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Speedometer reading high - driven gear question

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6.9-250

Speedometer reading high - driven gear question

I have a 1985 F-250, 6.9L auto, 4x4. The speedometer is reading high. When its showing 60km I am actually doing 55km. When its showing 100km, actual is 85km. When it shows 121km, actual is 100km. I believe the previous owner replaced the transfer case so not exactly sure what year its from (serial number on the transfer case says 10 30 84, so I would assume 85 if that is the date). I tried another speedometer head from a parts truck. It was reading high as well, very close to the same as the original one. When I bought the truck it had bigger tires on it. I have since replaced them with the oem tire size ( 235/85 R16 ). Rear axle is 3.55.

This takes me to the driven and drive gear at the transfer case. There is a 16 tooth (purple/wine) driven gear installed. It is either a 7 or 8 tooth (teal or possibly blue) drive gear. Transfer case is New Process Gear 208F.

Using the drive gear chart it shows a 8 tooth blue drive gear. The driven gear looks like I have the right one as well, wine/purple 16 tooth.

Using a gear calculator, it comes up with an 18 tooth driven gear to use.

1. Any suggestions on a driven gear I can use to get the speedometer to read a little closer?

2. Can you just change to any tooth driven gear and it will mesh properly with the drive gear that is installed?

Thanks

1985 Ford F-250, 6.9L, Auto C6, 4x4 208F, Extended Cab

Gary Lewis

Re: Speedometer reading high - driven gear question

Administrator

Changing the drive gear isn’t easy, but changing the driven gear is. So I would buy an 18 tooth one and put it on since that’s what the calculator calls for.

And yes, you can change The driven gear and not worry about the drive gear. They will mesh just fine.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Rembrant

Re: Speedometer reading high - driven gear question

Gary Lewis wrote

So I would buy an 18 tooth one and put it on since that’s what the calculator calls for.

Yup. I just did some quick math based on the tire size and gears, and with an 8 tooth drive gear and a 18 tooth driven gear, the speedo cable should be spinning 1003.5 revs per mile. Bang on.

1994 F150 4x2 Flareside. 5.0 w/MAF, 4R70W, stock.
1984 F150 4X2 Flareside. Mild 302 w/ 5spd. Sold.
1980 F150 4X4 Flareside. 300i6 w/ 5spd. Sold in 2021.
1980 F100 4X2 Flareside. 351w/2bbl w/NP435. Sold in 1995

Gary Lewis

Re: Speedometer reading high - driven gear question

Administrator

Yep, that is "bang on" as the Brit's would say.

As for calculating things, why not use the calculators here: Driveline/Speedometer Gears?

Gary, AKA "Gary fellow": Profile

Steve83

Re: Speedometer reading high - driven gear question

Banned User

In reply to this post by 6.9-250

The TSB in this caption is relevant:

Rembrant

Re: Speedometer reading high - driven gear question

In reply to this post by Gary Lewis

Gary Lewis wrote

Yep, that is "bang on" as the Brit's would say.

As for calculating things, why not use the calculators here: Driveline/Speedometer Gears?

Well we say it in Canada too, so I guess that's just another one of those terms we inherited from the Brits.

I don't think I even knew that calculator was there, but it's pretty quick to do it (manually) with a calculator anyway.

Gary Lewis

Re: Speedometer reading high - driven gear question

Administrator

Steve - That's a good TSB. But I'm surprised that it doesn't mention calibrating the speedometer.

And, speaking of that, with the 1000 RPM being 60 MPH fact, I've been planning to try calibrating one. I think I have a 1000 RPM gear on my lathe, so can chuck a short cable in and run it to the speedo.

Cory - The annoying intro to BBC television shows here in The States, which is a cartoon-style thing, says "bang on". And "quite". Reminds me of my time there. Liked it so much we are planning a trip back.

Yes, those calculators are hidden. In fact, there are several calculators scattered around - in what I think are logical places. But, should we also have a common place for them? We can put them in multiple places, easily.

Gary, AKA "Gary fellow": Profile

Steve83

Re: Speedometer reading high - driven gear question

Banned User

Gary Lewis wrote

But I'm surprised that it doesn't mention calibrating the speedometer.

Note the "Issue" statement - that kind of work is beyond the scope of dealership technicians. If the speedo head is out of spec, it simply gets replaced.

PetesPonies

Re: Speedometer reading high - driven gear question

I'm a math person, love math, formulas, etc. HOWEVER, with everyone having a GPS on there phone, this is how you do it now. See how much your speedometer speed and your GPS speed are off, in %. Example, 10% off, too fast. So you need to slow down the cable revolutions by 10%, . You do that by increasing the driven gear teeth by the same 10%. You just round it. So if you have a 16 tooth driven, you would need a 17.6 or an 18 tooth driven gear.

Pete's Ponies
Mustang RUSToration & Performance

1982 F100 Flareside
1983 Bronco

6.9-250

Re: Speedometer reading high - driven gear question

In reply to this post by 6.9-250

I am a "just in case" guy, so I ordered a set of 5 gears. They arrived today and i installed the yellow (18 teeth) one as per the calculator. Lower speeds were fairly close but it was still up to 5km out in the higher speeds. Since I had more to try, I installed the pink (19 teeth) gear. Now its "bang on"... at least according to my Garmin GPS. 0-100km is all reading accurately.

The tires on my truck have a very deep tread. This may contribute to requiring the 19 tooth gear instead of the 18 tooth gear. I should have gone to the tire manufacturer website for the revolutions per mile. This may have made the difference.

This is another "layer of the onion" that I may have peeled, but I don't think it will ever end, lol.

I have the remaining gears for sale if anyone requires except the pink (for changing tire size, etc) Shipping from Canada.

1985 Ford F-250, 6.9L, Auto C6, 4x4 208F, Extended Cab

Gary Lewis

Re: Speedometer reading high - driven gear question

Administrator

Congrat's! I like the idea of a pack of gears, letting you try several.

But I think you are right about the tread. Obviously yours is effectively taller than other tires that size.

Gary, AKA "Gary fellow": Profile