Incorrect Speedometer Reading

[my67c20]

TH350 trans, 3:73 rear gear, 29.5" tires, speedometer ~10 miles fast, verified by a GPS at different speeds. I tried some of the online tools, nothing made me feel confident in what they mentioned. Not 100% sure of size of output shaft on trans, apparently there is 1.84 & 1.76. Hard to tell colors from pictures, but looks like brown on trans (drive) and natural (driven) on end of speedo. However, no charts mention the brown drive gear, so it makes it difficult to calculate. Help is appreciated and I did search, but nothing really relevant was found.

[OregonNed]

Mine was off the same way at approximately 10 MPH fast. I can't remember what color gear I put in, but as soon as I did with that one change my problem was fixed. I bought the set of gears on Ebay, and I can look at what I have left by process of elimination.

[my67c20]

Did you just swap out the driven (small) gear until you achieved success? I see that i can get all drive and driven gear sets off of e-bay, but before I tear into the trans if I can achieve easy success first, that is always desirable.

[geezer#99]

I’ve always done the short cut.
Used a ratio adaptor.
Like this.
https://transmissioncenter.net/shop/...ratio-adapter/

[tdangle]

You can also use this calculator.

http://www.tciauto.com/tc/speedometer-gear-calculator/

[OregonNed]

Originally Posted by my67c20

Did you just swap out the driven (small) gear until you achieved success? I see that i can get all drive and driven gear sets off of e-bay, but before I tear into the trans if I can achieve easy success first, that is always desirable.

Yes, just the driven gear. I meant to look in the garage this morning at what I used, but the snow and ice event messed things up. Will take a look tonight, however..

[kwmech]

It's not really 10mph fast, it works on a percentage. So you'll have to reduce by that percentage that it is off. The speedo says you're doing 60 but Gps says 50 then you have to slow it down by 17%

[RichardJ]

Back in the day, speedometers on American cars were never very accurate. Car magazines doing road tests on the new cars as they came out, used a device they called a 5th wheel, to determine the speedometer accuracy. They did this routinely on every car and truck they test drove. They usually complained how European and Japanese cars faired better.
American manufacturers did this partly by design. Every knuckle head that bought a new car had to go out and do 100 mph for bragging rights. Speedometer error allowed them to not be as much of a dumb-ass as they thought they were. A built-safety margin. Besides, most European and Japanese cars in the 50s and 60s could only do 100 down hill. Ha !

Instead of going to the garage and finding an old car mag showing a 5th wheel picture to scan, I thought I could just do a simple google search. Google just could not get past 5th wheel trailers, but eventually I found these.

After steel bumpers went away in the 70s, they hung the 5th wheel off the rear wheel well openings on the side of the car. That finally gave way to radar guns and now GPS.

In 1970 if you found out your truck speedo was 10 mile off, you probably wouldn't give it a second thought, unless your glove box was full of speeding tickets.

[OregonNed]

I can’t seem to find the gear selection set I bought on EBay - but they are very easy to swap and road test. I used a GPS app to decide on the selection. I just can’t remember how many teeth were on my selection.

[my67c20]

FYI, for others. So i bought the gears shown in the picture. From what i can tell, my original was natural @19 teeth. With this gear and the brown drive gear, my speedo was reading high by about 10 mph. I swapped out the driven for the gray @ 22 teeth and now the speedo is reading high by about 5 mph, which I can live with, until I feel like I need to remove the tail shaft and change the drive gear to something with less teeth. Hope this info will help others.

[Canuckvetter]

I swapped my 3.73 for a 3.08. My speedo was reading 76 MPH when the gps said I was doing 60. I tried to get a driven gear to correct that but they didn’t exist with my drive gear. I determined the percentage the speedo was out and had an adapter made. Speedo is now bang on with the gps.

[rockyrivermark]

Originally Posted by my67c20

FYI, for others. So i bought the gears shown in the picture. From what i can tell, my original was natural @19 teeth. With this gear and the brown drive gear, my speedo was reading high by about 10 mph. I swapped out the driven for the gray @ 22 teeth and now the speedo is reading high by about 5 mph, which I can live with, until I feel like I need to remove the tail shaft and change the drive gear to something with less teeth. Hope this info will help others.

Yes. Just buy this set or similar which is cheap. The driven gear can be changed out in 5 minutes.
Download a gps speedometer app on your phone and change them out until you get one that's real close.
https://www.ebay.com/itm/TURBO-TH350...YAAOSwm79ceoEN

[rockyrivermark]

Originally Posted by RichardJ

Back in the day, speedometers on American cars were never very accurate. Car magazines doing road tests on the new cars as they came out, used a device they called a 5th wheel, to determine the speedometer accuracy. They did this routinely on every car and truck they test drove. They usually complained how European and Japanese cars faired better.
American manufacturers did this partly by design. Every knuckle head that bought a new car had to go out and do 100 mph for bragging rights. Speedometer error allowed them to not be as much of a dumb-ass as they thought they were. A built-safety margin. Besides, most European and Japanese cars in the 50s and 60s could only do 100 down hill. Ha !

Instead of going to the garage and finding an old car mag showing a 5th wheel picture to scan, I thought I could just do a simple google search. Google just could not get past 5th wheel trailers, but eventually I found these.

After steel bumpers went away in the 70s, they hung the 5th wheel off the rear wheel well openings on the side of the car. That finally gave way to radar guns and now GPS.

In 1970 if you found out your truck speedo was 10 mile off, you probably wouldn't give it a second thought, unless your glove box was full of speeding tickets.

Awesome pic. I just picked up a '67 cougar to restore last month

[KQQL IT]

Old blue came with this sweet conversion table.

[painterljp]

Subscribed for reference. Thanks.

[Canuckvetter]

Originally Posted by KQQL IT

Old blue came with this sweet conversion table.

Very close to mine. I slowed down the speedometer with an 18% reduction adaptor.

[Breadstick]

Reviving an old thread, my speedometer reads about the same percentage slow as your does fast in relation to the GPS. Will switching the speedo gear still fix it?

[TBONE1964]

I know this is old but will shed some light on the issue.

Number one and easiest way to check speedometer accuracy.

Find yourself a spot where you can drive a measured mile. ( like on a freeway with mile markers). Drive one measured mile. You should see odometer move exactly one mile. If the odometer reads more than or less than a exact mile, then your speedometer gears in the trans are the issue.

If it matches an exact mile, your trans gears, gear ratio and tire size are correct. If your speedometer (needle) reading is off, then the speedometer needs calibrated.

If you need more clarity on this, let me know.

[my67c20]

Yes, switching the gears did help, but then something went twang and now i just go slower than the fastest guy, so he can get the speeding tickets. When i am interested i use an app called Speed Box. When i do my Sniper upgrade, its on my list to tackle this issue as well.