I had my truck 4.11s replaced with 3.54s recently and it's been an absolute nightmare. First, the shop messed up and put 4.56s in it instead! The next day, it's dropped off to get the job done right. Well, they got the right gears it (and I LOVE being able to drive eighty down the freeway just barely over three grand now), but the speedometer reads twenty percent slower than my speed. If it says I'm driving sixty, I'm really going seventy-five! Okay, there's a speedometer gear on a cable that goes to the tailshaft... the mechanics at this shop (who I don't have a lot of faith in at this point) said there's only two options for that gear and they tried both. There's also a speedometer gear ON the tailshaft which they were prepared to change for me yesterday, but they told me when they took the tailshaft out, they discovered that the gear is PART OF the tailshaft and the whole tailshaft would need to be replaced. I guess I can adjust my driving to drive without a correct speedometer, but I really want an accurate odometer so I can check fuel mileage and keep the truck true.

My question for you guys is what can be done? Are they wrong and there's more options to try for speedometer gears? Is my speedometer a type that can be calibrated? I figure I can modify the actual gauge face, but that still leaves the odometer off. Help please!

The truck is a '77 F250 SuperCab RangerXLT Trailer Special with a 460, C6, and 3.54s with 9.5OR16.5LT tires... I believe they put a sixteen tooth speedometer gear on the cable.

 

You can try getting a speedometer cable (or gear) for a truck with a C6 and a 3:50 ratio. The 3:50 ratio was common among the 4x4 F150's and F250's. That should correct it. If not Stewart Warner makes (or used to) gear reducers for just such a problem.

 

seems like i saw on here a week or 2 ago a page about speedo gears. seems like they had all of them from like 15 to 20 teeth. maybe try doing a search about it?

 

I was about to post a similar question. I have done a search on speedo gears and could not come up with anything. I have 3.50 gears in my F100 and I have pulled the speedo gear and cable from a truck with 4.1?s. I was going to use the original speedo gear but I am unsure if it's the correct gear. The truck had an aftermarket speed-control and the speedo cable went from the speedometer to the controler in the engine bay then down to the bottom. So I am not sure if there is a reduction in the speed-control system. I was hoping to find a list saying use this gear with this ratio of rear end.

 

sorry i could not find that post. had good information in it. hope it magically shows back up soon. if not maybe numberdummy will jump on here quick and straighten this mess out.

 

With a C6, the speedometer DRIVE gear is manufactured as part of the output shaft.

The DRIVEN gear is on the end of the speedo cable.

Here's what the 1973/79 Ford Light Truck Parts Catalog lists for your application:

1977 F250 2WD / C6 / 3.54-1 ratio / 9.50R x 16 tires (engine size doesn't matter, but since it's a 460, I know the truck has 2WD):

C7VY17271A .. 19T Driven Gear / AVAILABLE from Ford

MSRP: $2.60 // FTEpartsguy.com price: $1.87.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Only dealer in UT that has it:

LARRY H. MILLER FORD in SLC has ONE: 800-950-5006.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
IZZYAZ: ALL the DRIVEN gears are still available from Ford: 16-17-18-19-20-21 teeth.

The DRIVE gears (if not a C6) are also available: 6-7-8 teeth.

To figure out the correct gear, the following info is required: TRANSMISSION TYPE / TIRE SIZE / REAR AXLE RATIO.

 

I went from H78-15's to 235/85-16's and there wasn't a gear for the
T18 to correct the difference, I was outside of the engineered range. LOL :)
I just lived with it.

My odometer (tested and figured both) was off 12.8% and figured it in
every time I checked the mileage. No big deal. ;)

The speedometer was off about ~5mph at ~65mph after I adjusted all I
could get out of it (yeah they're adjustable, just don't have much range).

Unlike the OP I do my own work and changed the rear gear ratio from
3.25 to 3.50 which just so happened made my speedometer read right
for the first time, ever. Even when brand spanking new it wasn't quite
right. LOL :)

http://www.panix.com/~alvinj/file12/SpeedTable.jpg
{courtesy of SPRR}

The odometer is still off about 4.7% and could be corrected with a new
driven speedometer gear but heck, I've lived with that gear for 34 years
I'm keepin' it as is. :) (17 teeth white)

When figuring this stuff out you can do it with a calculator but I prefer a
slide rule. Slide rules kick butt when it comes to figuring out proportion
problems and making changes in them. YMMV?

I'm the dumbest guy on FTE and even I didn't pay someone put the
wrong gear in my axle... you trying to take my title or somethin'? :/

Alvin in AZ
ps- Time your speedometer with the gear you have now and pull it out
and count the teeth (has a number on the end of it) and post that and
the color of the gear too. The 16 tooth out of my parts truck is red.
pps- 60:75 = 16:20 = ~15:19 but since the 60 to 75 is a guess and
the 16 is a guess too... My money's on Bill's 19. LOL :)

 

Cool, didn't knowed that already. :)
Hmmm... just another reason automatics suck. ;)

Alvin in AZ '75 F150 360FE T18 9inch

 

NumberDummy, how many teeth are on a C2DZ-17271-J speedo driven gear? That's the number on the package for what's in the truck right now. If it IS in fact a 16T, then a 19T wouldn't work, right? Doesn't a SMALLER driven gear make the speedometer read faster? If the one in it ISN'T a 16T, then the mechanics screwed up again!
If I can't obtain the results I want by changing the driven gear, are there other options for output shafts for my truck (C6 tranny)?

Thank you so much for your help.

 

I simply googled the number and it came up as a 16T on a Mustang site.

Sure enough! LOL :)
When posting the ratios I wanted to flip them around but was trying to
make them match up with Bill's post but I screwed up because I didn't
think it through even when something was telling me it wasn't quite right.

Good question. :)

Have you got a chance to time your speedometer yet?
http://www.panix.com/~alvinj/file12/SpeedTable.jpg
Timing it will eliminate tire size and internal speedometer errors.
Then the math will work it out better than guessing.
Might have to choose between two "wrong" gears when it's all done. :/
Timing it will help you decide which direction you want to go at that point.

Alvin in AZ

 

Thanks for finding the part number Alvin... never thought of Google for that. I was REALLY hoping that the mechanics had goofed again and put a 21T in or something. Too bad there's not a smaller driven gear available than 16T... I didn't want to replace an output shaft. I'll have to time a mile tomorrow I guess.

 

To confirm: C2DZ17271J = 16T Driven Gear

C7VY17271A = 19T Driven Gear.

This is the gear the 1973/79 Ford Light Truck Parts Catalog lists for your exact application.

The catalog does not list a 16T, 21T or anything else for your application.

 

I'm really sorry NumberDummy, I don't know what to say. The package for the gear they put in says it's a 16T and it reads slower than actual speed. I need a smaller gear to make the speedo read faster. Something is different in my truck that makes it an oddball I guess. Is there any reason a gear with more teeth would help get a more accurate reading?

 

Well, I'm trying the 19T... I work for a parts distributor and they have a base in Midvale. A Midvale driver will pick it up from the dealer for me and give it to me personally when I make one of the three transfer connections with them in Bountiful today. Working where I do sure makes getting parts easy (and cheap a lot of the time too).

 

I was about to post the same question. I found a formula for figuring out driven speedo gear size, but it requires that you know what your tire RPM's are. Does anyone know what driven gear I need for a 77 F250, NP435 trans, 315/75-16 tires and 4:10 gears?