I changed rearend gear ratios, how can I calibrate speedo?

12seccamino

I changed rear-end gear ratios, how can I get speedo gears, or calibrate my speedometer? I put a Grand National 3:42 rear in my Elco with a Th-350 trans. I need help getting the tailshaft/speedo cable gears. Any other tricks to calibrate my speedometer/odemter to reed the correct Miles/MPH? Thanks -Matt
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64SS427 · Posted: Mon Sep 06, 2004 9:44 pm Post subject:

You'll need to change the speedo drive gears in the transmission, where the speedo cable hooks up. Not a big deal to do. Might want to check, I don't know if GNs came with a TH350, but if they did they stock speedo gears should put you pretty close depending on tire size.

Devin

12seccamino · Posted: Mon Sep 06, 2004 11:10 pm Post subject:

I know I need to change the calibration gears, that is the easy part. I can't find the right gears to calibrate mine properly. The buick Grand National came with 200-4R transmission which is a 4 speed automatic, so those speedometer gears wont work. I can not find a factory GM car that came with a th-350(3 speed auto), and 3:42 rear ratio, otherwise I could just get those calibration gears and make them work. Thanks for the reply! -Matt
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acauth1 · Posted: Tue Sep 07, 2004 12:21 am Post subject:

http://www.transmissioncenter.net/speedometer_calibration_______va.htm
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87ElCamino · Posted: Tue Sep 07, 2004 8:15 am Post subject:

http://www.tciauto.com/tech_info/speedo_gears.htm
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87 El Camino
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Tommy · Posted: Fri Sep 10, 2004 1:44 pm Post subject:

When I switched to 3.23s, I used a silver gear from GM.
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Ebola · Posted: Wed Nov 17, 2004 11:31 pm Post subject:

I gave up on the calabration thing after 7-trips to different shops, and none of them could do it. Silly me switching the rear end out, and different tires. Now I just know that when I'm doing 30mph, 40mph 50mph, I have to add 12mph to that...oh well.

LeoC2 · Posted: Thu Nov 18, 2004 12:09 pm Post subject:

Funy this thread should pop up just when I needed it! I have a question... I've been suspicious about the accuracy of the speedo since I got the car. It has 16" S-10 rims and low profile tires. I drive along the road doing 70 as per the speedo and old ladies in Civics are passing me like I'm standing still. For a quick test I had my wife drive her car at exactly 55 and I followed, the Elky showed exactly 55.

So the question is this: If the speedo gears are wrong can the speedo be wrong geometrically, that is, it is not off by 10 mph across the board but off on a scale like 5 mph from 60 to 65, 8 mph from 65 to 70, etc. Therefore it may be close to accurate at 55 but the error increases the faster I go. I hope the question is clear enough.
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Ebola · Posted: Thu Nov 18, 2004 12:48 pm Post subject:

Good question, I believe it to be consistent with mine, although there is some variable in there, that is to say, at 0mph I'm not going 12mph....lol. But I find at 40 50 60 all are consistent with a 12mph difference. Another way to check is to get out on the highway and use mile markers (and stop watch) to test the speed. Plugging along through a school zone with everyone else at 20mph will let you know where you stand at low speeds (again mine was right around 12mph off). Now I might be totally off base here, but experience says it is consistent.

dabirdguy · Posted: Thu Nov 18, 2004 12:53 pm Post subject:

It is clear.
Because the gears are a mechanical RATIO (ie: 3 turns of shaft A = 1 turn of shaft B for example), the speed you will be off will be a percentage of that speed (IF you are off!) across the entire range of speed you travel.
So if you are 10% off you are 5 miles off at 50, 3 at 30, etc.
My Elky is off 12%. I used the odometer to read against a measured mile (5 miles in fact!) to detrmine the percentage I was off. Measuring against another speedometer is ok also, but what if THAT one is off too???
Most cops will do a speedo check with you if you ask them to and theirs are SUPPOSED to be measured and tested periodically.

Anyway, I will NEED this thread later when I get my ride finalized and my rim/tire/gear selections made.

Be well!

Glenn

87last1 · Posted: Thu Nov 18, 2004 5:07 pm Post subject:

I had the same problem when I changed the orig 256:1 to 342:1 rears.It seemed every thing I tried didn't help,So I called Powel speedometer in Lancaster PA,25mi away,They calabrate police cars as well as pass,and trucks.They used a transducer I think it was called,You use all of the original equipment,it screws on the tranny like the cable would and the cable screws onto this.They know the turns and ratios like Glen explained above, They hook up a counter to the tranny and test drive your car,this will tell what percent your mph is off.They than put two small gears in this transducer that will correct the mph.I later changed to a 373:1 and went back to Powls and had it corrected again.
The cost of the transducer was $40.00 and the labor about the same,so for around $80.00 I might of saved a traffic ticket. There should be some one in your area that does this kind of work.Call the police department if you have to and see where they get theirs done. Good Luck!
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87Last1;
87 El Camino Conquista 3:73 posi Stock 305
85 Monte Carlo SS 3:73 posi Stock 305 HO
86 Chevy Silverado Short Box, 350/300HP Crate Engine 700R4 3:08 Posi
1970 Monte Carlo 350/300HP 308:1 posi

TRIK-56 · Posted: Thu Nov 18, 2004 5:42 pm Post subject:

Just use one of the websites thar Acauth I or 87Elcamino recommended, follow the instructions, and you can straigthen it out yourself without having to spend big bucks on a transducer or a speedometer calibration shop to work on it.
I went from 2:41 rear gears to a 3:73 ratio. I used one of these tables to tell me what drive and driven gears to use in my 350 transmission, put those gears in, and my speedometer is now accurate.
Its not a big deal.
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87ElCamino · Posted: Sat Nov 20, 2004 10:21 am Post subject:

87last1 · Posted: Sat Nov 20, 2004 5:28 pm Post subject:

Thanks Ernie,
Thats exactly what it looks like,When I went from 342 to 373s all they did was changed the little gear inside.Its only 1mph off at 60, They check the speed with I think "Vascar'.
I would like to say if you can use the formula as posted it is a lot cheaper,My problem was I couldn'get GM parts department to order the right part,they even called a problem solver within the co. and it was still wrong.[They did really try] They keep sending gears for a 350 tranny instead of the 2004r so I got the Sh--s of it and went this way.
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87Last1;
87 El Camino Conquista 3:73 posi Stock 305
85 Monte Carlo SS 3:73 posi Stock 305 HO
86 Chevy Silverado Short Box, 350/300HP Crate Engine 700R4 3:08 Posi
1970 Monte Carlo 350/300HP 308:1 posi

Steve84Ec · Posted: Wed Dec 01, 2004 11:27 am Post subject:

I changed tire size on my car, i'm 99% sure my speedo is off, lol. it was little off before, now i can tell big time. I cruis along at 65 mph on 1-95 which is a 70 mph highway, and at night, me doing 65, i'm passing most people. No way people are doing that much under speed limit. so i usually just cruise with traffic to be safe.
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