speedo gear swap

[MITCH_89']

I just got back from the driveshaft shop to measure for my new shaft, and we started talking about the speedometer being so far off. My Blazer is an 89' with factory 31" tires, and 2.73 gears. I now have 35" tires, and 3.73 gears. The driveline guy did some figuring, and said I could go to the Chevy House and ask for a speedo gear for a 3.55 rear end, and it should be almost on. Any thoughts on this? Wouldnt they need my original gear to know how many teeth I have, or do I just get one for the 3.55 rear?

 

[pauly383]

Go to this thread , post # 10

 

[pvfjr]

Hmm, I've got some figuring to do.

 

[MITCH_89']

I'm not smart enough to figure this out, and was hoping some was. It seems whem Im going around 55, the speedo says around 75. I havent taken the gear out yet, so Dont know hich it has

 

[MITCH_89']

anyone??

bump

 

[pauly383]

Provided your 89 is an earlier MECHANICAL speedo cable , pull the speedo gear out and look inside .

If your gear on shaft inside is blue , with 3.73's and 35's ( actually around 34-34.5 ) your gear you need is 39 tooth brown .

If inner gear is gray , then it says 33 tooth , but closest is 34 tooth LIGHT green

 

[ssls6]

Distance traveled per revolution of input shaft

D = (1/gear ratio) * 3.1416 * tire diameter

D (old setup) = 35.67 inches
D (new setup) = 29.48 inches

Error = D(old) - D(new) / D(old) = -21%

Because of the tire change, your effective gear ratio is

((tire old)/(tire new)) * gear ratio new = 3.31

If 35s aren't really 35" then 3.55 may be the right answer.

You can calculate error using gear ratios

Error = G(old) - G(new) / G(old) = -21%

 

[jekbrown]

Ive had my speedo disconnected for atleast 2 years now... no big deal really. Don't see the point in going through the hassel of swapping speedo gears in/out.

j

 

[pauly383]

Ive had my speedo disconnected for atleast 2 years now... no big deal really. Don't see the point in going through the hassel of swapping speedo gears in/out.

j

Be lucky you haven't gotten pulled over . I still have yet to buy my gear either , and I sweat it when the sherrifs deputy's follow me for awhile , have to look at my tach and my speedo , and kinda guess , since its off different at different speeds

 

[jekbrown]

lol, I can't remember the last time my rig was traveling faster than all the other cars on the road... doubt if I get pulled over it will be due to excessive speed. Around town I cruise nice and slow (enjoy driving the beast to much to want it to end!) and on the highway cars are constantly wizzing by me, dont think I drive over 65mph, ever.

j

 

[Cricket]

Need Wisdom

Okay I have a question and I'm just confusing myself. What about manual trans specifically Sm465. When I look at the speedo gear link it shows all the automatics but I did not see anything on 4spd's.

What's really confusing my little brain is the relevance of the trans type since the speedo gear on a 4x4 is in the Tcase output shaft housing.

Is the speedo gear recommendation based on trans gear ratio then assuming the tcase will be in 2 high? Am I making any sense?

 

[pauly383]

The tailshaft on the transfercase , if your aluminum case like a 32 spline 208/241 to a 465 or 400, or 27 spline 208/241 with a 700r4 , uses 2wd 700r4 gears . Its basically tire size and gear ratio of rearend , and existing gears in case will dictate what you need

 

[dyeager535]

Mechanical speedometer setups just measure off of output shaft RPM's. GM is 1000 cable revolutions per mile for correct speedometer.

Changing the axle ratio or tire size is what changes the amount of revolutions per mile of the output shaft for a given MPH, (and the speedometer cable), thus why the speedometer is off.

Tranny means nothing then in a 4wd setup.

 

[pvfjr]

Do any of the gears on that site apply to 205's?