In the event that you've recently inter-changeable your tires or changed your axle ratios, you're possibly staring at your dash wondering exactly why your np208 speedometer gear is definitely suddenly lying in order to you. It's the common frustration for anybody running an older Chevy, Ford, or Dodge with this classic chain-driven move case. You're touring down the road, the flow of traffic feels such as 70, but your own needle is trembling its head at 55. Or maybe it's the other method around, and you're racking up "ghost miles" on the odometer while barely moving. Either way, obtaining that speedometer calibrated correctly isn't just about avoiding the speeding ticket; it's about knowing your actual fuel economic climate and keeping your own maintenance intervals on the right track.
The NP208 was a basic piece within the 80s regarding a reason—it's relatively lightweight and gets the job done—but its mechanical speedometer setup is certainly a product associated with its time. In contrast to modern rigs where one can just plug within a tuner plus change an electronic digital value, the NP208 relies on physical plastic material gears meshing together. If you modify the math associated with your drivetrain (tires or gears), you have to change the physical parts to fit.
Focusing on how the gears actually work
Before you go ordering parts, you've obtained to understand that there are actually two various gears involved here. You might have the drive gear , which sits right on the result shaft in the exchange case, and the driven gear , that is the particular small plastic piece attached to the particular end of the speedometer cable (or the VSS sensor, depending on your year).
Most of the time, when people talk about swapping a good np208 speedometer gear , they are mentioning the driven gear. It's the simple someone to get to. You just unbolt the housing from the tailpipe of the transfer case, plus there it will be. However, the powered gear only provides you a certain amount of modification. These gears have color codes based on the particular number of tooth they have. In the event that you find that you require a gear with, say, 45 teeth to make your own speedometer accurate, yet they only create them up in order to 43 for your specific housing, then you might possess to take a look at altering the drive gear on the shaft itself. That's a little more of a project, but for most tire swaps (like going from a 29-inch stock tire to a 33-inch or 35-inch), a simple driven gear swap generally does the technique.
Doing the math without shedding your mind
Figuring out which gear you need doesn't have to feel like a higher school algebra last. The basic logic is definitely simple: in case your speedometer is reading too fast, you need a gear along with more teeth to slow it down. If it's reading too slow, a person need fewer teeth to speed it up.
If you want to become precise, there's the classic formula individuals use: (Drive Gear The teeth x Axle Percentage x Tire Cycles per Mile) / 1000 = Powered Gear Teeth .
Now, most of all of us don't have the particular "tire revolutions per mile" tattooed on our arms, so that you can usually find that facts about the wheel manufacturer's website. Or even, if you want the "good enough for a trail rig" method, you can use the percentage associated with error. If your speedometer says sixty but GPS says 66, you're 10% off. You'll want a gear along with roughly 10% fewer teeth than what's currently in right now there. It's always the good idea to pull your present np208 speedometer gear out very first and count the teeth so a person actually know your kick off point. Don't just rely on the color; after thirty years in a very hot transfer case, a "green" gear may look suspiciously like a "yellow" one.
The housing issues more than a person think
1 thing that excursions a lot of people up is the gear housing. The NP208 uses the circular housing that holds the powered gear and connects into the transfer case. These housings are offset. Because the gears come in various diameters (a 40-tooth gear is physically larger than the 34-tooth gear), the particular housing is made to be rotated and balanced to different opportunities to ensure the particular gears mesh correctly.
In case you try to cram a huge gear into the housing setting supposed for a little gear, you're likely to gnaw up those plastic material teeth in regarding five miles. More over, if the gap is too huge, they won't touch whatsoever, and your own needle will simply sit at absolutely no. Most housings have got numbers stamped upon them indicating the number of teeth they will support (for instance, 34-39). If your new np208 speedometer gear drops outside that variety, you'll actually require a different housing to match. It's a little detail, but it's the difference between a smooth-running dash and a few of plastic bits.
Tips with regard to a leak-free set up
Since you're already pulling the speedometer gear away, this is the perfect time to address the particular "NP208 leak. " These things are usually notorious for weeping ATF from the particular speedometer area. Generally there are usually 2 seals you should look at. Very first, there's the large O-ring on the outdoors of the housing itself. Those get flat and brittle over time.
Second, and more significantly, there's a tiny small seal inside the housing where the gear base slides through. If your speedometer cable offers red fluid inside, that inner close off has failed, plus the transfer situation is literally pumping oil up your cable toward your dashboard. It's a mess you don't want to cope with. While you're swapping your np208 speedometer gear , spend the additional couple of dollars on a new inner seal and the outer O-ring. Your driveway will be glad.
When you move to put the gear back in, provide a little sprinkle of transmission fluid or assembly lubricant. You don't desire it to have its first "conversation" along with the drive gear while bone dried out. Once it's sitting, make sure the particular retainer clip is definitely tight. If that will clip is shed, the housing may back out simply enough to cause a massive leak or even strip the equipment.
Testing this out in the particular real-world
Once everything is buttoned up, don't simply take it regarding a spin across the block and call it good. Grab the GPS app on your own phone or the dedicated GPS device and head out to a straight stretch of road. Check your speed at 30, 45, and 60 advise. Mechanical speedometers aren't always perfectly linear, but you must be within 1 or even 2 mph throughout the board if you got the tooth count right.
If you're nevertheless off by the bit, don't sweating it. Sometimes a person have to increase or down a single tooth from what the "math" suggested mainly because tire manufacturers aren't always honest regarding their actual installed diameter. A 35-inch tire rarely actions a true 35 inches when the weight of a pickup truck is squishing this against the sidewalk.
In the particular end, swapping a good np208 speedometer gear any of those satisfying "Saturday morning" projects. It doesn't take much more than a several basic hand equipment and also a bit associated with patience, however the payoff is huge. There's just something good about looking at your own dash and also understanding how fast you're going—especially when you see those blue lights in the backview mirror and can with certainty say, "Officer, I actually know exactly how fast I had been traveling. "