2nd gen speedometer question

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Symon623

Symon623

10 Year 15 Year
Joined
Feb 25, 2007
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16
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225
Location
Peoria AZ
I have a 85 truck, with a 89 4runner gage cluster, and Dana 300 case, 5.38's and 37 MTR's. I hooked up the speedo to see how far it was off so I could order a new gear to slow it down. In my old rig I had a Stewart Warner speedo with the same t/c, gears and tires and it read 70 when my GPS said I was doin 55. Well now I have this set up in my Toy and when my GPS says 55the Toy speedo says 95. Why? are Toy speedo's geared? WTF? I can't get a gear big enough to compensate for this error.
Anyone have any ideas? Thanks
I know speedos are overrated, but I kinda want it to work.
 
I think the stock speedo cable is geared down from the drive shaft speed, maybe as much as 3 or 4 to 1, I don't think I've seen this ratio online, maybe someone with a spare case can pull it apart and count teeth.

The stock set up (Diff gearing) is about 21.45 inches of horizontal travel to one drive shaft rotation. You are at 21.6 So you (If the tires measure out at 37 inches) are a little under geared, but less than 1 percent off.

So if your speedo is off by 72 percent, the 300 case must only gear down the speedo cable about 0.6 of what the toy case does...(Or not at all, depending on the toy ratio)
 
I'm running the same diff gears and 37" Toyo's. When I read 70mph on the speedo out on the highway, I'm really doing 68mph according to GPS. Not far enough off to worry about for a truck on 37's... I have the speed being registered in my rearmost transfer case.
 
Toy speedo cable, I modified to fit into the 0.100" square hole of the 300 case gear. A local speedo shop can remedy with a reduction box but they want $180. I want to make sure something else isn't F'd up first.
 
Doesn't look like I'll make it out to my dad's this week.... Work schedule is all screwed up... Can you wait till next week? :D

In the mean time find out what the 300 case output shaft to speedo cable ratio is so we can compare.
 
Symon623 said:
Can't get any info on the tooth count on the 300 output shaft???
Click to expand...

You or me? I don't even know what type of vehicle the 300 is out of.... :D I'm a toy guy.

If there's a shop in town that can make gear boxes. They might have access to the case ratios.... both the toy and 300....? Worth a shot.
 
Me, no one could tell me, they all are like Speedometer, WTF? you don't need that, or mines 10 or 15 off......

Well I ain't lookin for exact, but I'd like it to be closer than +50?
 
Hehe, Anyone that says "Mine is 10 off" doesn't know how gears and tires work... If it worked that way, then you'd be going 10 mph when your speedo says 0 right? It doesn't work like that fella! :D

Once I get the toyota gear ratio, we should be able to reverse engineer and figure yours out with some math. Do me a favor and double check that 55 GPS 95 speedo numbers are as accurate as you can get. Also take a picture with your speedo pegged..... it'll be that last chance you get! :D
 
Will do, the speedo pic is a good idea :D but this thing will do it for real not sure I wanna drive it that fast but it will do it I bet....

I know that the difference with reation and tires multiplies as you go faster. As is the case here. My gas mileage looks real good but I'm really rackin up the miles..... WTF?:D
 
Update; GPS 30 = 55 on teh speedo, GPS 60 = 110 on the speedo.

I just noticed the trip meter doesn't work, could this be having an effect?

Didn't take a pic, pre-rush hour traffic, but was still able to go 60-65 most of the time.....
 
Ok, I pulled the t-case apart and it's a 3-1 ratio 6 teeth on the output shaft and 18 on the speedo cable, So the drive shaft goes around 3 times for every 1 the speedo cable does.... I'll get to work on the math later....
 
Thanks Jerod, I'm not certain what my output shaft tooth count is, but the speedo gear is 31 teeth, I'll research and see what I can come up with.
 
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