Odometer Not Even Close

[justagoodolboy]

My truck's interior is not finished yet so I don't drive it much but it seemed to me that the miles were not adding up on the odometer very fast but I didn't give it much thought till I checked it with my gps. I wrote down my mileage and drove into town. My gps said I went 5.1 miles and my truck said 2.2. My speedometer is 4 MPH slow- It makes no difference whether I drive 10 or 60 i'm going 4 MPH faster than it's showing, but that won't throw my odometer off that far will it? I tried using the search but couldn't find anything and I don't know how these odometers work- can someone please help me out here?

BTW, my guages are rebuilt white face ones from the guy on ebay. I don't know what it was like before I tore my truck apart-I never checked.

My truck is a 72 Chevy 4x4 3.73 350/350 on 15x8 ralley wheels 31x11.50's

[omodified]

You actually have a good problem. I remember a while back, I had a lease vehicle. I think I got 12,000 mile allowance. I bought some wheels and tires that were a smaller diameter than stock cause I lowered it a bunch. Well, the smaller tire made the speedometer 5 mph faster which of course put more miles on it than it should have. I really kicked myself for that one.

Your case is the opposite of mine. Yours is slower and it is putting less miles on it. I understand you probably just want it to be correct, I would want the same for my C10. So, you can either deal with it, or you can change out the speedometer gear. I can't tell you which one you will need as it will depend on your rear end gear as well. I would start with counting the teeth on your current gear and going from there.

Do an EBAY search for C10 NOS and a bunch of them will be listed there. You can also search 350 speedometer gear and there are a bunch more. Good luck!

[dsy1]

Changing the speedo gear at the transmission is easy but it will affect both the speedometer and the odometer. I don't see how the speedo can be pretty close (although 4 MPH at 40 is still 10% off) but the miles read less than half of what they should. If the wheels or in the odometer have wear it may be that they are not advancing the way they should.

Does your speedo needle hold steady, or does it bounce some when you are at a steady speed? I had a bad odometer and the gears on the number wheels would bind up and not turn correctly, but that made the speedo needle bounce whenever the odometer wheels would stick.

[oldgold70c10]

Go here: speedometer world they have a chart to help you figure out what speedometer gear combo in the transmission you need based on your tire size & rear axle ratio. The odometer part is pretty well fixed in the speedometer head, 1000 revolutions of the speedometer cable is 1 mile. The actual indicated speed is adjustable for calibration purposes, but doing any adjusting will be worthless if the odometer is off.

[PatrickGMC70]

I'm hardly an expert, but logic tells me that if at any speed, you're always off by 4mph, then how can it be the speedo gear/tires/rear end, etc? Changing those would make the speed off by a percentage, which means that the faster you went, the more the mph would be off - not a constant 4 mph. Instead, I would think that the gauge is not properly set at zero, but again, I'm no expert.

Interested to see what the opinions are.

[jocko]

I believe you are correct Patrick, i.e. it's a proportional percentage. For example, on my dd (an 01 chevy truck) I have slightly taller tires than stock - the result, as expected, is that I'm "showing" less than I'm "going"... NOT good if you're trying to avoid tickets. Anyhoo, to the point about percentage amount off - when I'm going 50-55, it's only off by about 3-4 mph, when I'm up at 75 mph, it's off by 5-6 mph (both on the slow side).

[70cst]

Originally Posted by oldgold70c10

Go here: speedometer world they have a chart to help you figure out what speedometer gear combo in the transmission you need based on your tire size & rear axle ratio. The odometer part is pretty well fixed in the speedometer head, 1000 revolutions of the speedometer cable is 1 mile. The actual indicated speed is adjustable for calibration purposes, but doing any adjusting will be worthless if the odometer is off.

Thanks for the LINK

[DaBigKahuna]

Yes it's proportional. Before I fixed it, mine was 10% over so when it read 40 I was doing 36 when it read 70 I was doing 63. At least according to my GPS.

[justagoodolboy]

Originally Posted by dsy1

Does your speedo needle hold steady, or does it bounce some when you are at a steady speed? I had a bad odometer and the gears on the number wheels would bind up and not turn correctly, but that made the speedo needle bounce whenever the odometer wheels would stick.

Good thought. My needle doesn't bounce a lot but when I speed up the needle will hesitate just a tiny bit then jump quickly to catch up and then then it bounces a little as it settles down- it's kinda like it sticks just a bit. I think it only does it when I speed up quickly but I can't remember for sure.

Thanks for the comments all. I don't have time to look at anything right now- gotta get to work. Maybe at lunch I can click on that link.

[48richard]

I chased around a similar problem. Found that 2 teeth on the speedo driven gear in the transmission were worn down to almost nothing. My speedo was also reading 10% low because I have 31" tires, so I replaced both gears in the transmission and solved all my issues - rock steady needle, accurate to 1 mph at 60, and correct odometer reading.

[justagoodolboy]

Originally Posted by 48richard

I chased around a similar problem. Found that 2 teeth on the speedo driven gear in the transmission were worn down to almost nothing. My speedo was also reading 10% low because I have 31" tires, so I replaced both gears in the transmission and solved all my issues - rock steady needle, accurate to 1 mph at 60, and correct odometer reading.

