Is it the speedo gear?

[68 ReNay]

Hey Guys,
i posted a long time ago about a bouncing speedo needle problem. you all offered several suggestions - thank you - unfortunately, nothing i tried fixed the problem. it still bounces from zero and tapers to steady by 42mph.

a couple months ago i discovered my speedometer is off. it's a new OEM speedo, but most likely the original one i replaced was off too, since my tires are larger than the original diameter tires.

but my question is this, should the speed be off a consistent number of MPH's, regardless of speed, or does the difference increase with speed?

i checked mine out and here's what i discovered:
(left side number is actual MPH, right side number is my speedo reading)
30mph - 30mph
35mph - 37mph
40mph - 43mph
45mph - 49mph
50mph - 55mph
55mph - 62mph
60mph - 67mph
65mph - 73mph
if that doesn't sound right, what else should i be looking for?
if that seems reasonable, how do i decide what number of teeth in the gear i need to buy? i know each added tooth adds an mph or two, etc. but do i 'gear' it for what speed i drive most often (ie. highway)?

Please help!
P.S. it has a TH350, if that matters.

[mrein3]

Looks like you're off by a constant 10%.
Think about it. At 40 mph, add 10% and you get 44 (your table says 43)
At 65 add 10% you get 72.5 (your table says 73).

This is why it is important to learn math. Your high school teacher was right, you really do use math every day.

For a bouncy speedo you may need to lubricate the cable. You can remove the cable from the back of the odometer, pull out the center, lubricate it, and stuff it back in there. If it gets cold where you are don't use a lube like white lithium grease - that stuff can get pretty stiff when it is cold.

[Richard H.]

Here is some help.
http://chevelleengineer.home.comcast...eer/speedo.htm
More help..
http://www.transmissioncenter.net/sp...n_______va.htm

And speed cable lube "graphite" at a parts store.

[Zeke's Garage]

A bouncy needle is going to be caused by the flex in the speedo cable shaft. You need to remove it completely (kind of a pain to get the hold down off below the master cylinder/booster), take out the flexi-shaft and clean the snot out of both pieces with your choice of liver damaging solvents. (Engine starting fluid is my favorite). Once you get all the old lube out of the cable housing and off the flexi-shaft, use a lube specifically for cables, or a graphite based lube so that it won't get gummed up again in the future.

As for the speedo off-set, you need to figure out your tire size, rear-end ratio and transmission type, then use one of the calculators found on the inter-webz and determine the color of the speedo gear drive you need.

Good Luck!

[Keith Seymore]

Or - since we've already determined you are off by 10% - just buy a speedo adapter and throw it in.

K

[67Slider]

My speedo was jumping but it was because the driven gear had a chip in the input end and was slipping. I took it out and tested it with a drill and it worked fine. I tested a couple of speedo's while I was at it. Before you buy some drive gears and driven gears let me know what you need. I ended up buying several gears until I found the right combo. I have an orange (8 tooth)drive gear and a 20 & 21 driven gear.

[68 ReNay]

Thanks for the replies guys! and sorry i'm so long in responding. i forgot to check the box to get notified when someone responded. then i just got swamped and side tracked.

the speedo cable and housing are new. that was the first fix i tried, to correct the bouncing needle. but i also pulled that new cable out and graphite oiled the crap out of it - no change. i bought the new speedometer as a 'last resort' - no change.

mrein3, i never did like math and i don't really know what the significance of being off by a constant 10% means (in order to fix the problem) in your post.

Keith, i hadn't heard of a speedo adapter before. i'll have to check into that.

Thanks 67Slider. that's about the only thing i haven't checked. i pulled out the drive gear at the end of the cable and it looked good to me.

Thanks again for everyone's input. i'll let ya know what i find out - when i find the time to look at it!

[mrein3]

Originally Posted by 68 ReNay

snip
but my question is this, should the speed be off a consistent number of MPH's, regardless of speed, or does the difference increase with speed?

i checked mine out and here's what i discovered:
(left side number is actual MPH, right side number is my speedo reading)
30mph - 30mph
35mph - 37mph
40mph - 43mph
45mph - 49mph
50mph - 55mph
55mph - 62mph
60mph - 67mph
65mph - 73mph
snip

Originally Posted by 68 ReNay

snip

mrein3, i never did like math and i don't really know what the significance of being off by a constant 10% means (in order to fix the problem) in your post.

snip

You were wondering why you're not off by a constant number of miles per hour. And you're right. You're not off by a constant number of miles per hour but you are off by a constant percentage of miles per hour. Most likely this is caused by you or a previous owner putting on the wrong sized set of tires.

This incorrect mph reading is totally correctible. The "signal" for the speedo is generated by a gear set in the transmission behind where your cable screws in. There are links on the internet where you type in your tire size and rear end gear ratio and the program or spread sheet tells you what two colors you need for the gears in the speedo signal generator.

Since you've replaced the cable and speedometer already, and since this gear set is the only moving part left in the system, if your needle still bounces, I'd revisit the cable.

Another thing to consider is the lube you used on the cable. I don't know how cold it gets in Kansas but here in Minnesota a good way to make your needle to bounce is to put lube on it that gets hard in the cold then drive it on a cold day.

[68 ReNay]

Thanks mrein3!
i purchased the truck with 275-60-15's already on it - which i'm sure are not the same diameter as the stock tires it once had.

i used graphite oil to lube the cable, after researching, it seemed the better choice. we get some nasty cold days in winter, but apparently it's not the lube gunking up cause it bounces the same whether winter or summer. i'll have to revisit the cable - while also checking the websites for correct color gears specific to my set up.

thanks again for the input!

[MARKDTN]

Have you replaced the "bullet" that holds the driven gear at the transmission? The aluminum bullets wear and can cause leaks or bouncing. As far as an adapter, why spend $75 when you can probably get the correct driven gear to match your drive gear for less than $10? I would figure out which gears you have, there are websites that have this information, and just buy a new bullet and a correct driven gear. Might fix both problems.

[eightbanger]

This is the driven gear, it's the brown plastic part....and where you'll find it on your tranny.

[68 ReNay]

With any luck (time availability) i'll crawl under the ol' beast this weekend and check things out. i'll post my findings. thanks again everyone!