Shouldn't speedos be:

Started by 47convert, March 25, 2010, 10:17:20 PM

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47convert

March 25, 2010, 10:17:20 PM
Linear? I have an old Stewart Warner speedo in my '34 and it's I checked out which driver and driven gears I should have in my 700R4 trans taking tire diameter and rear gear into account. At 30MPH it's close and at 40MPH it's right on the money but at 50 it's showing a little over 60 and it gets progressively worse the faster I go. How can it be perfect at 40 and out after that? I'm checking it against my GPS.

brucer

#1
March 25, 2010, 11:19:21 PM
Quote from: "47convert"Linear? I have an old Stewart Warner speedo in my '34 and it's I checked out which driver and driven gears I should have in my 700R4 trans taking tire diameter and rear gear into account. At 30MPH it's close and at 40MPH it's right on the money but at 50 it's showing a little over 60 and it gets progressively worse the faster I go. How can it be perfect at 40 and out after that? I'm checking it against my GPS.


the way ive always understood it is speedometers can be off as much as 10% either way

53stvblt

#2
March 26, 2010, 09:27:44 AM
Most mechanical speedometers, not all, have a number stamped on them stating the number of revolutions required per mile. Usually it's 1000. Some are different & that could throw off your calculations.
 Also, Iv'e been told to measure tire diameter correctly you should measure from the floor to the center line of the axle & double it. This measurement could differ as much as 2-3 inches from the actual tire diameter which could change the outcome of your calcs also.
These are just suggestions that may help.
Good Luck,  Bud

papastoyss

#3
March 26, 2010, 03:43:28 PM
Seems like I remember TCI in Memphis advertising speedo drive gears for sale. My local trans shop changed hands several times & in one of the cleanups the box of RWD speedo gears got pitched.The local GM dealers don't carry them any more.
grandchildren are your reward for not killing your teenagers!

UGLY OLDS

#4
March 26, 2010, 05:04:18 PM
Hey ......Lookit what I found ......  :shock:


http://www.tciauto.com/Products/TechInfo/speedo_gears.asp



Bob........... :wink:
1940 Oldsmobile- The "Ugly Olds"
1931 Ford sedan- Retirement project

***** First Member of Team Smart*****

enjenjo

#5
March 26, 2010, 06:02:38 PM
Quote from: "47convert"Linear? I have an old Stewart Warner speedo in my '34 and it's I checked out which driver and driven gears I should have in my 700R4 trans taking tire diameter and rear gear into account. At 30MPH it's close and at 40MPH it's right on the money but at 50 it's showing a little over 60 and it gets progressively worse the faster I go. How can it be perfect at 40 and out after that? I'm checking it against my GPS.

Speedos work on a spinning magnet. As they get old, the magnet can get weaker, and no longer read correctly at higher speeds. If you have a good speedo shop in town, not many left any more, they can remagnitise the speedo so it works right again.
Welcome to hell. Here's your accordion.

47convert

#6
March 26, 2010, 11:23:41 PM
AHH, This set of SW's I have in the car are pretty old. Never thought about that. My buddy collects old SW's and he never thought of that either. They go for big bucks but maybe they should stay as just collectibles. Good info on the tire diameter - never thought of that - I just went by the advertised diameter (31") Thanx all.

Rochie

#7
March 27, 2010, 08:33:57 AM
Mike,
There's a great speedo shop in Pontiac Mi Scott's Speedometer (if I remember correctly) on #59 west of town.  The old boy can do * near anything with speedos, but he's NOT cheap.

248-338-4148 ...-800-208-3030

phat46

#8
March 29, 2010, 06:31:11 PM
Quote from: "enjenjo"
Quote from: "47convert"Linear? I have an old Stewart Warner speedo in my '34 and it's I checked out which driver and driven gears I should have in my 700R4 trans taking tire diameter and rear gear into account. At 30MPH it's close and at 40MPH it's right on the money but at 50 it's showing a little over 60 and it gets progressively worse the faster I go. How can it be perfect at 40 and out after that? I'm checking it against my GPS.

Speedos work on a spinning magnet. As they get old, the magnet can get weaker, and no longer read correctly at higher speeds. If you have a good speedo shop in town, not many left any more, they can remagnitise the speedo so it works right again.

Ahha, so that's why my brother swears he was only doing 80 when the cop pulled him over and ticketed him for 101 last week..... :lol:  :roll:

Old Blue

#9
March 29, 2010, 07:01:36 PM
I used to look really good in a Speedo when I was about 18 and a lifeguard -swimming- diving Instructor  :lol:

Fat Cat

#11
March 31, 2010, 10:40:14 PM
Quote from: "phat46"Ahha, so that's why my brother swears he was only doing 80 when the cop pulled him over and ticketed him for 101 last week..... :lol:  :roll:

No your brother says that because he is lying. :lol:
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