I just changed rear end in my 69 camaro.
is there any good reliable speedo gear caculators out there?
of course ill need it for a GM vehicle .
original rear gear ratio was 3:08
new gear ratio is 3:73

http://www.bgsoflex.com/speedo1.html

http://www.tciauto.com/tech_info/speedo_gears.htm

What transmission do you have? and. . .
What tire size are you running?

I can get you part numbers of what you "should have" and what you need to correct your speedo.

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www.novaresource.org/speedo.htm
www.novaresource.org/speedo2.htm

You'll need tire diameter & gear ratio for this site.

The only problem I see with that site (I tried to use it myself when I needed this done with my own) is they don't specify any individual transmission OR it's available gears OR their part numbers OR a source to purchase these gears.

I will most likely start another sticky post for speedometers, and this thread may end up being a source.

If anyone has anything to add to a speedometer sticky post PM me with the info.

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so if this is is right i will need for my th350
and 4.10 rear is this
4.10 10 41
ratio t r

Not at all, and here's why.

Your speedometer, the cable to drive it, and the driven gear all turn at a slower rate than your drive gear, the output shaft and driveshaft.

Then the complicated part, your axles and tires turn at an even slower rate yet.

Lets assume some unchanging variables in an example equation.
Engine turning at 2200 RPM's.
1 to 1 transmission high gear ratio (using a GM TH-400 for example).
235/60/15 tires (measuring 26.1 inches diameter & 82 inches circumference, turning 773 revolutions per mile).

With these gear ratios you will run the posted miles per hour at the 2200 engine RPM speed listed above.
2.73-63 mph
3.08-55 mph
3.31-52 mph
3.42-50mph
3.73-46 mph
4.10-42 mph

To set your speedometer straight with each of these gears in our otherwise unchanging mock vehicle.
The following gears will need the listed (D) drive & (DR) driven gears in our transmission to correct our speedometer. These go by tooth count just like axle gears.
2.73-18 D,- 37 DR, or 2.05:1 gear reduction
3.08-19 D,- 45 DR, or 2.37:1
3.31-17 D,- 43 DR, or 2.53:1
3.42-15 D,- 39 DR, or 2.60:1
3.73-15 D,- 43 DR, or 2.86:1
4.10-15 D,- 45 DR, or 3.00:1

When I recently swapped the gears in the axle in my Malibu from 2.29's to 3.73's to be able to fully utilize my TH-700R4's .70:1 overdrive ratio, I used the calculator FOUND HERE: to determine that the 19-tooth drive gear and 34-tooth driven gear giving me an accurate speedometer reading prior to my gear swap to 3.73 just wasn't going to cut it, and I needed to swap the speedometer gears or I was just going to have to guess how fast I was going. NOT an option.

Some drive gear and driven gear tooth count gears are not availablefor all transmissions and to compensate for this they sell gear ratio converters that screw on the speedometer driven-gear housing and either increase or decrease by a percentage.
They look like this: and are found HERE:

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I got a TH350
tire size is P255/60S R15

Your combination of 3.73 gears and 255/60/15 tires will require a 15-tooth drive gear, and a 41-tooth driven gear, which gives a gear reduction ratio of 2.73:1


BUT, Houston we have a problem. The 15 tooth drive gear is not available, don't worry we can still be of help here.

TCI's website lists a 7 tooth drive gear (pink), and 19-tooth driven gear (natural color) 19/7=2.71.

TCI also lists a 8-tooth drive, and 22-tooth driven for 2.75:1

Either of these are as close as you are going to get without more math, more money, and having to purchase the gear reduction unit.


Here is TCI's link for speedometer gears, just scroll down to find the transmission you have.

Here's another link for the speedometer gears, scroll down about 3/4 way to the TH-350.

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