im doing 60, and at 50 Im doing 68. Its off a pretty consistent 26%. Since I already have a 17T gear, I cant make that up by going to a 16T.

Im either going to need short tires, a new rear end, or a new drive gear, or just leave it alone.

There are a number of places that make adapters to slow down, or speed up the speedometer readout. Usually used on 4x4's with big tires/lower gears, here is one I found using google, scroll ~1/4 way down, and watch for GM / Dodge / Ford Ratio Adapter

http://www.partshp.com/speedometer_gears.htm

I think the ratio adapter is the way to go. Since I have a 17T gear, if I ever go with shorter tires, I have plenty of other gears to keep the speedo accurate.

To bring this back up...

Ive always though the rear end under my truck doesn't belong there. I think it has 3.10 gears and it looks to be older than the truck.

Tonight I was taking some measurements to use while sway bar shopping and I noticed that the diff is offset about 2 inches to the passenger side. The diff in my 66 was dead center.

The truck drives straight with no vibration at any speed.

This complicates the entire sway bar situation.

viewtopic.php?f=5&t=26278

this will give you info on rear swaybars i used addco #340 for rear and a addco #113 for the front and yes a 64 the pumpkin is offset to the passanger side.

Danny