[montea]

in my 82 grand prix

stock
TH350
2.29 (26.10 overall diameter inchs)
205/75R14

now
TH350 (original but rebuilt)
2.73
295/50R15 (26.61 overall diameter inchs)

i found a shop that deals just with spedometers in the city but they cannot tell me wich gear i need.

how do i figure out wich gear i need to buy? do i need to find the teeth on the current gears and go from there or is it possible to just find out with my new setup?

i read the sticky thread from m&mcustom, i didnt find anything on there i needed tho, or maybe i missed?

[Triaged]

This is a quote from Steve Chin on CK5

The correct way for a speedo shop to calibrate a speedo is to ensure that the odometer reads correct at 1000 revolutions per mile, then make the speedometer head agree with that by either moving an adjuster in the speedometer head or by changing the magnetism of the flying magnets in the head.

That said, you first need to figure out what gears you have in your transfer case (you already know your tire OD and final drive ratio). To calculate what gears you need, you first need to know the Speedometer Gear Ratio (SGR).

Calculate that by:

FDR = Final Drive Ratio
TOD = Tire Outside Diameter

SGR = 63360*FDR/(TOD*3141.597)

In this case, with 38" tires and a 4.10:1 final drive, the SGR = 2.176030384

Next, you need to know the tooth counts on the drive gear and driven gear. The only way to find out is to remove them and count.

If you divide the number of teeth on the driven (output) gear by the number of teeth on the drive gear (the gear on the output shaft) and that result does not agree somewhat closely with the SGR, you need a ratio adapter (or you need to change one or both of the gears).

In order to determine the ratio of the drive adapter you need, you simply:

SOG = Speedometer Output (driven) Gear
SIG = Speedometer Input Gear
SGR = Speedometer Gear Ratio

Adapter Ratio = SOG/SIG/SGR

Note that some adapter ratios might be overdrive (greater than one) and some might be underdrive (less than one).

Clear as mud?

http://coloradok5.com/forums/showthread.php?t=66439

Another

Speedometers and odometers are calibrated for 10,000 revolutions in a mile. You need to calibrate the gears in the transfer case to obtain a gear ratio of 2.35:1 to get the correct odometer reading, then calibrate the speedometer head to jive with that. Most likely, you will not be able to achieve the exact gear ratio and you may need to go with something that's somewhat out of whack and use a drive ratio adapter (I have a 20.2% overdrive adapter on mine that was installed by the factory because the factory set my TC up for 3.73s and 30" tires).

FWIW, this is the formula for speedometer gear ratio: (63360 * Axle Ratio) / (3141.6 * Tire OD in inches) = SGR.

Take the SGR and multiply it by the number of teeth on the drive gear to obtain how many teeth you need to have on the driven (output) gear: SGR * DGT = OGT

http://coloradok5.com/forums/showthread.php?t=55995

[M&M CUSTOM]

The old gears you had were at or close to a 1.79 reduction ratio.

The new gears will need a gear reduction of 2.05 to 1.

This will be MUCH easier if you knew what gears you already have in the transmission.

If there is any way you can see either one to tell me the color (they are color coded) you could save the hassle of pulling the tailshaft or speedometer housing off just to take a tooth count.

The 10 tooth (red OR purple drive gear on the transmission output shaft) and 18 tooth brown driven gear give a ratio of 1.8 (close enough for the necessary 1.79 needed).

Either way you will need to change both gears (just in case the drive gear is already worn and the cost isn't that much) to a 10-tooth red or purple drive gear, and either a 20-tooth blue driven gear, or a 21-tooth red driven gear.

Both gears are equally split in the difference from technically "exactly" what you need, and the 20 tooth gear will read "slightly" higher around 1.5 mph faster at 55 mph, and the 21 tooth gear will read the same only 1.5 mph less at 55 mph.

It's just easier knowing what you already have, because the TH-350 has two different gear sets, you can read more on this at TCI's speedometer information page. You stated an 82 Grand Prix, so this does make a difference- the Chevy and the BOP transmissions have different speedometer housings.

[montea]

ok finnaly got time to check out the gears in it... the small gear is 19tooth and light brown i believe and the inside gear on the tailshaft i thought i would remeber but soon as i put the spedo cable back in i forgot! but it looked like it was grey or light pink or somthing, hard to tell when its all covered in tranny fluid. but i do think its a 7 tooth

i got the car saftied today and is on the road tommarow, untill i can swap out those gears aproxx how much is my spedo going to be off the way it is right now? it will show i am going slower then i really am correct? i just dont want to get a speeding ticket, i can follow all other cars and go same speed but no one goes the speed limit.

[montea]

ok i read your post over a couple times and checked that website out wich has a awesome calculator thing and i think it would be best for be to go with a 10 tooth drive gear and a 20 tooth driven, id rather it say faster then slower.

my tranny is the original in my pontiac but i belive its the exact same as a chevy one because its a chevy bellhousing pattern and only 1 pattern plus it was originaly hooked to a chevy 267

if you could just confirm the 10T drive and 20T driven before i go pick them up that would be awesome! thanks M&M!