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#1 |
Registered User
Join Date: Mar 2006
Location: east syracuse ny
Posts: 371
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where to find a transmission ratio adapter
I'm looking for a adapter for my truck ac part number 1565208 ratio 1:0.833 no luck finding it can someone give me some places to look I Googled it and checked eBay with no luck thanks
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1981 K-5 Blazer 2wd 1985 C-10 Short step 1988 C-10 Suburban ![]() |
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#2 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,599
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Re: where to find a transmission ratio adapter
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#3 |
Registered User
Join Date: Aug 2014
Location: New Bern NC
Posts: 142
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Re: where to find a transmission ratio adapter
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#4 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: where to find a transmission ratio adapter
I have used both of the vendors listed above with no issues.
I can't find the AC Delco part either, which means getting an aftermarket part. Be careful figuring percentages. .833 is 16.7% less than 1, but 1 is 20% more than .833. GM lists ratio as the speedometer side, so .833 out to the speedometer for 1.000 in from the xfer case. So you should be reading 20% high on speed (reading 60 mph at an actual of 50) to need a 17% step-down adapter. The first vendor listed above has a 17% slow-down adapter. (From 60 down to 50 is 17% slower.) The other option is ACDelco #3950352, which is 1:.8241. This will be 1% low, or 1/2 mph at 50. I couldn't find one of those either.
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Rich Weyand 1978 K10 RCSB DD. |
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#5 |
Registered User
Join Date: Jan 2015
Location: Los Angeles California
Posts: 927
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Re: where to find a transmission ratio adapter
Would these adapters be the same for 4x4? Im running a NP205 T/C and the speedo reads 7mph slower due to larger tires, anyone have a part number?
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#6 |
Registered User
Join Date: Mar 2014
Location: Blossvale NY
Posts: 202
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Re: where to find a transmission ratio adapter
Wish mine was only 7mph slow... Reading about 48 I'm actually going 60.
I was kicking around here yesterday and saw another speedo ratio adapter thread... Gave myself a brain-ache trying to figure out the math to determine what % to speed up my guage.... I give up... Just read the tach! LOL!
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#7 |
Moderator
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Location: Wentworth, NH
Posts: 4,951
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Re: where to find a transmission ratio adapter
I put together a spreadsheet to take some of the pain out of it. You can find it on the manual page.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
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#8 |
Registered User
Join Date: Jan 2010
Location: Kansas City, Mo
Posts: 284
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Re: where to find a transmission ratio adapter
Speedometer shops can build them.
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Thank a Veteran. They have sacrificed more than you will ever know! 2003 Chevy Z71 Tahoe 190,500 miles 2002 Chevy Silverado 1500 4x4 230,000 miles 1987 custom built Silverado 4x4 435,764 miles 1985 K5 Blazer 396 bb 4x4 project 1982 K5 Blazer Silverado 4x4 519,348 miles |
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#9 |
Registered User
Join Date: Mar 2014
Location: Blossvale NY
Posts: 202
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Re: where to find a transmission ratio adapter
I'll check it out Hatzie. Thanks!
![]() Woops... Downloaded the Apache files and I couldn't open it due to "sharing violation"....
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Who needs a Hemi when ya got a Chevy! Last edited by RIDE-RED 350r; 03-17-2015 at 06:24 PM. |
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#10 |
Registered User
Join Date: Mar 2014
Location: Blossvale NY
Posts: 202
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Re: where to find a transmission ratio adapter
Just got it figured out and got the spreadsheet open... WOW that's confusing! Never used a spreadsheet before.
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Who needs a Hemi when ya got a Chevy! |
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#11 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: where to find a transmission ratio adapter
To figure a new ratio, and look up the part number, I need 1) the speed indicated, 2) the actual speed, and 3) the ratio of the adapter that is already on there. The existing adapter should have the ratio stamped on the housing.
An alternative to the speed indicated and actual speed is to drive 10 mile markers on the Interstate somewhere, recording the odometer reading (to tenths) at both ends, then subtract them. You will come up with something like 10.7 or 9.5. Anyway, give me that info and I will calculate it and look it up.
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Rich Weyand 1978 K10 RCSB DD. |
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#12 |
Registered User
Join Date: Mar 2006
Location: east syracuse ny
Posts: 371
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Re: where to find a transmission ratio adapter
I got mine working for about 50 miles but then stopped and the Speedo was off also so what ratio would I need my GPS said I was going 50 the trucks Speedo said 56 and that was with the 1:833 adapter
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#13 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: where to find a transmission ratio adapter
To get it to display 50/56 of what it is displaying now, you need a 50/56 step-down, or something like 1:.893. You already have 1:.833 on there, so to get one ratio adapter that incorporates what you have plus the additional you need, that is .893 * .833 = .744. So you need a 25.6% slow-down adapter.
Closest period GM part is 3952220, at .7333. This will be about 1% slower than what you need, so will show 49.5 at 50. This was a common adapter for Corvette's with low rear-end ratios, like 4.56:1. I found a used one here: http://www.parts123.com/parts123/yb....5Z50000050n~~~ That same exact ratio is also available here as "slow down 26.7%": http://www.transmissioncenter.net/sp...n_______va.htm
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Rich Weyand 1978 K10 RCSB DD. |
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#14 |
Registered User
Join Date: Oct 2014
Location: Southern Michigan
Posts: 85
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Re: where to find a transmission ratio adapter
Hi Rich. Where do you find the ratio stamped? Is it on the little box between the transmission and the speedo cable? Didn't see anything on mine but I could take another look.
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#15 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: where to find a transmission ratio adapter
The little box between the transmission and the speedo cable is probably a ratio adapter. That should have a ratio stamped on it if it is a GM/AC Delco item.
I think they normally did the speedometer ratios in the drive and driven gears of the transmission without ratio adapters in the C trucks. In the K trucks, there were fewer ratios available, and so they had factory speedo ratio adapters.
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Rich Weyand 1978 K10 RCSB DD. |
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#16 |
Registered User
Join Date: Oct 2014
Location: Southern Michigan
Posts: 85
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Re: where to find a transmission ratio adapter
I'll get a better look at it tomorrow. I'm sure the truck didn't come with the rally wheels and big tires. It's an 86 Custom Deluxe. I know the engine was originally a V-6 but now is a 305. I checked the speedometer this morning against my GPS and when the speedometer said 65 the GPS said exactly 60. Down and around 25 there didn't seem to be any difference. Thanks for the info as usual.
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#17 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: where to find a transmission ratio adapter
So it is probably an aftermarket ratio adapter to correct for some changes along the way. Best is to take that off and put the cable directly into the tranny and see what you have. While you do that note the color of the driven gear the cable plugs into. They are color coded, and a new gear is cheap and easy to replace. On a transfer case, there are fewer choices, and the driven gears are expensive and unobtainable, so there you normally go with the adapter.
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Rich Weyand 1978 K10 RCSB DD. |
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