Quote:
Originally Posted by CUSTOM/10
Hi Dan
Would you know what speedo gear count I would need for a truck with 3:73s and 33" tall tires ? ( and does the gear inside the out-put housing need to be changed at the same time ? )
Or
Do you have a speedometer ratio adapter that screws on where the speedometer cable goes that would correct my speedometer ?
Gary
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Gary,
According to a couple different speedometer calculators (internet search of "speedometer gear calculator" should bring up several to choose from) if you have the red 7 tooth plastic drive gear, 3.73 axle ratio and true 33" diameter tires you would need a driven gear with 15.94 teeth.
As far as I know the NP203 or 205 used in GM vehicles never had a 16 tooth speedometer gear (Dodge/international did, but the rear output housing is different). I have 18 to 22 tooth driven gears for GM.
If they make an 8 tooth plastic drive gear for GM then the 18 tooth would get you close.
Your best option is probably going to be a ratio box like you mentioned. Some transmission shops will custom make them for your exact (or as close as possible) ratio.
I do have several and could sell you one if I happen to have the ratio you need.
The best way to determine that would be drive the truck on the highway at 60 mph, (set cruise control if you have it) and have a passenger monitor your speed on their phone via an app, or simply using Google map directions (or similar). This will give you a GPS speed (assuming you have cell service etc). You will need to go long/far enough to have a steady reading on both your speedometer and GPS.
The GPS speed will be pretty close to actual speed (in my work trucks if I drive 67.5 mph per the speedometer, the GPS reads 65).
Use the GPS speed on your phone to determine the difference between what your speedometer reads and actual. This will be the ratio you need to adjust to get it to read correctly.
Note: if you have any type of ratio box already (doesn't sound like you do) you will need to remove it before attempting the process listed above.
After you determine that, I'll look thru the ratio boxes I have to see if there's anything close.