new gears, wrong speedo reading

[mfinney]

Hello, on my 72 K20 the previous owner rebuilt the differentials and put 3:55's in instead of like 4:10's. My guess is that then my speedo reads 52-53 i'm going around 65 or so. I'm also running 255/85/R16 tires with new tread. Where is the best place to get the little gear for the speedo cable that goes into the tranny and what one might I need? Thanks.

[hounddogs!]

I have an external gearset between the cable and the trans. If you can tell them that your odometer shows 0.7 miles over 1 road mile (or better, 7.2 miles over 10) a shop could set you up.

My (semi)local shop is Metro Speedometer in Independence, MO 816-478-0116

[68gmsee]

Yeah... If you have a shop that can calibrate speedometers that's the best way to do it. Otherwise, you can get a fairly close percentage of your speed by having someone who has a good speedometer follow you and tell you how fast you're going. I used my Trailblazer.

Another way, like hounddogs said, is to test it between with mile markers. That would be the best way actaully if you know for sure that the markers are set up correctly.

Then go to some of the sites that help you select the correct gears. On the second one, cursor down to your trans.

http://www.transmissioncenter.net/sp...n_______va.htm

http://www.transmissioncenter.org/GM...eter_Gears.htm

[69GMCLonghorn]

Originally Posted by 68gmsee

Yeah... If you have a shop that can calibrate speedometers that's the best way to do it. Otherwise, you can get a fairly close percentage of your speed by having someone who has a good speedometer follow you and tell you how fast you're going. I used my Trailblazer.

Another way, like hounddogs said, is to test it between with mile markers. That would be the best way actaully if you know for sure that the markers are set up correctly.

Then go to some of the sites that help you select the correct gears. On the second one, cursor down to your trans.

http://www.transmissioncenter.net/sp...n_______va.htm

http://www.transmissioncenter.org/GM...eter_Gears.htm

Also if you have AAA, at least around these parts, they have a roving speedo calibration machine that makes its rounds at the various offices. I believe members can get a free calibration. What's that saying? "Membership has its privledges..."

[geezer#99]

I just corrected my speedo using a ratio adaptor I bought on ebay from patctrans. Was 60 bucks. They have various adaptors depending on how slow or fast you want to change your speedo.

[unclewatts]

Cheapest way to go is to buy the right speedometer gear located in the transmission. Classic parts has them for $6.95 to 10.95 depending on the number of teeth.

[mfinney]

Thanks for the input. It's a np205 and I can pull the cable off and check the # of teeth of the drive gear but can someone just tell me the # of teeth on the drive teeth? Thanks!

[Legolas894]

Also try your local GM dealer. I got my speedometer gear from them. I think it was less than $10.00. You need to know the right number of teeth though and the colour if you're trying to find a replacement.

[LockDoc]

Originally Posted by mfinney

Thanks for the input. It's a np205 and I can pull the cable off and check the # of teeth of the drive gear but can someone just tell me the # of teeth on the drive teeth? Thanks!

I can't but I bet someone here can if the on site calculators don't help.

http://www.transmissioncenter.net/sp...n_______va.htm

LockDoc

[unclewatts]

Originally Posted by mfinney

Thanks for the input. It's a np205 and I can pull the cable off and check the # of teeth of the drive gear but can someone just tell me the # of teeth on the drive teeth? Thanks!

That's the problem. You might call Classic Parts Tech department at 1-816-741-8029 and see if they can help you out.

[texanidiot25]

Learn what each speed on the speedometer really is like I've done... LOL

[geezer#99]

Sure it's about 10 bucks for a new gear if they have one with enough teeth on it to correct your speedo. If not then you're into changing the drive gear in the tailshaft. Bin there, done that. If you can't do it yourself you're gonna pay someone. Suddenly we're over a 100 bucks or more. Not to mention the u-joint and tailshaft bearing you'll change too. I found it cheaper and easier to buy the ratio adaptor. JMHO

[mfinney]

Thanks guys, I'm gonna check it out today. I'd like it to be a $10 fix. But we'll see! If it's too expensive i'd just leave it and get used to how off it is.

[mrein3]

You may be in luck.

All the still factory NP205s I've seen that came with 3.73 gears had NO 'adapter' on the speedo cable off of the transfercase. The one 3.07 4x4 I've dealt with had an adapter screwed into the transfer case that the speedo cable then screwed into.

The difference between 3.73 and 3.07 is approximately 20%. The difference between 4.10 and 3.55 is approximately 20%. Now if for some reason that 205 has its speedo output geared for 4.10s, you may be able to get one of these 'adaptors', place it in-line, and be done.

Now I'm really guessing here but if your truck came with 4.10s it probably had an adapter on the speedo cable that changed the speed of the cable to compensate for 4.10s - again remember I've seen many 3.73s with no adapter and perfectly reading speedos.

I'll try to get a picture of my speedo adapter so you know what to look for. Mine isn't for sale - I'm not allowed to sell stuff here. Perhaps after seeing the piece I'm talking about you'll know what to look for.