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How The Speedo Works Question.... zeroing in on it


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I looked up the part numbers for the speedo driven gear on opposedforces.com for an '00 Forester and a '96 OBW and they are the same. So if I were to use the '96 manual trans in the previously automatic Forester, it should still work correctly with the Forester wheels and tires or do the auto/manual Foresters have different gauge clusters?

 

How does the speedo needle actually move? I noticed that the Forester's speedo goes to 120 while the OBW's goes to 140, so 60mph (for example) is in a different place on the dial for each cluster. Is the sender that the driven gear is attached to different for each car and thus sends a different kind/strength of signal? , or is this again a manual/auto trans thing? The catalog shows the same part number for what I'm thought was the sender, but it just calls it a "speedometer shaft" so is this not the sender at all?

 

Thanks!

 

Will-

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I looked up the part numbers for the speedo driven gear on opposedforces.com for an '00 Forester and a '96 OBW and they are the same. So if I were to use the '96 manual trans in the previously automatic Forester, it should still work correctly with the Forester wheels and tires or do the auto/manual Foresters have different gauge clusters?

 

How does the speedo needle actually move? I noticed that the Forester's speedo goes to 120 while the OBW's goes to 140, so 60mph (for example) is in a different place on the dial for each cluster. Is the sender that the driven gear is attached to different for each car and thus sends a different kind/strength of signal? , or is this again a manual/auto trans thing? The catalog shows the same part number for what I'm thought was the sender, but it just calls it a "speedometer shaft" so is this not the sender at all?

 

Thanks!

 

Will-

 

i don't know all of what you are asking but i can tell you what i do know and what i think.

 

the engine and trans gearing are meaningless until you get to the speedo gear which is matched to speedo faceplate and the stock tire size. tires revs per mile have to match the speedo gear. change tire diameter 10% the speedo will be off 10%.

 

when i swapped a speedo (96 legacy i think) into a 97 gt, both auto trans, the speedo was off. it was off a percentage, about 10%; 1 mile at 10 mph, 6 miles at 60 mph.

2 things to remember:

 

1. the 96 lego and the 97 gt have basically the same size tire diameter, meaning the same tire revs per mile. (not exactly the same but very close, way closer than lego and outback.)

 

2. the gt auto trans has the 4.44 final drive (it is the outback auto trans and i think speedo drive gears) and the lego auto trans has the 4.11 final drive.

(i'm not real sure how important this is except that it may be an indicator of the speedo gears. ? )

 

i also noted when i made the swap that the face plates on the speedo were different, BUT when actually going 66? miles per hour, both speedo needles were pointing straight up, even though the the indicated speeds were different (66 on one and 77 on the other????). from this i concluded the the speedos were the same but that the face plates were different. one was calibrated to match the the gt, the gt speedo only went to 120, the lego went to 140.

 

so i swapped the face plates. much harder than you would think since you have to remove the needle to do it and reinstalling the needle so it reads correctly is a challenge / tedious. i have another post where i talk more about that.

 

as far as i know, there is no adjustment on the speedos. the mechanical units are all the same, the faceplates and speedo drive gears are different and match the tire size. when i first removed the gt cluster, i noticed that the wire connector for the gt speedo from the speed sensor was a different shape and color than the lego wire connector.

 

i also searched for speedo drive gear part numbers and i think i found that the gt was an odd duck but i'm not sure. i didn't bother to remember since i have mine with in 1.5mph.

 

if the vehicles in question have the same size tires and are in the same modle year group i think there is a good chance it is a straight swap. if the face plates are different probably not. when you get around to swapping the face plates a GPS comes in really handy so you know exactly how fast you are going. my solution was to drive the car 65 mph and then stick on the needle. it is very dangerous to do this on the hiway at 65 mph in traffic. you can do the same thing without swapping the face plates, but the speedo will only be accurate at that speed. not 30 miles slower or 30 miles faster.

 

the info from the speed sensor goes directly to the speedo, it think. and i assume that the odometer is also matched to the tire size. i dont think either data stream is adjusted by the ECU or the TCU before it is displayed at the cluster. ( itried swapping the TCU befor swapping the spedo face plates.

 

i hope this helps.

Edited by johnceggleston
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Do you have a mechanical or electronic display for the odometer? Mechanical is certainly different. Electronic gets VSS pulses from a sender in the transmission which got to the speedometer as well as the ECU.

 

Mechanical

 

speedo gear which is matched to speedo faceplate and the stock tire size.

 

i also noted when i made the swap that the face plates on the speedo were different, BUT when actually going 66? miles per hour, both speedo needles were pointing straight up, even though the the indicated speeds were different (66 on one and 77 on the other????). from this i concluded the the speedos were the same but that the face plates were different. one was calibrated to match the the gt, the gt speedo only went to 120, the lego went to 140.

 

 

i hope this helps.

 

Yes, this does help. The OBW has 205/70 15" and the Forester has 215/60 16" which makes them just about the same OD, which would explain why they have the same speedo driven gear since they both also have 4.11 final drive (in the manual trans versions).

 

So my options are to either put in the '96 cluster which probably won't fit connection wise, swap faceplates which sounds like a bear, or obtain a cluster from a manual trans Forester which should be calibrated the same as the '96 and just deal with the odometer difference which is probably what I'll do.

 

Thanks!!

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Hmmm.... so looking at some clusters on ebay, the manual trans clusters also only go to 120 with 60mph being in a different place then on my manual trans outback.

