Yet another dumb question!
 

How can you use the same speedometer regardless of rear end ratio, tranny type etc. Are sending units (on tranny) geared to correct for these things or what? Just something that crossed my mind.:p

Your rear-end ratio will affect the reading big-time. If you're running 4:11 gears or lower, your speedo could say 75 when you're only doing 50! Tire size will affect this as well.

There is a gear in the transmission that runs the speed of the speedo cable. That gear an be changed.

gearing on the end of the speedo bolted into the trans via a steel cased cable on most of these trucks can be changed out for different gear ratio's. if and i mean IF you have a good GM Garage the parts guy will go out in the heat and muggiest of days and view your hotrod/truck/ or goofy project nad tell you a good idea of size and help get the right one for ya

When I changed from 3.73 to 3.08 it made my speedometer
15 mph slow. I was already using 60 series tires and it
was a little slow to begin with.

I had an article that Hot Rod Magazine did on this problem
and it was full of all the formulas and gears available.
There are actually 2. They are ,or were, color coded.

I figured out what I needed and went to the dealer. He
had the cable gear and showed the inside gear. When
he tried to order it, it came back as discontinued.

There was a fellow who advertised in Hemmings that
could build a unit to fix any of these problems. I ordered
one of the units after giving him all the measurments
he requested. The unit made the speedo 15 mph fast.

I gave up out of frustration and decided to live with it.
There are advantages to this. I know that when I am
showing 55, I am actually doing 70. It is a cobweb in
my truck and does bother me a little. I would like to
hear what others have done.

EdB

krue, if you have an adaptor on the trans end of the cable, it will have the ratio stamped on it, that it is matched to. You will need to know the ratio of the gear set, and then get a matching adaptor for it. Much easier than changing the drive gear in the trans, which requires removal of the tail housing from the trans. Around here, the adaptors are available from speedo repair shops, or they sometimes change the gears in the existing adaptor to match the gear set, and tire diameter.

Yeah what class of 69 said.Look in the yellow pages for a speedo shop.I use one in Lancaster Pa.I think they charged $75 to build a new ratio box for my big truck.When they where done it was off by less than 1 mph.

These guys can give some insight on what is needed to correct speedometers.

Gears

forget those other speedometers. they make gps speedometers now! they work great apparently as long as you have a line of sight to 2 satellites. no matter what you do it's always right.

There is a guy here in salinas, that changes the gears in the box that attaches to the transmission. I had him do the one for my '72 before I tore it apart. It had 33" tires 3.73 gears and stock 350/205, It was off by like 1 mph mabe 2. I will get his # if anyone is interested. It didn't cost $75.00 though, I think it was like $40 or something. but that was two or three years ago. I am going to be getting the one for jenna's 71 redone, mabe next month or so. After our camping trips, I am running 55 at 2000 rpm and i think it is off, so I am going to them to have them fix I'll let you guys know how much $$$ it is.