1980 1/2 ton speedometer question

[ahatchemt]

Thank you for reading my post. I am new to the forum and hope that someone can help. I have a 1980 1/2 ton (Big 10) with a 350 engine and 350 trans. I recently had the transmission rebuilt and now the speedometer reads 45 when I am going 60. I have taken it back to the transmission shop and they told me i needed to find someone to recalibrate the speedometer. Any idea if that is something that can be done by an individual or if I need to find a speedometer shop? I do not know if this was a problem before the transmission repair as I bought the truck with a bad transmission so I never drove it before. Any help or suggestion would be greatly appreciated. Joe.

[silverhairdeere]

sounds like they had rebuild someone else's transmission and put it in your truck , which had a different rear gear than your truck did , what this boils down to is finding what rear gear ratio you have and getting the right gear to put in the speedo chamber in the transmission , a speedometer shop can help .

shd

[leolkfrm]

check with a dealer and see if tehy can tell by the gear, it used to be by color

[hotrodpc]

Thats a load of crud !!! I'd be very mad. Your speedo was correct before you took it in to them??? IF so, I would not settle for their explanation of gettnig the speedo recalibrated. For #1, it is not cheap to have done, maybe $100 or so. #2, the device they will use in my opinion is not very durable. My dad has this done at one time for a rear gear change, and it seemed his speedo quit working about once a year, and he had to go back and have that part replaced and recalibrated again. I would take it back and make them put the same gears you have on it when you came in. If your speedo was working when the trans went bad, do not let them tell you the gears were bad. They need to make that right for you. I will probably do it another post, but I can get you some info on getting the right gears for your speedo. This will be a Turbo 350C with lockup I suppose, so hopefully they also gave you the right tailshaft because there is 2 differant speedo types and if you have the wrong tailshaft, you will have to change that too, other than just the 2 plastic gears. Just be sure to do it the right way with the gears you had originally, don't settle for one of those calibartion parts. JUNK in many cases. I am sure they are some that are better than others, but not sure what those would be. Good Luck with this, take it back and chew some tail. BTW, Welcome to the Forum Newbie !!!

This link happens to be for 700R4, but the math and the gear color combos should be the same for your application. Hope this helps. You will need to know your tire size, and gear ratio. Hopefully you do not need this stuff, and you can make those idiots make it right like you paid them to do.


Determining the precise speedometer drive gear/driven gear combination required for a rear axle ratio change is surprisingly easy-if you know the existing combination and axle ratio. Whip out your calculator and follow along. Let's assume that your truck is currently equipped with 2.73 gears and that you're planning to install a 3.73 ring and pinion. If you didn't change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what's required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed. The speedo gear combination for a 2.73 rear (assuming 29-inch diameter tires) is a 18-tooth drive gear and 35-tooth driven gear. Switching to a 45-tooth driven gear (the most teeth available) would translate to the vehicle traveling at 78 percent of indicated speed - close but not perfect.
Obviously, the drive gear must be changed to one with 15 teeth if the 73.1 percent ratio is to be achieved. Going back to the original 35-tooth driven gear for illustration purposes, the 15-tooth drive gear would result in the vehicle traveling at 83 percent of indicated speed - that seems like a step in the wrong direction, until the driven gear is changed. (This is where it gets a little tricky because you have to work with a percentage of a percentage). Matching the 15-tooth drive speedometer gear up with a 40-tooth driven gear brings the drive ratio to 72.8 percent (15/18=.83; 35/40=.88; .83 x .88 = .7304, which rounds to .73, or 73 percent). There you go.
My truck came from the factory with 2.73 gears and 29-inch tires and as the above example shows it had a 18 tooth drive gear and a 35 tooth driven gear like the ones pictured here. I changed to 28-inch tires and I needed a 37 tooth driven gear to go with the original 18 tooth drive gear. When I changed to 3.73 gears I needed a 15 tooth drive gear and a 41 tooth driven gear with the 28-inch tires. Also I had to change the sleeve to #25512339 for 40 tooth up driven gears.
In case you haven't put all the pieces together, there is a distinct limitation regarding speedometer accuracy and steep rear gearing. Unless driven gears with more than 45 teeth, and/or drive gears with fewer than 15 teeth become available, it will be impossible to have an accurate speedometer with rearend gear ratios lower (higher numerically) than 3.90:1 - unless 28-inch or larger tires are installed.

Here is a site that has a Speedometer Calibration Program you can use to help calculate what gears you might need.


#Drive Gear Teeth X Axle Ratio X (20168/Tire Diameter in inches)
-------------------------------------------------------------------- = # Driven Gear Teeth
1001 Example: 15 teeth drive gear, 3.73 axle ratio, 28" tire diameter.
15 x 3.73 x (20168/28) = 40300
40300/1001 = 40.26 or 41 teeth

Blue Drive Gear Combinations
With drive gear number 8640518, (blue, 18 teeth) the following combinations apply: (The 18 tooth gear I had in my 85 C10 was green, they changed the color to blue)
Axle Tire Dia. Driven Gear
Ratio (Nominal) (Tooth Count)
2.73 29.0 35
2.73 28.0 37
2.73 27.0 38
3.08 29.0 39
3.08 28.0 41
3.08 27.0 42
3.23 29.0 41
3.23 28.0 43
3.23 27.0 44
3.42 29.0 44
3.42 28.0 45
Red Drive Gear Combinations
With drive gear number 8640517, (red, 17 teeth) the following combinations apply:
Axle Tire Dia. Driven Gear
Ratio (Nominal) (Tooth Count)
3.08 29.0 37
3.08 28.0 39
3.08 27.0 40
3.23 29.0 39
3.23 28.0 41
3.23 27.0 42
3.42 29.0 41
3.42 28.0 43
3.42 27.0 44
3.73 29.0 45
Gray Drive Gear Combinations
With drive gear number 8642620, (gray, 15 teeth) the following combinations apply:
Axle Tire Dia. Driven Gear
Ratio (Nominal) (Tooth Count)
3.08 27.0 35
3.23 29.0 35
3.23 28.0 36
3.23 27.0 37
3.42 29.0 37
3.42 28.0 38
3.42 27.0 39
3.73 29.0 40
3.73 28.0 41
3.73 27.0 43
4.10 29.0 44
4.10 28.0 45
Lower ratio (higher numerically) rear or smaller diameter tire requires higher driven gear tooth count. Increasing or decreasing driven gear tooth count by one will usually accommodate a 1/2" to 1" change in tire diameter.

Here is the link.
Speedo Gears

[ahatchemt]

I am not sure if the speedo was correct before as i bought the truck with a bad trans. However I do know that they rebuilt my transmission rather than install one already built as they did not have one ready to drop in for me. Thank you for all of the info so far and I will try to get the gear ratio from the truck as soon as i can.

[hotrodpc]

OK then, that sounds better. My blood pressure can go back down then. Whoever you bought the truck from had apparently swapped the trans at one time then. So yes you are on your own. But, I still do recommend trying to get the right speedo gears for your application and do it that way, the right way. Don't go to a speedo shop and spend all that money for an POS adapter. Is your RPO sticker in the glove box on the truck? If the rear has not been changed out, that RPO sticker will tell you your gear ratio. Get me that and your tire size and I can probably tell you which gears you need. To do this swap, you will have to take the drive line out, and the tailshaft off the trans. Once you get those 2 tasks done, its a 10 minute job and put it back together. Also look under the truck at where the speedo hooks up and let me know whether you have the small diameter drive gear, about the size of a quarter, or the large one, about the size of a handball in diameter.

[ahatchemt]

i will get that info to you as soon as i can thanks