Speedometer Gear Replacement / Calibration

[StevePT]

So this weekend I worked on recalibrating the speedo in my '82 4x4. I know that when I'm on the highway and the speedometer says I'm doing 55mph, I'm actually doing more like 65mph. I'm running P225/75R15 tires on it, which are apparently considerably larger than stock so my speedo is off by quite a bit.

I was able to find on http://www.michelin.com the specs for my tire and it gives the "revolutions per mile." Using this number is more accurate than calculating driveline speed based on the tire size. I'll post the rest of the equation later, but I was able to find out that I needed a 3:1 ratio between the driveline speed and the speedometer speed. It just so happens that the gear from my '86 Turbo/D project is an 18x6 or 3:1 ratio. When I took the gear out of the '82 P'up 4x4 I found it was marked 20x6 which makes sense that it wasn't spinning fast enough to make my speedometer read correctly.

I got it all buttoned up and haven't tested it yet for accuracy, but based on the math I should only be 1% off and based on driving it a few miles I can tell that the engine noise is the same at 65mph as it was before at 55mph when it was off.

I'm not sure how many gears out there are available, but for anyone that is running larger tires on a 4x4 like me, you can get your speedo back to where it should be by getting the gear out of a 2WD w/ 3.73 gears.

Here's some pics to show the process:

P'up jacked up in the rear to keep the oil from draining out and blocked in the front for safety.

Tools needed for the job. Oil pan to catch any oil that might come out, rubber headed hammer to tap in the collar during reinstallation and a 12mm wrench to remove the bolt that holds down the collar.

Speedometer drive gear installed in the tailshaft housing. This is cast iron on a 4x4 or aluminum on a 4x2.

Empty hole where the speedometer drive gear belongs.

Speedometer drive gear out of the transmission.

Work the collar in by hand as far as possible and then tap it in until it's flush with a rubber headed hammer/mallet.

reinstall the cable making sure to get the tab lined up with the slot in the gear's drive shaft. The flange of the cable should sit flush with the aluminum collar.

[JoeIsuzu]

Steve,

Great step-by-step. I can't stress this one enough:

Work the collar in by hand as far as possible and then tap it in until it's flush with a rubber headed hammer/mallet.

If you don't have it flush, you will break off the retainer tab when you tighten the bolt (like I did!).

Moving this to the "How-to" section.

One question. The housings have different numbers embossed on them: "1", "2", etc. Here's mine, before I broke it, but the one I got from Larry (quicknotslow) had a "1" or a "3" on it (can't remember).

Since some of the drive gears have more teeth, are thus larger in diameter, in order for the teeth to still mesh with the gear in the transmission, is it possible that they used different collars with different amounts of offset? I think the collars are paired with the gears, and you should change BOTH, not just the gear. Can someone speak with authority on this?

Jack

[StevePT]

Jack,

My guess is that the number you see on the mounting ear, in your case it's a "2" is just a tool mark that shows which cavity it came from. Often on molded or cast parts they will have more than one cavity so they can build more parts faster. My guess is that's all this is.

There's no reason the steel drive shaft should be larger. Yes, the nylon gear looked to be a touch larger on the 20-tooth gear compared to the 18-tooth gear, but the I.D. of the nylon gears are the same and they both used the same size steel drive shaft.

For the record I have an '82 and an '86 P'up and both collars had o-rings in them. I've seen pictures of the lip seal that is suppose to supersede the o-ring design, but if it wasn't used in my '86 then I don't know when they changed to that design... Also, I used the 18x6 gear in the same collar that was in my '82 P'up and it's working great. I didn't pay attention to the number on the housing, but I'll take a look at that. Though again, my guess is that it's just a tooling mark to indicate which tool it came from.

[quicknotslow]

Jack,

I put My gear back in the housing that I got back from you. It slid in and meshed with both gears. I thank what Steve said is right, with the the numbers being just casting marks. But and this is a Big But. What year can you use the seal that we got from Jerry?

[crazyyth]

does the 3:1 only work for that size tire? or should i say that many revs per mile?
Britt

[JoeIsuzu]

What year can you use the seal that we got from Jerry?

I don't know, but I have the seal and I'm hoping to eventually swap out the housing. I understand it also requires a different drive gear, too.

Jack

[StevePT]

does the 3:1 only work for that size tire? or should i say that many revs per mile?
Britt

Britt,

If you have a 4.10 differential, which I expect you should because you have a 4x4, then yes a 3:1 ratio speedometer gear will work perfectly with P225/75R15 tires. If you can justify the money, I would highly recommend the Michelin LTX M/S tires. These things balanced properly make the thing ride like its on glass and they show no wear at all. I've heard of numerous people running the Michelin LTX tires and getting well over 60K miles out of a set.

