Speedometer reading and gear selection

swisherred

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I recently changed my rear diff from 2.76 to 3.23 and my speedometer is off...quite a bit actually...or maybe...I cant tell because I cant read the damn thing. Do I follow the numbers or the lines? they only line up around 60mph...otherwise its like the background is from a different car...horrible.

1st...how do i read this thing?
2nd, what gear do i need....when im doing 47mph (gps) the needle is steadyish on the long tick mark over 60mph
currently running 215/70r14" tires, but will be switching to 225/70r15 when i find the right rims.

20200421_171211.jpg
 
I also have no idea how to ensure the odometer is correct...honestly...not really worried about it unless it needs to be correct. I dont know how to adjust a mag speedo...so Id be happy in the needle said the right number. I'm thinking 31 or 33 teeth depending on which tire i roll.
 
What he said in that comment up there (1).jpg

And even with that you can have problems. Like you, my brother changed rear gear ratios and did his homework about the different size gears. He has the correct one but it's still way off. I haven't really studied transmissions but do know about different size gears. I have a feeling that they have to be indexed like the reluctor in a distributor. Yes, tire size will play with those numbers too. Sorry to say that that is a really F-ed up speedo you got, I'd pass on a car with a speedo like that. Like you say, How do you read that? Good Luck
 
I replaced my speedometer and odometer with NOS. Everything is correct the only thing that is holding me up is wheels and tires, currently it's 5mph off (fast)

The old speedometer had a weak magnet and the odometer pretty much crapped out
 
I used a gps and noted on my tach the rpms because my speedo is off. One day I will fix it. I believe my tires have a larger diameter than stock and is why my speedo reads slow.
 
The 14" tires on the car are too small----compared to the original bias ply O.D. of 28 inches.
Hence the reason many like you and me go with a 15 inch tire.
22570R15 works very well on our C's but still .5 inch smaller than the 8.55-14's believe it or not,LOL!!
The 31 tooth will bring you much closer to being accurate but I prefer GPS myself.
I do a lot of cross border driving and my GPS I can set it back and forth to MPH and KPH. No tickets....yet!!
 
The 14" tires on the car are too small----compared to the original bias ply O.D. of 28 inches.
Hence the reason many like you and me go with a 15 inch tire.
22570R15 works very well on our C's but still .5 inch smaller than the 8.55-14's believe it or not,LOL!!
The 31 tooth will bring you much closer to being accurate but I prefer GPS myself.
I do a lot of cross border driving and my GPS I can set it back and forth to MPH and KPH. No tickets....yet!!
I really dont know what the original rolling diameter was...these are the tires that came on the car....and its the correct speedo...just poor design.....as far as indexing the gear (above post) there is an adjustment that has to be lined up depending on what gear tooth count you have...that much i do know.
 
I believe there is, my FSM is boxed up and like I said. I don't know them that well, plus I've never had to do this. My brother took his tranny & other parts to a shop and had them do it, so they may have borked something up, or not done right???? They have since closed. It's just a bug from building a car that needs to be worked out for him. I can't help any more then I have. Good Luck
 
on the subject of it being a crappy design....should the needle follow the tick marks or the numbers when operating correctly?
 
I drive a little faster than the cars around me. very seldom look at the speedo. no real need to. I have not had a speeding ticket in 40 years. once got pulled over for doing 70 k in a 80 k zone. cop said he thought I had been drinking because I was doing less than the speed limit. go figure. anyhow hope this helps.
 
on the subject of it being a crappy design....should the needle follow the tick marks or the numbers when operating correctly?
The first tick on the left is 10, not zero. Nobody cared about 0-9 mph then (or now). So, the big ticks are 10, 20, 30, etc. The 60 tick is straight down, pointing to.....60!

Which brings me to my second point or suggestion. I'd swap pinions around until your GPS and the speedo are about the same at 60. This puts you in the basic range for everything else, with 55-60 being the most important to have kinda accurate. Pinions are plentiful and cheap, so you could buy a handful around the size you "think" you'll need and swap them around to target 60 mph.

As for adjusting the pinion adapter vs tooth count, the adapter doesn't hold the pinion in its center. The eccentric orientation allows for different diameter pinions (meaning different tooth numbers) to be used. Rotating the adapter to the correct "line up" mark per tooth-count puts the teeth (of any size pinion) properly against the worm gear on the output shaft. You can see this in the FSM.
 
The first tick on the left is 10, not zero. Nobody cared about 0-9 mph then (or now). So, the big ticks are 10, 20, 30, etc. The 60 tick is straight down, pointing to.....60!
ok...follow me for a second...what you say makes perfect sense...until you start at the 60 and count large ticks back to the first one.....which lands at 0....the 60 and 70 are the only ones that even remotely line up. That is what I dont understand..
 
it just occurred to me...The Large tick marks do line up....with the bottom most edge of the numbers. If you hold a straight edge up to the picture it touches the bottom center of each number set....stupid...counter-intuitive...
 
P225/70R-15, previously GR70x15, usually have a revs/nile of about 755, similar to the P215/75R-15 or the old G78x15 size. Should be able to find that correct gear combination in the '72 FSM, or there abouts.

Each of the long marks corresponds to the speed under it. Might appear to be some mis-match on the linear speedometer displays, by observation.

As I mentioned in the other post, get the distance calibration correct FIRST, then worry about the indicated speed. Distance is a direct measurement, as speed is done via magnetism of the speed cup on the back of the speedometer head. The odometer is geared from the speedo cable, back there.

Enjoy!
CBODY67
 
.I cant tell because I cant read the damn thing. Do I follow the numbers or the lines? they only line up around 60mph...otherwise its like the background is from a different car...horrible.

View attachment 370499

All things considered, the '67-'68 Chrysler "linear" speedometer comes out pretty decent. in the '68 or so Satellites, they had a similar display (needle pivoted from the bottom, rather the top), in a less-wide space, that went to 150mph on the GTX and possibly Road Runner. CAR LIFE magazine did a test and got a shot of the speedometer face. About the only time the needle got really close to the numbers' placement was in the 75mph range, in the middle of the display. This pre-dated the '69 round gauge/speedo panel that became very popular on the B-body performance cars. As the major lines still pointed, pretty much, toward the numbers they represented.

Enjoy!
CBODY67
 
As the major lines still pointed, pretty much, toward the numbers they represented.
Not only did I chuckle at this, I also realized that "major" lines and "pretty much" is absolutely the case when talking about these old speedos. I vote this for the Quote Of The Day. :thumbsup:

I stand corrected....I went back and looked at swishered's speedo, and yes, the 50 is oddly positioned between two of the major ticks....certainly a bit confusing. We can wonder why the gage designer did that, long ago.

Here's my 70 300 speedo. It's a bit clearer as to what goes where. Sorta!
IMG_1203.JPG
 
If the hotrodders can get GPS speedos,why cant we?
There has got to be a way to upgrade our old speedo heads with GPS guts.....
Put any size tire on the car,swap any size rear gears any time you want without the hassle of a speedo being "out"
Eliminate the speedo cable which can bind and make the needle jump.
Whoever pursues this can get rich..just not me.
 
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