Rear-end ratio formula?

[slategrove]

I am trying to figure out what speedometer gearing I need to get my speedometer to read correctly...I think it's 15-20 mph faster than I'm really going. I've done the searches on here, and read a lot of posts about this, but I need someone to tell me if there is a formula to figure out my rear end gearing?

For 10 revolutions of the drive shaft, I get between 5.25 and 5.5 revolutions of my tires. The two ways I could think to do this math straight up give me either a 1.90 rear end or a .525 rear end...neither of which I believe to be anywhere near correct.

PLEASE Help

[BADPURPLE70CHEVY]

Originally Posted by slategrove

I am trying to figure out what speedometer gearing I need to get my speedometer to read correctly...I think it's 15-20 mph faster than I'm really going. I've done the searches on here, and read a lot of posts about this, but I need someone to tell me if there is a formula to figure out my rear end gearing?

For 10 revolutions of the drive shaft, I get between 5.25 and 5.5 revolutions of my tires. The two ways I could think to do this math straight up give me either a 1.90 rear end or a .525 rear end...neither of which I believe to be anywhere near correct.

PLEASE Help

Do you have a way to remove the cover? If so it should be stamped on the gear. You can always count ring gear and pinion gear teeth and divide if you have the rear cover off.

[leddzepp]

Rotate your driveshaft once and count the number of times your tire makes a complete revolution. This will give you the gear ratio. I did this on mine, and my tires turned 3 3/4 turns to one turn of the driveshaft, leading me to the conclusion I have 3.73 gears.

[Rooster's 67]

Uh that would be near a 2.0 gear ratio, not too realistic, but not impossible. Was only one wheel spinning? You need to raise both wheels off the ground, and they must spin in the same direction at the same speed to get an accurate count.

use white-out or shoe polish to make a mark on the tires/shaft for reference.

Or jack up one wheel and count the number of revolutions of the tire in the air for one revolution of the driveshaft.

If you have both wheels in the air and can get an accurate count (posi diff or spool).

Divide the number of shaft revolutions into the wheel revs for Ratio

if only one tire in air(can only do with an open carrier, or non-posi):

Divide number of tire revs by 2
then divide shaft revolutions into that figure

For instance, one tire in air, 2 driveshaft revolution & approx. 18 revs of the tire in air.

18/2=9
9/2= 4.5 or 1:4.5 or most likely a 4.56 gear

Hope this helps.

[Rooster's 67]

divide the ring gear tooth count by pinion tooth count, or Tire revs by shaft revs.

without pulling cover and counting teeth, see my post above.

[BADPURPLE70CHEVY]

If you have never had the rear end cover off this is a good time to give oyur rear end some fresh gear oil as well as count teeth to be sure. Divide ring gear teeth by pinion gear teeth. example 37 ring gear teeth divided by 9 pinion gear teeth is a ratio of 4.11

[slategrove]

The first time, I only had one tire off the ground, which I understand from the above posts is wrong with a posi rearend...

tried it again, both tires off the ground and turning the tire (both turning same direction at same speed) while watching the driveshaft revolutiuons...10 driveshaft revs = 2.6 or 2.7 tire revs

Im wondering if turning the drive shaft would produce the same results in opposite (10 wheel revs & 2.6ish driveshaft revs)???? Because above posts say 2 revs of driveshaft and 9 revs of tires

Am I correct in thinking that my rear end ration is 3.73?

[lakeroadster]

The method for a posi -vs- an open differential is different....

To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

[C-10 simplex]

i went thru this fiasco myself, and, in my usual anal style, i made it real complicated, drawn-out, but thourough:

http://forum.73-87chevytrucks.com/sm...p?topic=3785.0


1) Is there a sticker in your glove box or inner fenderwell---it should state what gear ration you got.

2) you can also go to your local gm dealer(preferrably chevy) and they can type in your vin# and find out what gear ratio you got---ASSuming the gear is still stock. From this, they have charts to match up whichever speedo gears go with your combo----don't forget tire size. In other words, which tires came stock on your truck and what size do you got now? This is what screwed me up.