If that worked for you hopefully it'll fix my problem. I'll have to check the teeth first chance I get. the speedo being off doesn't bother me that much but the odometer drives me nuts.

[425HP409]

The odometer is a direct mechanical drive off of the speedometer cable. You can change the gears in your transmission, or get a reducer adapter to correct this, but any change you make with the odometer will also change what the speedometer reads by the same proportional amount. The problem you have is twofold. 1. The speedometer gears are wrong for your truck with the gearing you have. 2. The speedometer is way off of calibration. If you change your odometer to read correct, the speedo will be really way off. You will not be able to make both of them read correctly with the existing speedometer that you have unless you take the speedometer out and have it calibrated magnetically at a speedo repair shop.

[FL71C10]

The good news is that once you fix it your gas mileage will go up significantly!

[justagoodolboy]

Originally Posted by FL71C10

The good news is that once you fix it your gas mileage will go up significantly!

Good point. If I ever start driving it daily I'll have to fix it cause I write my miles down for tax purposes. But I'll prolly try to fix it anyway. I'd rather it didn't work at all than to have it not working right. Stuff like this bugs me like crazy.

[Palf70Step]

I think you have a mechanical issue in your speed head, mainly the odometer. probably a wore gear in there. I think before I would messing with the gears to get the speed accurate, I would tray gauge head to see if your odometer acts correctly (or at least closer). Then I would dive into getting the speedometer accurate.

[justagoodolboy]

Originally Posted by Palf70Step

I think you have a mechanical issue in your speed head, mainly the odometer. probably a wore gear in there. I think before I would messing with the gears to get the speed accurate, I would tray gauge head to see if your odometer acts correctly (or at least closer). Then I would dive into getting the speedometer accurate.

Makes sense cause I drove it to work today and I'm noticing that the odometer is not consistant at all- last night driving home it was turning over just about dead on with the gps. Like 1-2 tenths off in a couple miles. and this morn it was real close part of the time but when I got to work my gps said 5.6 and the truck said 3.2. So I drove half a mile more than in my first post but my truck thought it was a mile more.

[justagoodolboy]

On second thought, are you saying I should replace the gauge itself? I have "new" guages from the guy on ebay but I guess that doesn't garentee mine's not bad.

[old Chevy guy]

Just because its new dosn't mean it can't be broke.

[Palf70Step]

Originally Posted by justagoodolboy

On second thought, are you saying I should replace the gauge itself? I have "new" guages from the guy on ebay but I guess that doesn't garentee mine's not bad.

That is what I was thinking. I didn't realize you had new gauges. Thought they might have been you originals with new facing put on them.
If you have another guage I would try it just to see if you get a different reading.

[justagoodolboy]

I was reading the ad on ebay and sounds like the guy uses original parts for the clusters he sells so could be mine is wore out. He says he'll replace any defective parts for free but I'd like to know for sure if that's the problem. I don't have my old cluster any more but maybe I can ask around and get one. I sprained my ankle really bad last night so could be a while till I'm going again

[justagoodolboy]

Ok I called the guy that sold me the guages and he was sure its in the odometer so he sent me another one of those roller things and it does the same thing. drove around the block last night and my gps said 7.2 miles and the truck with the new odometer installed said 3.1. any ideas?

[midniteblues]

My speedo needle bounced too.hopefully this new gear fixes that. not only was some of the teeth really worn but the gear had a concave wear pattern to it.

[oldgold70c10]

OK, do this then:
find something that you can use as a handle like an allen wrench & put it in the back where the speedo cable goes on the speedo. Turn it 100 turns. You've got to turn it forwards, because there is a ratchet-like device in the odometer reel that prevents miles from being subtracted in reverse (remember Farris Beuller's Day Off?) The odometer should move 1/10th of a mile. You could do this 10 times if you've got the patience and every gear in the thing would have gone round at least 1 turn & it would be 1 mile from where you started.
Anyway, if it passes that, get a short piece of old speedo cable or something that will let you spin it with a drill motor without damaging the square hole that the speedo cable goes in. I like using a small flat blade screwdriver bit. Run it at about 60 mph and watch the odometer. It should do one mile in one minute. If the odometer pauses or does less than 1 mile, somethings wrong. If everything checks out OK it your speedo cable or speedo gears in the transmission.

[justagoodolboy]

Originally Posted by oldgold70c10

OK, do this then:
find something that you can use as a handle like an allen wrench & put it in the back where the speedo cable goes on the speedo. Turn it 100 turns. You've got to turn it forwards, because there is a ratchet-like device in the odometer reel that prevents miles from being subtracted in reverse (remember Farris Beuller's Day Off?) The odometer should move 1/10th of a mile. You could do this 10 times if you've got the patience and every gear in the thing would have gone round at least 1 turn & it would be 1 mile from where you started.
Anyway, if it passes that, get a short piece of old speedo cable or something that will let you spin it with a drill motor without damaging the square hole that the speedo cable goes in. I like using a small flat blade screwdriver bit. Run it at about 60 mph and watch the odometer. It should do one mile in one minute. If the odometer pauses or does less than 1 mile, somethings wrong. If everything checks out OK it your speedo cable or speedo gears in the transmission.

Thanks a lot for your advice.