Speedometer does not read actual speed...

[68gmsee]

I have a 68 GMC with a stock 250. Some year ago I removed the 3 speed non synchronized trans for a synchronized one that came off a V8 truck.

I found out my speedometer is 10 mph off!!! Shows 20 mph and it should be 30 mph and 25 mph and it should be 35 mph (measured with another newer vehicle). Don't drive it much and usually on country roads. Also, I usually drive slow but I'm lucky I haven't been caught speeding.

I read where they make gears to fix this, but can't find out where to get them.

Any help appreciated.

[jhow66]

GM dealers still have them.

[BAGGERSOCAL]

Would also like to know my speedo is off a bit

[kwmech]

That is quite a bit for the speedo to be off by 30 percent. Still have the old trans? Should be able to swap out the drive gear.

[Longhorn Man]

30% ewouldn't surprise me... mihne used to read 100 MPH at about 75, and now, with a blind gear swap, it reads 35 at about 70

[68gmsee]

Originally Posted by Longhorn Man

30% ewouldn't surprise me... mihne used to read 100 MPH at about 75, and now, with a blind gear swap, it reads 35 at about 70

Holy mackerel, that's off quite a bit! I found a site on the interenet that shows you how to figure your speed vs speedometer. Sort of complicated but I may try it.

kwmech, I don't have the old trans anymore. I think I left it behind in one of my moves about 10 years ago.

I have another question, if I find the gears, do I have to also replace the internal drive gear as well as the driven gear? I would suspect I will since it looks like they're a matched set...

[Premium72]

you can go to any transmission shop and get one off them. i lost my gear during tear down, i went to the local shop he just gave me one for free. go ask em theyll dig around and find sumthn.

[kwmech]

Originally Posted by Longhorn Man

30% ewouldn't surprise me... mihne used to read 100 MPH at about 75, and now, with a blind gear swap, it reads 35 at about 70

sounds like you went the wrong way

[70C10Toejam]

being off a set amount seems kinda odd, not what i thought gears would cause. would think gearing would cause a set percent. mine is off 10% because of oversized tires (not by much) 50 is really 55, 60 is really 66 ad nausium.

I came across a website on checking speedo vs. real speed with a stopwatch. basically it is 60 seconds for one mile at 60 (duh) and then one second more for each mph below 60 and one second less for each mph over 60. about every 5 mph it jumps 2 seconds

66 sec. @ 55mph
64 sec. @ 56mph
63 sec. @ 57mph
62 sec. @ 58mph
61 sec. @ 59mph
60 sec. @ 60mph
59 sec. @ 61mph
58 sec. @ 62mph
57 sec. @ 63mph
56 sec. @ 64mph
54 sec. @ 65mph

I printed it off, but can't find it and I think it went further beyond this range. The more miles you cover the more accurate it will be. Spreads out any inconsistancies.

[68gmsee]

Originally Posted by 70C10Toejam

being off a set amount seems kinda odd, not what i thought gears would cause. would think gearing would cause a set percent. mine is off 10% because of oversized tires (not by much) 50 is really 55, 60 is really 66 ad nausium.

This may very well be the case with my truck. My wife was behind me when I was going 20 mph. She says she was seeing 30 and when I went to 25 she said she saw 35. However, it could have been approximate. We were on a gravel road and some bouncing going on...

So, it may not be as exactly as she read it. I'll try it on a straight highway before I do any gear changing...

[trucky72]

LOL that happened to me when I bought my truck. It was a rainy day, not too many other cars on the road, I drove it and man it was quick! Like a scalded dog quick... I drove it back to the guy and he leaned toward me and tapped me on the arm and said, "What'd ya think? huh? huh?" I said, "Man that thing sure is fast!" The next day driving it home the speedo said 70 but cars were passing me left and right! I have been told it's the same thing as your problem. I was going to take it to the speedo calibration shop tho... the trans guy around me is kinda greedy!

[LockDoc]

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Here are a couple of web sites you might take a look at....

http://www.bgsoflex.com/speedo1.html

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

Actually everything you ever wanted to know about speedo gears.....

LockDoc

[68gmsee]

LockDoc, thanks for the sites... That's what I was looking for.

[LockDoc]

Originally Posted by 68gmsee

LockDoc, thanks for the sites... That's what I was looking for.

-
No problem... Great board 'aint it.....

LockDoc

[68gmsee]

Don't mean to keep this one going, but I thought I'd keep some of you that are having problems with speedometer not reading correctly updated.

I just stopped by the chevy dealer and threw the old 22 teeth driven gear on the counter and asked the guy if had some of these. He said, yes, he had them. I asked if he had one with fewer teeth on the driven gear (mine has 22 teeth). I figured less teeth = faster rotation on the driven gear which will increase my speedometer reading. He brought back a white one with 19 teeth. Cost was $6.00.

Since I was messing with all this, I noticed the rear end drive shaft seal and the transmission seal were leaking. And since I was going to remove the shaft to replace them, I went ahead and got u-joints.

When I get it all back together again I will try it to see if that brings the speedometer closer to the correct speed. I'm not worried about a few mph over or under.

[jimfulco]

http://novaresource.org/speedo.htm has the math and charts to figure out exactly what you need.

[Scott Webster]

hmmmm - suddenly my speedo at 35 when I'm doing 55 soesn't seem so bad. I went to the dealer down the road and they said it was too old to get the gear. I never bothered look anywhere else since I don't drive it out of town a whole lot

[68gmsee]

Continuing with update.

Got the rear end and transmission seals in, new u-joints, new speedometer cable (the old one was cracked) and put the new 19 tooth speedometer driven gear into the transmission.

Had my wife follow me on the same road and this time I was 5 mph off at 25 instead of 10 mph off. In other words, when I was going 20 my wife was doing 25.

I'll have to try it at faster speeds later on. I can live with this for right now...

[Longhorn Man]

Try another dealership, the parts guy at the one you went to is either a lazy bum, or is afraid to open a book.
They are still avalible.

Originally Posted by Scott Webster

hmmmm - suddenly my speedo at 35 when I'm doing 55 soesn't seem so bad. I went to the dealer down the road and they said it was too old to get the gear. I never bothered look anywhere else since I don't drive it out of town a whole lot

[retodd7711]

Originally Posted by Longhorn Man

Try another dealership, the parts guy at the one you went to is either a lazy bum, or is afraid to open a book.
They are still avalible.

And that right there ticks me off. Remember when parts stores were helpful?

[68gmsee]

Originally Posted by Longhorn Man

Try another dealership, the parts guy at the one you went to is either a lazy bum, or is afraid to open a book.
They are still avalible.

Longhorn Man is correct. Sometimes of these guys say that they're obsolete as soon as you say the year of our trucks. It's true that most parts are, but some of them applied to other year vehicles as well and are still available...