Speedometer off by 20 mph HELP!

[ranger_68_f100]

First of all i know there are threads that help out with the jumpy speedomoter that i have.
But before i can fix that problem my jumpy speedometer is off by 20 when i drive.
My transmission is a C6 and engine is 390 if any of this information helps.

[fordman]

once you get it to stop jumping by greasing the cable. figure out your rear end ratio and tire size and get the correct peeedogear for your transmission.

[91Bear]

I still haven't figured out how to cure mine (10 mph fast). 235/75R15 and 3.25.

[ranger_68_f100]

And how are you supposed to figure out ratio anyway?

[BobbyFord]

I still haven't figured out how to cure mine (10 mph fast). 235/75R15 and 3.25.

Larger speedo driven gear.

[basketcase0302]

ranger,

There is an "old school" way to determine which tooth speedo gear you need. I had my C-6 rebuilt a few months ago and my tranny guy is a "Ford only" retired technician of over 30 years working on them.

The procedure had to do with counting seconds while driving at a constant (certain) speed and a certain distance. And then subtract the amount of teeth from the current gear using the results, (to determine which gear you needed).
Sure wish I could remember it...
You might want to search here:
http://www.network54.com/Forum/260730/

[fordman]

the gear ratio is marked on the vin door tag. by decoding the axle numbers. i think 17 is a 3.25 ratio.

[Jasper1975]

check these guys out also do a search for speedometer calculator and pick one it should be ease to figure out once you have the bounce taken care of http://www.partshp.com/speedometer_gears.htm

[ranger_68_f100]

Guy at local parts store was trying to get me to buy a new speedometer cable.
He's full of tihs, then i asked about some replacement blades as mine are the brushed metal look,
and his were black and he was like "you can take of rubber and slide them on yours, 8 bills a piece"...
hell no...

[JMcTurnan]

Hey man just deal with it, mine is about 15 mph off at 30mph, and like 30mph off at 100. I dont know what type of rear end I have so I cant do anything.

[ForingaMex]

First off, these questions have been asked and answered on here before and answered better than I can but I'll try to help.

once you get it to stop jumping by greasing the cable

I agree with fordman. First fix the bouncing issue and than go from there.

I still haven't figured out how to cure mine (10 mph fast). 235/75R15 and 3.25.

And how are you supposed to figure out ratio anyway?

To determine your gear ratio just jack up both rear wheels and put jackstands under the axle. Turn both wheels on turn (exaclty) forward or backward (You'll need someone to help you with this) and have your helper count the number of revolutions the driveshaft makes. For example if the driveshaft turns 3 1/2 turns that means you have 3.55 gears, 3 3/4 means 3.73 and so on. Depends what kind of rear end you have and what gear ratios are available for that perticular rear end. Just some very simple math.
I personaly would never trust the VIN tag. What is the chance that a 40 year old truck will still have the original rear end under it. At least verify it and than you know if the gear ratio matches the VIN tag.

Hey man just deal with it

You DO NOT have to just deal with it. You might not get it 100% accurate but you should be able to get it to within 2-3 miles -/+.

[71cc]

If you have a gps you can use it to determine exactly how far off your speed is. The more accurate way is to reference your driven miles shown against the mile markers on the interstate. The greater number of miles driven will provide a more accurate sample. So if your odometer shows 10 miles and 5 tenths driven compared to a known 10 mile stretch you can then work out percentage of error. The only thing you will need to know is speedo gear count. That will be the basis for percentage correction. Even with a bouncing needle the odometer will be close. Do the check, fix the bounce, and count the gear teeth while it is out and come back. Then we can get you very close.

[cdeal28078]

I check my speed by the mile markers also. I drive at a steady 60mph and it should take 60 seconds between markers if it is accuraite
clint

[ranger_68_f100]

My speedometer cable was damaged, i have yet to install it as it got late. I didn't get to check the gears or teeth on transmission. Is it easy to take out to count teeth? How do i take it out?

[ForingaMex]

The small plastic gear, who's teeth need to be counted, is on the end of the speedo cable. If you take the cable out of the trans you'll see it. I don't exactly understand your question. Are you asking about how to remove the cable from the trans or how to take the gear off of the cable? There are two types of cables, I think. The one I have has a retainer with a small bolt that holds the cable into the trans. Remove the bolt and pull and wiggle the cable. I'm not sure how to remove the other type of cable, never seen one. To take the gear off of the cable, just take the small metal clip off the end of the plastic gear and it will slide of. I hope this helps.