3) i think the easiest(but not nessesarily cheapest) way is to get a GPS speedometer.

[slategrove]

As usual...I'm a complete idiot!!! I ordered the driven gear from the local Chevy dealer...got it in today, and it doesn't fit. I hadn't really given my transmission a second thought...knew the pan didn't look like a TH 350, so assumed it was a TH400 (why I didn't look at the pan compared to images on the internet I don't know)......

Long story short, I think I have a THM200-R4 here are the pics, what do you guys think?

[C-10 simplex]

Yes, it does look like a 200r4. This is interesting; Did you notice the trans shifting at least 3 times while driving? i say 3, but it could feel like 4 if the lockup converter is working.

Don't worry---you'll get sorted out.

[hounddogs!]

200R4. It'll have "METRIC" in inch high letters stamped into the pan.

[slategrove]

hounddogs!----"METRIC" is stamped in the pan, and you can make it out in a couple of the pics posted above

The PO said it was a 4 speed Transmission with overdrive, but the overdrive was disconnected. It definitely shifts 3 times (1-2, 2-3, and 3-4)...the longer I have the truck, the less I think he knew...

C-10Simplex----There is a cable (kinda looks like a spark plug boot where it plugs into the passenger side of the tranny) that is just tied off on top of the motor...would this be the lockup converter you're talking about?

[Rooster's 67]

that cable is caled a T.V. cable(throttle valve). It is supposed to hook up to the carburator linkage and controls the line pressure of the transmission in relation to the throttle position. it will affect shift points, both upshifts and downshifts. there will be an electrical connection for the lock up unless the 2004-R's where mechanical lock up [scratching head trying to remember last time I worked on a 2004-R]

[slategrove]

I went to the local Chevy dealership, and ordered the gear that an online calculator told me I needed for my 200r4 transmisison...a 36 tooth...I didn't fit either. It has a diameter of about 1.5" while the 28 tooth I'm removing is about 1" diameter...the larger gear won't fit through the hole in the housing. Spent an hour with the parts guy at the dealership and the best he can figure out is that my 200r4 came from an 84 camaro, that's the only car in his computer that shows the 200r4 with a white (19 tooth) drive gear (none of the other cars with a 200r4 even show the white drive gear as an option). He said the 84 camaro is also the only 200r4 that shows the yellow 28 tooth driven gear (which is currently in my tranny)...this info he gave me about the white drive gear being 19 teeth doesn't match up with the info on http://chevelleengineer.home.comcast...eer/speedo.htm

????????????????????????????

I think I'll order a ratio adapter and forget any of this ever happened

[OhOneWS6]

Originally Posted by C-10 simplex

i went thru this fiasco myself, and, in my usual anal style, i made it real complicated, drawn-out, but thourough:

http://forum.73-87chevytrucks.com/sm...p?topic=3785.0


1) Is there a sticker in your glove box or inner fenderwell---it should state what gear ration you got.

2) you can also go to your local gm dealer(preferrably chevy) and they can type in your vin# and find out what gear ratio you got---ASSuming the gear is still stock. From this, they have charts to match up whichever speedo gears go with your combo----don't forget tire size. In other words, which tires came stock on your truck and what size do you got now? This is what screwed me up.

3) i think the easiest(but not nessesarily cheapest) way is to get a GPS speedometer.

Options 1 & 2 do not work for the 67 - 72 year models. AFAIK The SPID does not show standard gear ratios and GM did not keep records on gear ratio by VIN that far back.

[C-10 simplex]

duly noted.

thanks for the info.

[cdowns]

no such tranny as a 200 r4 there is a 2004r tranny like pictured

[slategrove]

Originally Posted by cdowns

no such tranny as a 200 r4 there is a 2004r tranny like pictured

Sorry...please replace every "200r4" with "200-4R"

[Keith Seymore]

Originally Posted by slategrove

I think I'll order a ratio adapter and forget any of this ever happened

Good call -

that cuts through all of this "middle math" and gets right at the result...

K