Speedo gear change

[wagonman]

After going from 273s w/a 31.50 15 to 373s with 33s the speedo is way off,like 85 mph @ 55. Local speedo shop wants $100 to get it right.Isint there a formula to get me in the ballpark like what color gear to use.Speedo guy says there is a unit to go inline with cable.Anyone done this swap?

 

[Mr Clean]

Yes there is. I just can't remember were I found the info. Let me do a little looking around.

 

[wagonman]

Did some research and found a site that has mucho info,just did a search gm speedo calibration.Any how heres what I learned.Find the tire revs per mile= a bfg 33x12.50x15 has a circumfrance of 102 inches when new.Divide the 102 into 20168=197.725. Take the # of theeth on drive gear,in my case 18 x 3.73 the gear ratio x 197.725= 13243.3 divide that by 1001 = 13.2 This is supposed to be the # of teeth on the driven gear. Well no such gear exists so you need an adaptor that goes in between the cable end and trans gear housing.Found one in my parts stash and screwed it on and took it for a spin. Better but still off by approx 20 mph.The site I was on can build one of these puppies in any ratio ya want. Gonna go scrounge pick n pull and see what i can find.If I can remember I will post the site I found the info.At least the speedo aint burried past 85 anymore...

 

[wagonman]

Well I screwed up the calculation,went back to the site( transmission center.net) and did some more reading.@ the end of the page they have a calculation chart.Just input tire size,gear ratio,drive gear size hit enter and they give you the # of theeth on the driven gear,a yellow 41 gear.I think my mistake was in the tire size calculation.Diameter not circumfrance.Doh.

 

[HotRodPC]

Yes, the speedo shop wants $100 and they will use a junky adapter. I recently posted a link to figure out what color gears to use. Did you find that one? I'll dig it up.

 

[HotRodPC]

http://www.transmissioncenter.net/s...__va.htm?gclid=COX6w6Llv5kCFSEgDQod4CZ1uw#400

Once you use this stuff to find out which parts you need, you can usually find them cheaper elsewhere. What transmission or transfer case are you needing gears for?

 

[wagonman]

Thanks Hot Rod,Yes I did find the gear I need on that site,its a yellow 41 tooth that should mate with the blue 18t drive gear.I am going to the wrecking yard this am.I might need the housing with the gear,will see what I can find and keep yall posted..

 

[bucket]

Yes, the speedo shop wants $100 and they will use a junky adapter. I recently posted a link to figure out what color gears to use. Did you find that one? I'll dig it up.

What's wrong with the little ratio boxes? Even GM used them.

 

[HotRodPC]

What's wrong with the little ratio boxes? Even GM used them.

Could be the brand I guess. I know 2 people, one being my dad, that I did a gear and tire change to his truck. Ended up with the ratio box from the speedo shop and it broke within 1000 miles. Took it back, they put another on it, about 4000 miles later it broke again. Then we got some exsuse BS it was our fault and they wouldn't replace it, so he went without a speedo since he knew by the tach how fast he was going now after having an accurate speedo for 4000-5000 miles. The other guy, his didn't last 2000 miles and they wanted to charge him for a new one. So I don't have much good to say about them. I usually do the math and get the gears. Usually at worst, you can be 2.5% off in that is in rare occasions so next time you get tires, get a size smaller or larger whatever is needed to correct it. But 2.5 miles off at 100mph or 1.75 miles off at 50mph is not so bad.

 

[wagonman]

Back from the wreckers,found a 84 with the t-case on the ground next to the truck.Picked the speedo gear and housing,black gear with the correct housing.Seems like there are 2 differant housings,one for 30s series tooth gears and one for 40 series.Needed an axle so I poped the cover and its got factory 3.73 gears.So the speedo issue is now fixed.Will go back and pick the gears,$55 sure beats Summits $170 plus they are OEM.Couldent get the gears as I didnot have the Big socket for the pinion nut.Hopefully they will be there when I get back after Memorial day weekend.Oh yeah I eyeballed the 2 gear housings and didnot see a differance??? Used the picked unit though.

 

[wagonman]

Yeah them adaptors are crap,I tried the one I have had laying around.Pulled it apart,cleaned and lubed it,installed it and its noisy and speedo bounces. Its in the trash can now.And they want like $75 for it,JUNK.

 

[HotRodPC]

Back from the wreckers,found a 84 with the t-case on the ground next to the truck.Picked the speedo gear and housing,black gear with the correct housing.Seems like there are 2 differant housings,one for 30s series tooth gears and one for 40 series.Needed an axle so I poped the cover and its got factory 3.73 gears.So the speedo issue is now fixed.Will go back and pick the gears,$55 sure beats Summits $170 plus they are OEM.Couldent get the gears as I didnot have the Big socket for the pinion nut.Hopefully they will be there when I get back after Memorial day weekend.Oh yeah I eyeballed the 2 gear housings and didnot see a differance??? Used the picked unit though.

When I find stuff I want to go back and get, I stash it. I'll carry it over to say a foreign car trunk where someone looking for foreign parts woudln't be interested in the parts I want.

 

[usar17]

How simple is it to do this? Does this require pulling apart the transfer case? I just swapped to 4.10s and need to fix the speedo. I have a 700r4 with np208. 33" tires

 

[HotRodPC]

In most cases you can get by with changing the driven gear and not the drive gear on the tailshaft. And I'm not even certain if NP203 had 2 different drive gear options or not. Might have only been 1, so you'd just change the driven gear.

 

[usar17]

...i said NP208...

And i have no clue what your talking about. I don't know **** about this. Is it something in the cable or in the transfer case?