Replacing speedometer driven gear

I have a transmission that came from the factory specified for a 3.23 gear ratio car. The transmission is going to be in front of a 3.42 ratio rear. Is it fairly simple to just change the gear that drives the speedometer cable to get the correct speedometer reading ?

 

Yes, can be done in the car. I would check how accurate the car is now,
correct any error as well as take care of gear change in one step. It
might require both drive and driven gear changes, possible in the car
on most earlier transmissions. A extreme gear ratio sometime needs
an external gear box to get in range. Bruce Roe

 

Transmission is currently out of the car so now would be the time to make the adjustments if possible.

 

http://www.v8buick.com/index.php?threads/speedometer-gear-help.342103/#post-2895353

 

Thank you, that tells me everything I need to know and save me a ton of trial and error.

 

The formula 20168/ tire height = Tire Revs per mile (TRM) would be true
if your tire made contact as a perfectly round disc. However tires are FLAT
where they contact the road, flexing (varying length) coming off contact.
The real ROLLING circumference is determine by the belt inside, so the
above formula will always err.

There is no need for trial and error with a working setup. Just make a
precise measurement of current error (GPS, calibrated mile markers, etc)
and then calculate the correction needed on your current gears, you will
be precisely on the first time. Bruce Roe

 

Well, they're rubber! Air pressure, speed, tread wear.... all minor unless you're talking top fuel!
I had figured 1,000 speedo cable revolutions per mile, but I think the auto makers use a slightly different figure, maybe for slippage?

Bob, note that the gear holder (sleeve) is meant for a certain range of driven gear sizes. Sometimes you have to change the sleeve if the gear is too big/small.

 

I will try to explain in detail when I get back from mailing some owners
car parts. In the mean time, some research might turn up "rolling
diameter" that some tire makers release. Bruce Roe

 

For our purposes, the calculators available at TCI website are more than sufficient . Jim/Rott

 

Just found this, may be helpful.....Jim

 

the outside circumference of the tire isnt going to change enough to make the speedo reading change. so a 26 inch tire has 81.61 inches of tread and that stays the same even if the tire went oval or heaven forbid ...square.

 

1001 speedo cable turns per mile is to make the odometer correct,
which has reduction gearing 13:1, 11:1, and 7:1. Multiplies to 1001:1
As I recall the target for the Metric version is 616:1

We know the factory was not super keen on accuracy, and I am not
telling anyone how accurate is good enough. But I will tell how I made
my car as accurate as possible.

Regarding the flat side of a tire. Imagine a vehicle with tracks instead
of wheels, it always has a flat section of track on the ground, the rest of
the track shape does not matter. Imagine a pin was pulled and the track
laid out flat on the ground. If we measure the exact distance between the
holes the pin goes in at each end, we could agree that is the distance
travelled when the track goes around exactly once.

THEN consider this. The above said nothing about how high the pin is
above the ground. In fact, if the track wore down so the pin was an inch
closer to the ground, the measured length would be EXACTLY THE SAME.

That is how tires work. The "PIN" is at the level of the fiber belt in the
tire, not at ground level. The tread is not continuous, but flexes as it
meets and leaves the ground. The outer surface of the tread is not at
the effective dia, the belt is. I came to realize this messing with transmissions
in the 70s. A car I modified came mostly by accident, to be about a decimal
place more accurate than normal. I could (did) drive 50 miles by mile markers,
and they would still be exactly in sync with my odometer. But surprisingly,
when those new tires wore down, THE ACCURACY WAS UNCHANGED.
The final evidence was when a different brand of tire (same size) made a
measurable shift in my carefully checked readings, because its belt was not
done exactly the same way.

In time I measured some local roads to 1/100 mile, so I did not need to go
on a toll road for each check. Using the usual formula will get you in the
ball park, except any input error will blow it. How do you get the tire dia
anyway? But if you measure error of what you have, the required
correction is easily and exactly calculated. My PHOTOBUCKET album
SPEEDOMETERS AND GAUGES shows some of this, including how to
roll your car forward one tire rev and measure its effective circumference.
Probably not as precise as at speed.

http://s93.photobucket.com/albums/L71/bcroe/

click on an Album
click on a picture to enlarge + description

Bruce Roe

 

Thanks for the explanation Bruce. When I was a toddler, I always asked 'why?'.... I never out grew that!

Nice solar array! Have I seen you post on the SolarPanelTalk forum? Thought I saw your name there.

When changing gears to correct speedo error, I usually figure 1 tooth on the driven gear for 3 mph. Is that a decent ballpark for a TH300/400?

 

I can not give you direct teeth per mph, it would depend on the
drive gear and also the axle ratio. Look in there and get the tooth
count on both speedo gears to get the ratio. Measure the error on
the road, figure the ratio you really need, and pick new gears to get
the new ratio.

Yes my solar supplies all my heat and electricity here west of Chicago.
Bruce Roe
.

 

what about cable stretch and flex? If you burn rubber a lot you will make your tire smaller too. Thats why you need STEEL TIRES!. Yes the new tire that stays hard always.