68RocketAction

I figured I would share this since I know allot of us have issues with our speedometer reading the incorrect MPH.

Several months back I replaced the tires on my 68 Toronado. I assumed that since the owner’s manual indicated a radial tire size of 235 R15 then that is what I needed. Well apparently, the world today does not measure tires the same way that they did in 1968. They simply do not have the same height and obviously not having the same; height is going to cause the speedometer to read incorrectly… Well over the last few months, I have been trying to understand this complicated speedometer gear teeth thing. Well I learned that the first and second generation Toronado’s (Also assuming any vehicle with a THM425 transmission) have 32 teeth, which cannot simply just be changed out…

Well then, I started reading about these Speedometer Ratio Adapters, which turned out to be a very simple solution to my problem. Therefore, I simply downloaded a GPS speedometer app on the old Android and took the old girl out to see how off she was…

Therefore, I determined when the speedometer set solid at 70 mph my speedometer app indicated that I was actually going 63 mph… So 70 mph – 63 mph = 7 mph… So 7 mph of 70 mph would mean that it is 10% off… Therefore, I went ahead and ordered a Speedometer Gear Ratio Adapter that indicated that it would reduce my speedometer reading by 10%...

BTW, here is the site I worked from. They charged me $62.00 + $12.66 S/H, but you can probably can find them cheaper somewhere else online…

http://www.transmissioncenter.net/sp...n_______va.htm

Well I received it in the mail today and I think it took about two minutes to hook up. Took the old girl out for a drive and now when she sets at 70 mph my speedometer app indicated that I am actually going 70 mph…

midrange

I've been looking into this unit also for my Olds since I went from a 2.73 to a 3.73. How accurate are other speeds?
Say, 35, 45, 55, etc?
The reason I ask, is that on my truck, after a change of tires, at 25 (according to a roadside 'Your Speed' is ___ trailer) my speedo reads 30. At 40, the speedo reads 47, at 65, it reads 74. It isn't a steady 5 mph or 10% off.

68RocketAction

Its seems to be pretty accurate at any speed. However, there is a little bit of a delay in speed changes, but I think that is just because of the barrel rolling style speedometer on the first generation Toronado's.

Paladin31

That is a sharp car 68', like the color. What is the color code?

MDchanic

What size tires did you get?

235R15 should be 235/75R15.

You wouldn't happen to have gotten 235/60R15s, would you?
They've got a circumference of 82", versus 90.7" for the 235/75s, which is a 10% difference.



Wrongo.

(Well, maybe a little bit right. It's not a steady 5mph, and it's not 10% off.)

It's 20% off.

25 x 20% = 5... 25 + 5 = 30
40 x 20% = 8... 40 + 8 = 48
65 x 20% = 13... 65 + 13 = 73

- Eric

copper128

I used a ratio adapter on my '67 442 when I changed the rearend from 3.08 to 3.90:1.
It worked great and it's tons easier than changing speedo gears in the trans.

Of course when I changed the rear in my Delmont from 2.78 to 3.36 I didn't have to do a thing.

...the speedo drives off the front hub, LOL.

midrange

Thanks Eric. I never bothered to try to figure out the % since I knew what the new speeds are. Gotta love Big Brother and their 'speed reminding' radar trailers. LOL

Now, when the car comes out of storage in the spring, I'll use the GPS app on my phone and see what % I need for it.

MDchanic

Glad to help out.

Should be no problem to get it sorted when you want to, meanwhile, it's not hurting anything being a bit off.

- Eric

68RocketAction

Well thank you very much! The 68 GM Code was K and the PPG Paint Code is 13517. I know Oldsmobile called it “Ocean Turquoise”, but I believe Chevrolet referred to it as “Tripoli Turquoise”.

I had been looking for a 66 Toronado for a long time, but when I came across this 68 I absolutely fell in love with the color. It had 54,000 original miles, supporting service records, was almost 3/4th's of the way restored, had matching turquoise interior, no vinyl top, and was exactly in my price range. Therefore, without any kind of twisting of the arm I bought it!


Well I went out to have a look and they are Hankook 235/75R15 sized tires. I know it is defiantly shorter because I have the original pinstriped nylon spare tire in the trunk, and it defiantly sets higher than these do. When I went to the tire place, I was hoping they could at least order me a set of tires with a 3/8th inch whitewall. The person had no idea what I was talking about, so I showed him the spare tire, and I remember him saying “Well that tire has never even been on the ground, it still has printing on the tread”. Unfortunately, he said the only thing they could get was an orange or red pinstripe and they would be $375 to $500 a tire depending on the brand. Therefore, I just went with something a little more conventional and reasonably priced…

MDchanic

That's just weird.

Tire sizes are supposed to be uniform, and those old radials are supposed to be 75s.

All I can think of is next time measure the spare's circumference and work backward - you'll end up with a slightly wider tire, but hopefully one that's about the right size.

Anyone else have a god explanation for this?

- Eric

starfire

Well, I've always heard that God works in mysterious ways. Maybe this is an example of that.

twintracks

Cool solution. I've heard of these, but not seen one installed. Nice! I have the same issue with the speedo reading too fast on our '68. It's currently on the same 235 R75 tires.
The old J-78 tire designation does not convert to any tire size currently available, at least not in a white striped tire. I just did an exhaustive search to try to get the proper sized tires for my '68 Toro. The closest match to a J-78 is 255 R70 15. This not currently being manufactured in a white stripe 15 inch, unless you want tires for a Rolls Royce at around $1,000 each. The only R70 series options are the raised white letter BF Goodrich tires made for muscle car applications. The Toros ride & corner so much better on a R70 series tire vs. an R75. These R75s lay over & squeal so much more in cornering than I remember from the old days. Excessive tire wear is another side effect on these 75s.
Maybe someday Coker or someone else will bring back the proper "TFD" designated tires that these Toros were originally designed to have.
Jerry

442nut

Thanks for this post, I bought a Ratio adapter fro PATC and installed it with no issues in 10 mins. Speedo is finally correct!!

68RocketAction

That's Great!!! I know it feels extremely nice to have an accurate working speedometer...

deaddds

Just FYI, Diamondback will reproduce the correct whitewall on a Firestone radial for a reasonable price. If i remember it was a little over 200 a tire. Not dirt cheap but for a custom ordered tire, I thought it fair.