Speedo gear?

[c1084]

Hi, new to this site. I recently acquired a 1984 C10, longbed fleetside. The speedometer is reading way too fast, off maybe 18-20 mph at highway speeds. I suspect the trans (700R4) has been replaced with a unit from another vehicle that had a different axle ratio or tire size. The tires are the size that came on it stock (235/75/15) and the rear axle ratio is 2.73, also correct according to the option codes. Can't I just change the speedo gear to correct the inaccurate reading? Counting teeth isn't any help since I'm pretty sure that gear is wrong for this vehicle, and most of the ones specified as possibilities are obsolete according to the local GM dealer. How do I specify the correct one, and where can I locate it once I do? Any info appreciated. I hate speeding tickets.

 

[SlickGTP]

I would pull the speedo gear and count both the drive and driven gears so that you know what you have to start with. Then TCI has a nifty little calculator.

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

With that you can see if you need one gear, the other, or both... You can typically find the gears in a variety of places including eBay.

This place usually has a decent array of driven gears.
http://www.transmissioncenter.net/

And so does TCI Auto
http://www.tciauto.com/

 

[c1084]

Thanks for the info. What I keep hoping to find is what originally came in that truck, so I could just remove and replace all in one step rather than identifying what's there first since I already know it's wrong. I do, however, understand that I may have to start with pulling the old one and proceeding from there.

 

[SlickGTP]

It'll be a lot easier just to see what you have and start from there...

 

[HotRodPC]

Thanks for the info. What I keep hoping to find is what originally came in that truck, so I could just remove and replace all in one step rather than identifying what's there first since I already know it's wrong. I do, however, understand that I may have to start with pulling the old one and proceeding from there.

It'll be a lot easier just to see what you have and start from there...

I'm with c1084. Myself, I'd prefer to know what was in the truck OEM and get it. Chances are it's common. In most cases, I find, you only need to change 1 gear. But, sometimes that isn't the case. Sometimes someone may have swapped in a Th350 with the small drive gear tailshaft. In that case, you don't have a choice but to do it the way Slick suggests.

Thread moved from Intro to Auto Trans category. It could go in Tech Discussion too, but I've put here for a reason.

 

[c1084]

Thanks. It's definitely still a 700r4, it still has overdrive. I just think due to finding water in the rear diff and rusted brake lines that sometime in it's life this truck was partially flooded, ruining the trans that was in it, necessitating its replacement. Just piecing together evidence. The trans that's in it now functions just fine other than the speedometer reading too fast, leading me to believe the donor vehicle had larger tires or different rear axle ratio. Being that the vehicle now has the original combination of engine, trans, tires and rear diff it would seem the only step necessary to correct the problem would be to install the factory spec speedo gear.

 

[HotRodPC]

Yes, I'd go with the OEM factory spec gears. Once you figure out which ones you need, you can find them fairly easily, either on eBay or some trans shops. Usually about $12-$15 a gear and chances are you'll only need the one for the output shaft.