Speedo Calibration

banjojelly · Post by **banjojelly** » December 10, 2008, 1:18 am

Hi guys, long time no update on my part. (ill add pics later, im almost done)

anyway i was wondering if someone out there was experienced enough to tell me how to do this:

original my truck had 4:11 gears in the rear. i have driven enough tractors in my life so i switched them for a set of lincoln 2:50's. so now obviously my speedo is WAY off.

any idea how i know how many teeth i have to change up or down from the old speedo gear to the new one to make it register correctly? ive been racking my brain but finals are over so it is shut off.

Any help would be awesome. thanks.

fmartin_gila · Post by **fmartin_gila** » December 10, 2008, 7:49 am

Pull out your cable end from the tranny and count the teeth. Then multiply that number by .61 and you will be very close.

Fred

R Pope · Post by **R Pope** » December 10, 2008, 7:52 am

You have to slow your cable speed down by the same amount as you changed the rear gears. Roughly 5/8, that's 250/411, or 500/822 which is , like I say, about 5/8. To oversimplify, if your speedo gear now has 5 teeth, go to an 8 tooth one. It never works out this easy in real life, though.

Unibodyguy1 · Post by **Unibodyguy1** » December 10, 2008, 10:07 am

Fred,
if I'm understanding you correctly counting the existing speedo gear teeth,(from the original gearing) and multipy by .61, and that figure should give you a idea, how many teeth the "new" gear for the new gearing should be. Let me know if this is correct on this,

Michael

ICEMAN6166 · Post by **ICEMAN6166** » December 10, 2008, 11:50 am

one problem being the trucks did not have 2.50 gears to begin with,you should look for a nylon gear from a lincoln or other car with 2.50

dwbr · Post by **dwbr** » December 10, 2008, 1:55 pm

your tire diameter matters as well, so if you have a tall tires just calculating the teeth wont get you there, my spedo is about 30 mph off, i just use my tac
________
Ford Ranch Wagon picture

ICEMAN6166 · Post by **ICEMAN6166** » December 10, 2008, 2:07 pm

of course you know this messes with the odometer too .

best way to fake it is to drive 1 marked mile (the xway markers are good for this) see where you come out, for intance if you do a mile and it turns 1.2, you are putting 20% more miles on the odometer or showing 120 mph @ 100 actual ,
if you are the other way, .8 for every mile then you are slow the same 20%, so at 100 mph you are showing 80mph on the speedo.

2 sets of tires for my truck, winter and summer, one set is correct within many miles, the other is taller therefore it is at .9 to each true mile.

i calculate this in my head between mile markers after i change the tires and adjust my speed accordingly.

banjojelly · Post by **banjojelly** » December 10, 2008, 5:08 pm

thanks for all the info guys. i can use the tach for now, but it would be nice to have a speedo. plus the odometer was the gas gauge. lol.

what was the highest gearing that the trucks came with? if it was something like 3.23's that wouldnt help me either. would a lincoln gear fit on the cable? is it a universal thing (among fomoco)?

ICEMAN6166 · Post by **ICEMAN6166** » December 10, 2008, 6:36 pm

not completely universal
what trans do you have?
c4/c6 should trade right out.plenty of cars other than lincoln. fairlane ,galaxie etc.
nylon gears are color coded and marked with #of teeth.
several different ones for manual/auto between 16 and 21 teeth.
take yours out and note the color and # of teeth.
more teeth= turns slower
less teeth=turns faster

banjojelly · Post by **banjojelly** » December 10, 2008, 8:32 pm

original 3 speed manual.

ICEMAN6166 · Post by **ICEMAN6166** » December 10, 2008, 8:42 pm

might be a little harder to find a 3 speed car in the boneyard these days.
www.dearbornclassics.com
has the gears for fairlane, galaxy etc.
worth ordering the free catalog .

fmartin_gila · Post by **fmartin_gila** » December 11, 2008, 8:42 am

Sorry to be so late, but I don't usually log on but once a day. Yes, you need one with less teeth than the one you now have. If it has been close with the tires you are running, you should be close using that multiplication. If it has been somewhat off, now would be the time to bring it in. If the number of teeth you require is not availeable, you may have to change the driving gear inside the tailhousing.

Fred

36truck · Post by **36truck** » December 11, 2008, 9:20 am

When I had my 79 Bronco I put bigger tires on that changed the speedo about 5 mph I changed the gear 1 tooth & that made it correct. So for every 5 mph 1 tooth to correct. Your driven gear can only go so many teeth different before you need to change the tailshaft gear like Fred said. A older good Ford partsman should be able to help.

DanSanDiego2000 · Post by **DanSanDiego2000** » December 11, 2008, 11:14 am

You can go 'round and 'round with this problem, and it might drive you nuts. It's not just a matter of changing the driven gear (the gear in the speedo bullet), but also changing the drive gear (INSIDE the transmission, which means pulling the driveshaft, removing the extension housing, changing the drive gear on the output shaft, etc.)

I have done this many many times.... and you CAN get it very close.

Biggest problem though, as mentioned, is that the trucks never came with a 2.50 gear, so a Drive gear or Driven gear combo would be tough to find.

*Best Solution* I think is a correction/adapter. San Diego Speedometer and Tach offers a service in which they dyno the vehicle, then build a little gear reduction box to go onto the transmission speedo bullet, then the cable screws back into the adapter. The gears inside the adapter are chosen to underdrive or overdrive the speedo cable and correct the speedo.

Call around and see if anyone in your area offers this type of service.

My speedo reads only 69% of true speed, because I run 3.00 gears and taller tires than original. For the hassle and expense I would have to go to to correct the speedo, I'd rather go the dyno and adapter route than pulling the parts.

Regards.
Dan

Fanatic · Post by **Fanatic** » December 11, 2008, 5:51 pm

Heres a little tidbit that can come in handy. Ford still has the gears available by the way. Looks like trying a wine colored one -16 tooth--would be a start.
C0DZ-17271-A

cdherman · Post by **cdherman** » December 11, 2008, 11:59 pm

In the process of getting my speedo correct, I scrounged all the teeth #s from 15 to 19. I keep them around.

Beware!

C4, C6, t18 all have the speedo cable and gear coming into the driver side of the tranny.

Some other trannies (3 speed???) have the speedo coming into the passenger side of the tranny.

This results in the need for the spiral threads on the "Driven" gear (on the end of the speedo cable), to be in the other direction. So a left handed spiral instead of a right handed spiral.

They are most certainly NOT interchangeable.

banjojelly · Post by **banjojelly** » December 12, 2008, 12:34 am

you guys rock!

thanks for all the help. the under/overdrive borrection box is interesting to me. i have access to a cnc lathe and mill... i wonder if i could make one. im thinking of one like the variable speed setup on old belt driven combines. then it could be dialed in to the nats ass.....

cdherman · Post by **cdherman** » December 12, 2008, 7:53 am

banjojelly wrote:you guys rock!

thanks for all the help. the under/overdrive borrection box is interesting to me. i have access to a cnc lathe and mill... i wonder if i could make one. im thinking of one like the variable speed setup on old belt driven combines. then it could be dialed in to the nats ass.....

You figure our how to make a reliable, adjustable correction box, there would be a market for it.

R Pope · Post by **R Pope** » December 12, 2008, 8:31 am

S10's have a gearbox that attaches directly to the speedo drive connection. They come with lots of ratios, both up and down. They might fit your application, easier than building one.

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