 

So maybe resetting the needle is the thing to do.

 

To make this even more complicated, I'll be putting in a 3.7 final drive setup. So how about this plan:

 

Instead of trying to reset the needle while driving on the freeway like John Evil Knievel Ceggleston did (hehe) I could obtain an RX instrument cluster (what the trans and diff are out of) pop the driven gear off and spin the shaft with a drill and note the speed. Then do the same for the Forester setup and pop the needle on at the appropriate place. Think that would actually work?:rolleyes:

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Mechanical

 

Yes, this does help. The OBW has 205/70 15" and the Forester has 215/60 16" which makes them just about the same OD, which would explain why they have the same speedo driven gear since they both also have 4.11 final drive (in the manual trans versions).

 

Thanks!!

 

i'd keep the speedo you have now since the tire sizes are a near match, until you are sure it is wrong. test it by following some one else at 60mph, or use a GPS or time your self on a measured 1/4 mile (15 sec. = 60 mph).

 

215/60/16 is about 0.14" smaller than 205/70/15, slightly more than 1/8", not much.

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To make this even more complicated, I'll be putting in a 3.7 final drive setup.

 

so the error, without any other adjustment, will be the size difference between the 3.7 stock tire and the 215/60/16 tire.

 

swapping the speedo drive gear is the solution as long as the 'driven gears' are the same. they are hard to change........ unless you are putting a 3.7 in a 4.11 trans housing, in which case it will be easy, you will already have it open.

Edited by johnceggleston
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so the error, without any other adjustment, will be the size difference between the 3.7 stock tire and the 215/60/16 tire.

 

swapping the speedo drive gear is the solution as long as the 'driven gears' are the same. they are hard to change........ unless you are putting a 3.7 in a 4.11 trans housing, in which case it will be easy, you will already have it open.

 

The drive gear is the pinion shaft, which I've been told is the same as far as the speedo gearing section goes. It's still bothering me that the manual and auto versions of '00 Foresters have the same speedo face which makes me think that the drive gear (pinion shaft) is different which doesn't make sense. the drive gear is basically a worm gear and if the driven gear is the same part number for '96 and an '00 then that worm must have the same pitch even if they were different diameters.

 

But I guess the speedo could just be calibrated differently ie the needle is stuck on at a different place between the two.

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stock on my 97 gt is 205/50/16, stock on the 96 lego is 185/70/14. the GT is about 0.13" (1/8") smaller in diameter than the lego.

 

also in this general tire size is 195/60/15, diameter is 0.01 inch larger than 96 lego.

 

stock 205 55 16 is listed as the stock GT size? which is a huge 2.8% difference :lol:

 

man, it's amazing how much smaller those 14" tires look verses the 16". they look much smaller than they are.

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The drive gear is the pinion shaft, which I've been told is the same as far as the speedo gearing section goes. It's still bothering me that the manual and auto versions of '00 Foresters have the same speedo face which makes me think that the drive gear (pinion shaft) is different which doesn't make sense. the drive gear is basically a worm gear and if the driven gear is the same part number for '96 and an '00 then that worm must have the same pitch even if they were different diameters.

 

But I guess the speedo could just be calibrated differently ie the needle is stuck on at a different place between the two.

 

lets clarify, the drive gear of the front diff is the pinion shaft gear. the drive gear of the speedo is mounted on the carrier which rotates the same as the ring gear and axle.

the driven gear of the speedo extends down into the front diff below the speed sensor on the front right of the trans.

 

as far as the speedo goes, the drive gear is a large ring gear type of thing and the driven gear is small and spins on a shaft more like a pinion. so the take off for all speedo gears is after any final drive gearing. to put it another way, the take comes from the axle, just before it exits the trans housing.

 

the speedo drive gears are different in different models, and probably all of the smaller 'pinion like' driven gears, mounted below the speed sensor, are the same, but i do not know for sure.

 

 

some one posted a jpeg of a "speedo gear table" some time ago. i don't remember if only the drive gears are different or the driven gears are diferent as well.

 

and some one commented that these speedo gears come in different colors, maybe corresponding to the color on the speedo connector at the speedo. 97 gt and 96 lego were different, one was white and one was blueish.

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lets clarify, the drive gear of the front diff is the pinion shaft gear. the drive gear of the speedo is mounted on the carrier which rotates the same as the ring gear and axle.

the driven gear of the speedo extends down into the front diff below the speed sensor on the front right of the trans.

 

as far as the speedo goes, the drive gear is a large ring gear type of thing and the driven gear is small and spins on a shaft more like a pinion. so the take off for all speedo gears is after any final drive gearing. to put it another way, the take comes from the axle, just before it exits the trans housing.

 

This is true of the Auto trans. The manual trans, the driven gear is located on the side of the trans case and is driven off of the section of the pinion shaft pointed at in this pic,

 

IMG_0108.jpg

 

Here's the driven gear on the other case half

 

IMG_0109.jpg

 

A close up of the pinion gear section in question,

 

IMG_0113.jpg

 

 

some one posted a jpeg of a "speedo gear table" some time ago. i don't remember if only the drive gears are different or the driven gears are diferent as well.

 

That sounds handy, I'll try and find it in a search.

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Odd, I replaced my 3.900 transmission/rear diff with a 4.111 pair in my '97 Impreza. Speedometer error was about 2mph high across the range with both set-ups. So no apparent change with that change. My '97 has a mechanical cable running the speedo/odo.

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