Speedometer Ratio Gear Box Help!

Hello,

I've got a '66 Oldsmobile Toronado that the speedometer is on since I put on new tires.

I figured was off by about 25% of my actual speed. IIRC: I went past one of those speed signs and it said I was going 30 when my speedo said 40.

Now, I went to a site and did all the math with my tire size, final drive ratio, and the speedometer drive gear, and came out with I would need a 42.5 tooth gear to compensate. My original gear is a 32 tooth. I cannot get a 42 tooth gear with the particular system on my THM425, so I looked into those little gear-ratio boxes. I divided 32 by 42.5 and got 24.7% difference between the two... And that's the size gear ratio adapter I bought.

Now, I drive to and from work on a 55mph speed limit, and usually try to cruise at about 60, it's the speed virtually everyone goes (with my other car) Now, with my oldsmobile, I've got to go about 70-ish to maintain the same traffic. So my 25% above 60 does work out to be 70.... but, I think I'm looking at this backwards....

If the gear reduces my speedometer by 25%, then 25% of 70 is 17.5, meaning I am going 53, which is not possible on this road with how people drive. But, 25% of the necessary speedo gear of 42.5 teeth does work out to 32 (ish)

Soooo.... Which version of my math is right? x.x My brain hurts.

Twilight Fenrir

First thing, I'd do is confirm your speed with a GPS unit weather it's a hand held or dash mounted one.

Good luck,

Okay, I used a GPS speedometer in my phone. It's got me doing 57 when my speedometer says 65. And 60 at about 69-sh Which works out to 12-13% error off of the speedometer reading. Meaning I ordered the incorrect part. Right? >_>

FIRST . . . worry about DISTANCE. THAT's the geared portion of the speedometer items!

GM used "ratio adjusters" to fit applications were the normal drive/driven gear ratio didn't work well enough. There were a multitude of boxes, with the ratio stamped on their side. Like ".9500" or "1.053".

Once you get the gear ratio in the transaxle for the drive/driven gears what they need to be, with or without the ratio box, THEN you worry about the speed part of things. The speedometer cable's shaft spins a "speed cup" (think . . . freeze plug with a bar magnet in it) that fits inside of a slightly larger speed cup which is attached to the speedometer needle. The faster the magnet spins, the faster the speedometer needle indicates "speed". The magnetism in the bar magnet can be adjusted with a "zap" gun, which makes it a cut and try procedure at the speedometer shop, when I saw it being done.

In the earlier years, the drive gear was something like 8 teeth (the gear on the trans output shaft), which ran the driven gear with something like 17 teeth. These would be the numbers you'd find in the parts book charts. The OEM listings were also configured around both axle ratio and then-current tire sizes . . . NOT "diameters" as many in the aftermarket want to use.

The "adapter" housing which the driven gear slides into will have some markings on it, around the outer circumference. These numbers (and their placement) are indicative of the acceptable driven gear teeth which the adapter is designed to use. You'll notice that the hole the gear slides into is NOT centered, but somewhat offset in the adapter, to compensate for the different size/teeth driven gears. This should give you a general idea of which driven gear you need . . . as a range of sizes.

So, my recommendation is to go back and check the traveled distance vs. indicated distance on the odometer. When you know THAT number, then you can possibly seek out a ratio box to get THAT item dialed-in FIRST. When that's done, the speed reading should then be "in spec", somewhat, or at least give you a correction factor you can deal with.

What size tires did you put on the car? Just curious.

NTX5467

So... it's got an early version of a hal effect sensor built into it? O.o I was not aware of that... I guess I just assumed it was accomplished with gears. But, thinking about it more, that wouldn't make sense...

The problem with using the Odometer though, is that using anything to determine actual distance traveled is nullified by the fact that the road is not flat. GPS doesn't take into account ups and downs. Even something as simple as pot-holes means the wheels turn further than the car travels.

The tires I put on were 235 70 R15's.... I looked on the side of my original tires, and they weren't marked... as they were measured differently back then apparently. Someone on a forum suggested this would be the right size, and I wish I had researched it a bit more and avoided this headache...

I had gone ahead and ordered the gear, and the appropriate housing for a TH-400, upon which the THM-425 is based. However, when I went to install them I realized I had made a greivous error in not having looked at the parts on my car before ordering them. There is no way to get the proper ratio within the confines of the stock equipment. And, it actually has a metal speedo gear, which surprised me.

Anywho, after consulting my GPS, I ordered the 10% speed reducing box. It's within 1-2% of accurate, I'm not worried about the extra mile or two for every hundred. It will be nice when I can go back to not having to do math every day... I graduated from Highschool, I shouldn't have to do math anymore

I am very happy I read this. I have the exact same problem with my 68 Toronado. When my speedometer reads 70 mph my phone GPS application says I am going 62-63 mph. I also purchased R15 235 sized tires since that's what was listed in the owners manual. I am going to go ahead and order one of those Ratio Adapter that reduces speedometer reading by 10%.