Need help with speedo gear in 208 transfercase....

[K5blaze454]

I have a k5 blazer with 350/700r4/208 with gm14 bolt  rear and gm10 bolt 4.11 gears sitting on 36s. as of right now i have 15 tooth output shaft gear and 35 tooth drive gear. all of the calculations i have done, thats what it calls for... i cant seemed to figure it out, can you guys help me? its doing 70 mph at 55 mph on my gps. Thanks in advance!!

[frotosride]

I have yet to replace my speedo gears and I have the exact same set up and close to the same speed at 55 mph. If you sent an email to TCI like the did they will tell you what gear you need to get it on track. If I find the email I will upload it.

[1967KaiserM715]

What rpms are you spinning? And gps speed is 55, the speedo is fast at 70 correct?


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[Captain Swampy]

Did you actually measure your tires height? Measure from the ground to the center of the axle and multiply by 2. It will probably only be 32-33 inches tall.  You have the gray 15 tooth gear and the orange 35 tooth gear? Does your cable connect directly or to one of those offset adapters?  If it connects directly you need a 38,39, or 40 tooth gear

[K5blaze454]

Yes 55 on my gps and 70 on factory speedo. It hooks directly to my transfer case tail shaft. No I haven't measured the tires exact. Thanks guys for you input and help. I hope we can solve this problem because my gps is not practical. And I like to get my fuel mileage on my new motor.

[K5blaze454]

What rpms are you spinning? And gps speed is 55, the speedo is fast at 70 correct?

1500 at 55


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[Captain Swampy]

20168/ Tire diameter (inches)= tire revolutions per mile

15 x 4.11 x tire rpm (answer from above) =     

take the answer from above / 1001= # of teeth on driven gear


so if your tires are 32.5 inches...... 20168/32.5= 620.55
15 x 4.11 x 620.55= 38,256.9
38,256.9/1001=38.2   you would need a 38 tooth driven gear
You need to measure your tire height, Brooke's pickup is only 1 MPH off the GPS after doing this

in the formula above..... if anyone else wanted to use it, the #15 is the teeth on the drive gear, so would need to change to the # of teeth on your drive gear.

[rich weyand]

Or you could put a 1:0.800  speedometer ratio adapter between the speedo cable and the transfer case.