[zank2]

Hi, I bought the 84 pup that was on ebay the 1st of the month. The blue one with the turbo and 5 speed. The speedometer is off bad. At 25 I and going 35 and at 46 I am doing 60.
Just checked the tires and they are p205/ 70R/ 14/ Any guess what gear I have and should change it to. Thanks ED

[JoeIsuzu]

Hi, I bought the 84 pup that was on ebay the 1st of the month. The blue one with the turbo and 5 speed. The speedometer is off bad. At 25 I and going 35 and at 46 I am doing 60.
Just checked the tires and they are p205/ 70R/ 14/ Any guess what gear I have and should change it to. Thanks ED

We had some discussion about that truck, because it had some mismatched equipment. Is this it?
viewtopic.php?f=3&t=4269

Sounds like you have the rear diff from a 4-speed truck. According to some old posts here, the drive gear on the tailshaft is also different, so simply changing the driven gear (on the cable) may not put things completely right to match the 3.42 rear end.

My driven gear has "6x18" stamped on the shaft, and has 18 teeth (I have a 3.73 diff). Larry (quicknotslow) sent me one that had "6x20" on the shaft, and I don't know which diff it had. But if you have a 5-speed with a 3.42 rear, and the original 18-tooth gear, your speedometer should read 9% too low (actual speed is 9% higher than indicated), with stock tires. Swap the gear out with the 20-tooth gear, and it would change the speedometer reading by about 11%.

There's also a Borg-Warner device that can be installed for speedometer calibration. I think it's called a "speedometer correction gear" or something similar.

Jack

[zank2]

Yes that is the truck and I was told it was a 4 speed and they put a 5 speed in it.
Runs great so far and I got oil and filter for it and will change shortly. Little puff when starts but that is all. Starts easy and great so far. Thanks for asking.
Now who did you say mite have the gear.

Thanks ED

[JoeIsuzu]

Now who did you say mite have the gear.

That's a tricky one. You'd need to know what year the transmission came out of. They changed the seal design sometime after '83, because the seals would wear, and the cable pulled transmission oil right into the cab. With the new seal design came a different drive gear. I think the one in this tutorial is the "later" version (right Steven?). Mine, in this thread is the older one. Can you tell which one is more like yours?

If you know specifically which one you're looking for, that might help.

Jack

[quicknotslow]

Hi, I bought the 84 pup that was on ebay the 1st of the month. The blue one with the turbo and 5 speed. The speedometer is off bad. At 25 I and going 35 and at 46 I am doing 60.
Just checked the tires and they are p205/ 70R/ 14/ Any guess what gear I have and should change it to. Thanks ED

We had some discussion about that truck, because it had some mismatched equipment. Is this it?
http://www.isuzupup.com/viewtopic.php?f=3&t=4269

Sounds like you have the rear diff from a 4-speed truck. According to some old posts here, the drive gear on the tailshaft is also different, so simply changing the driven gear (on the cable) may not put things completely right to match the 3.42 rear end.

My driven gear has "6x18" stamped on the shaft, and has 18 teeth (I have a 3.73 diff). Larry (quicknotslow) sent me one that had "6x20" on the shaft, and I don't know which diff it had. But if you have a 5-speed with a 3.42 rear, and the original 18-tooth gear, your speedometer should read 9% too low (actual speed is 9% higher than indicated), with stock tires. Swap the gear out with the 20-tooth gear, and it would change the speedometer reading by about 11%.

There's also a Borg-Warner device that can be installed for speedometer calibration. I think it's called a "speedometer correction gear" or something similar.

Jack

Jack,

That gear was out of a 86 L/S long bed with a 5 speed gas engine. Mabe some can jump in a tell us what the rear end is.

[Paul]

Hi All--

I have posted to this subject before but it has been a long time and things get lost. Here is a link that references my post and elaborates a bit:

viewtopic.php?f=18&t=199&hilit=+speedometer

I think there is some latitude in the use of various driven gears with any one drive gear but it is probably pretty limited. I have one driven gear here that is close to worn out and it looks like it was not fully meshed.

Also, the selection of ratios is limited so if you have changed rear end ratio and /or tire size it will be tough to get an Isuzu gear set that will give accurate speedo readings. Also, at one point I thought I had figured out what Isuzu ratio (and parts) that I needed but was told that one of the gears was discontinued.

So, I am building a case for using the Stewart Warner 'Drive Joint'. This an excellent gadget and is easy to use. It will cost somewhere between $50 and $100 new. But they are available used. Try ebay. If you get a used one you will probably need to get different gears for it. The speedo shops carry these and they cost about $2 each and there are four gears.

To pick the gears you will just need to drive between mile markers and take note of your speedo error in percent. Then go to the speedo shop and they have a chart for which gears to use for any given error. Good to go. Corrections to less than 1%.

Paul

[JoeIsuzu]

It just so happens that the gear from my '86 Turbo/D project is an 18x6 or 3:1 ratio.

Wow, I just noticed what you said. I was under the assumption that the "6" referred to the gears' "pitch", not a ratio...18 teeth, with a pitch of 6 (teeth per centimeter?). It would need to be mated with another gear to have a ratio. Am I off here?

Jack

[JoeIsuzu]

I'm reposting this, since it looks like we never resolved it...

What year can you use the seal that we got from Jerry?

To clarify for any latecomers: My '83 had an O-ring, which seems to be the original style. Jerry said that there was a redesigned style that used a different seal. I have one of those seals, but I'd say that neither the housing, seal, or the gear are cross-compatible with old style.

Does anyone know what vehicles came with the newer design? Or were they just replacements for the people who asked for them?

Jack