hi
i have
79 f150 4x4 300 i6
4-speed
3.50 gears and 31'' tires
my question is that my speedo is off by about 5mph
i'm going to install 33'' tires will this help or get worse?
and if i have to change a gear in transfer case,what size?
thank u for any info.phil

 

That's going to depend on whether or not it's reading slow or fast, but assuming that you just have the stock speedo gear, it'll make it worse.

 

yes,its slow
im not sure about the gear,just bought truck.

 

Bigger tires will make it read slower so you need a driven gear ( the one on the end of the speedo cable) with fewer teeth so it will spin the cable faster. Plug in your info to this calculator. Enter 7 for the number of drive gear teeth and 1000 for the speedo revs per mile.

Speedo Gear Calculator - Wallace Racing

It says that you will need a 15 tooth driven gear with 33s which, unfortunately, does not exist. The lowest tooth count for that gear is 16. If by chance you have an 8 tooth drive gear in your transfer case you're in luck because then you would need a 17 tooth driven gear which is available all day long. Unfortunately, it probably has a 7 tooth gear and it's hard to tell for sure without removing it from the case. If you order a driven gear remember that you need a right handed gear. Using the wrong one will ruin them quickly, ask me how I know.

The teeth should be pitched like this.

 

Maybe a ratio adaptor will help.
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4

 

thank u all,for the info.i have a dennis carpenter catalog and
it says for the 4-speed i need the left angle gear.and that the 3-speed and automatic takes the right angle?

 

I'm not exactly sure if that is true because the speed-o gear goes in the same place no matter what. Maybe they are talking about the 2wd maybe not? I'm not 100% but I wait for a better answer. Sorry.

 

Not true. Some plug into the driver's side and some, like the top loaders, plug into the pass side. That determines which pitch you need.

To the OP, just pull yours out and check to see which way the teeth point. The one in my pic is right handed. It should even say RH or LH and the tooth count on it.

 

OH DUH! sorry about that totally forgot about which side.

 

It's all goood.

 

According to the truck parts catalog, with this application, the original DRIVEN gear is nylon, blue in color with 20 teeth (C2DZ-17271-H).

You'll have to remove the gear, count its teeth to verify. Removing one tooth will speed up the speedo, so if present gear has 20T, you'll need a 19T gear (C7VY-17271-A ~ tan).

16-21 teeth are offered, the same nylon gears have been used since 1960, all are available from Ford for less than five bucks each.

 

i have a tan i think 17t