Issues with Speedometer reading slow, please help

[Jonathan R.]

I have a 1976 Chevy K10 that has had a rebuild engine and transmission put into it. the transmission is a TH350 (For what pictures and other posts are showing) and the engine is a 5.7 350. I am running the truck with a 2" lift and 33x0.50x15 BF Goodrich A/T KO2 tires. My issue is that my Speedometer reads about 15-20 Mph under what i am really going. I was stopped for doing 65 in a 45 when my speedometer was reading 45. i need help figuring out what steps i need to take to find the correct speedometer gear.

I hope someone out there is able to help me figure this out before i get stopped again.

Thank you For looking

 

[1987 GMC Jimmy]

What's your gear ratio?

 

[Jonathan R.]

I checked it today and for every 1 turn of the driveshaft i had 2 3/4 turn of the wheel. I do not know what that would come out to be. From what i know the gears should be the same as stock. I was following poor instructions on how to get my gear ratio so it could be wrong.

 

[1987 GMC Jimmy]

You have to know for sure what it is, and that plus your tire size (33") will tell you what speedometer gear you need so your speedometer will show the right speed. According to the GM Heritage Center, the tallest ratio, which was also the standard one, was 3.07, but that doesn't mean anything. The way you mention of finding the gear ratio is one, but here's one that removes the guesswork.

 

[Jonathan R.]

Is there any way of counting the teeth on the pinion gear without having to remove the differential case?

 

[1987 GMC Jimmy]

You should be able to pull the cover, drain the gear oil, clean it up, mark the spot on the ring gear where you'll start counting, and mark the spot where you'll start on the pinion gear. It'll probably be a little tough to see, but it should be visible kinda under and behind where the c clips, side gears, axle shafts, and all that are. It'd give you a good excuse to clean her up, inspect the teeth, and put in fresh gear oil.

 

[Craig 85]

Once you pull the diff cover, spin the gear and you will find numbers similar to this:

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The 15:41 stamping is a 2.73 gear. 41/15=2.73.

I know when I swapped axles in my truck 3.08 to 4.10 (33" tall tires), I had to remove the speedo adapter on the transfer case. Once removed, the the speedo was dead on with the new gear set.