I have a VDO mechanical speedo that reads about 1/2 to 2/3 of the actual speed traveled. I have a TH400 transmission and checked the drive and driven gear for the ratio values and found that the drive gear is 15 teeth and the driven is 43. The tire size is 32" and the rear end gear is a 3.70. Doing the math and checking the charts available online, the drive gear is correct, but the driven should be 36 teeth. I am being told at a local speedo shop that the difference in each tooth accounts for about 2% increase/decrease, so by decreasing the 43 tooth to a 36 tooth driven gear, I will increase the speed seen on the speedometer 14%. Now with all of that said, I am confused as to why the speedo really needs to be increased about 60% to read correctly. Does anyone have any ideas before I go and have a ratio box made? Is there anything else to check?
Don't believe the old wives tale.
Just install the 36 ... and be HAPPY ...
No box needed.
The online chart calculator is here ...
For as inexpensive as the gear is (about $6.00), I will give it a shot. I spoke with another Speedo Company today in LA and they stated the same concern about the ratio box need.
I drove side by side next to my wife today and when she was going 40 MPH, my speedo was registering about 25. I will give it a try and see what happens.
Driven gear changes will not make up for that difference.
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