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Pedal Smasher
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Discussion Starter #1 (Edited)
I've tried looking for this information on the forum and I haven't found a specific thread that lists all the different speedometers that appeared in a GT. This could be a very useful thread to cover the different W numbers, what years of the GT they could be found, and KPH or MPH. Talk about the calculation involved to understand error, etc. This could be a good thread to sticky in this forum.

Speedo details I know for sure about...

My 1973 GT, produced in January of '73, has a W1020 MPH speedo and a 4 speed manual. I don't know if the transmission type mattered in the last year of production. I also have a W897 MPH speedo from Gordo (thanks BTW). I've seen W1062 MPH, W1094 MPH, and W660 km/h speedos. I don't know what years you could find these in a GT and if they were transmission specific. I've seen other potential W numbers listed online but I can't confirm what vehicle you'd find them in.

To calculate the W number, I believe the following equation works? I pulled it out of Dieter's speedo calculator. -This has been verified, see thread.

W No. = unit x rear axle gear ratio x gearbox screw wheel / tire circumference / speedo drive teeth.

Note: this equation can also be done as (unit x rear axle gear ratio x gearbox screw wheel) / (tire circumference x speedo drive teeth). Both ways will get you the same answer, following the order of operations.

Unit number - 1000 for km/h or 1609 for MPH. This value is specific to how many meters per hour. 1609 more accurately would be 1609.344 meters in 1 mile.
Rear axle gear ratio - stock GT's will be 3.44, a rare Kadett ratio is 3.18 and a common performance ratios are 3.67, 3.89, and 4.11.
Gearbox screw wheel - 7 for the GT and Manta, the Kadett has a 5 I believe.
Speedo drive teeth - this is the plastic gear located in the transmission which drives the speedo cable. There are 3 options, always done by color. Red have 18 teeth, yellow have 19 teeth, and blue have 20 teeth.
Tire circumference - you could add tire slippage to this value, or use the actual tire circumference. For choosing a speedo and speedo drive gear, it's probably best to not factor slippage into the calculation because slippage is not constant. Wet versus dry driving conditions alone would be different. Tire slippage can also vary a lot by the exact tire being used, too hard to know how much slippage there would be to help setup a speedo. For this calculation, tire circumference needs to be in meters.

% of Error = ((calculated - actual) / calculated) x 100

I don't know how easy or hard it would be to modify a speedo to better match the calculated W number. It might be easier to create a different speedo drive gear option that has more or less teeth than the standard 3 options for this, no one has done it that I know of however. I'm adding this to my gear calculator which focuses on tires, transmission gears, final gears, and vehicle speed. I'll upload it to this thread when I'm done.
 

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Pedal Smasher
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Discussion Starter #2 (Edited)
I've finished with an update to my old gear calculator. It now includes a section at the bottom for the speedometer. This excel file is the result of various calculators I've used, all related to gear ratios, tire sizes and speed. Any cell that is green can be edited, the rest of the excel is protected simply to prevent accidental deletion of formulas. Should you feel the need to edit a protected cell, the password to unlock it is autoholic. The info currently in it is for most stock US spec 1.9L GT's. A stock GT had 165/80R13 tires, the W1062 speedo seems pretty common, and it would have a red speedo drive gear.

*check below in post 13 for the newest link*

Some common Opel transmission information...

1.1L Diff Ratio: 3.89
Manual Transmission Gear Ratios
1st gear 3.867
2nd gear 2.215
3rd gear 1.432
4th gear 1.000
reverse 3.900

1.9L Diff Ratio: 3.44
Manual Transmission Gear Ratios
1st gear - 3.428
2nd gear - 2.156
3rd gear - 1.366
4th gear - 1.000
reverse - 3.317

TH180 Automatic Transmission Gear Ratios
1st gear - 2.40
2nd gear - 1.48
3rd gear - 1.00
reverse - 1.92

Getrag 240 Manual Transmission Gear Ratios
1st gear - 3.717
2nd gear - 2.019
3rd gear - 1.316
4th gear - 1.000
5th gear - 0.804
reverse - 3.445

Other Diff Ratios: 3.18, 3.67, 4.11, 4.22, 4.30, 4.75, 5.28
 

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The Getrag 240's fifth gear ratio is 0,804, not 0,81. There are also stock 4,22:1 and 4,30:1 rear gears for the torque tube axle, plus 4,75:1 and 5,28:1 for rallies and racing.
 

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Pedal Smasher
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Discussion Starter #4 (Edited)
The Getrag 240's fifth gear ratio is 0,804, not 0,81. There are also stock 4,22:1 and 4,30:1 rear gears for the torque tube axle, plus 4,75:1 and 5,28:1 for rallies and racing.
Thanks for the info, I've edited my previous post. Do you have any info on speedos to share by chance?

I'm looking through my large photo collection and I've found a GT that has a W557 km/h speedo, the car has a manual transmission. There is an orange GT/J, manual trans with a W660 km/h speedo out there. I have photos on a manual trans GT produced in May of '73 with a W1062 MPH speedo, so it seems that W1020 and W1062 both were used in '73. I have photos of a manual trans GT produced in May of '70 with a W897 MPH speedo. The 107 mile blue '73 GT that was bought by Suselbeek had a W1062 speedo.

Has anyone else seen a W1020 speedo? I'm wondering if this is a rare speedo. I've found it mentioned over on the other thread linked in the first post. But, I don't know why Opel would have used a W1020 and a W1062 speedo for the US spec GT's late production.
 

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I've finished with an update to my old gear calculator. It now includes a section at the bottom for the speedometer. This excel file is the result of various calculators I've used, all related to gear ratios, tire sizes and speed. Any cell that is green can be edited, the rest of the excel is protected simply to prevent accidental deletion of formulas. Should you feel the need to edit a protected cell, the password to unlock it is autoholic. The info currently in it is for most stock US spec 1.9L GT's. A stock GT had 165/80R13 tires, the W1062 speedo seems pretty common, and it would have a red speedo drive gear.

https://drive.google.com/open?id=1UMd45XaL_S1HBJRqcPv9OFg_m2w-ZQIr

Some common Opel transmission information...

1.1L Diff Ratio: 3.89
Manual Transmission Gear Ratios
1st gear 3.867
2nd gear 2.215
3rd gear 1.432
4th gear 1.000
reverse 3.900

1.9L Diff Ratio: 3.44
Manual Transmission Gear Ratios
1st gear - 3.428
2nd gear - 2.156
3rd gear - 1.366
4th gear - 1.000
reverse - 3.317

TH180 Automatic Transmission Gear Ratios
1st gear - 2.40
2nd gear - 1.48
3rd gear - 1.00
reverse - 1.92

Getrag 240 Manual Transmission Gear Ratios
1st gear - 3.72 3.717
2nd gear - 2.02 2.019
3rd gear - 1.32 1.316
4th gear - 1.00 1.000
5th gear - 0.804 0.804
reverse - 3.45 3.445

Other Diff Ratios: 3.18, 3.67, 4.11, 4.22, 4.30, 4.75, 5.28
I added the Getrag gear ratios in 3 digits after the decimal point to match the same precision you have for the stock 4 speed.
 

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Thanks for the info, I've edited my previous post. Do you have any info on speedos to share by chance?

I'm looking through my large photo collection and I've found a GT that has a W557 km/h speedo, the car has a manual transmission. There is an orange GT/J, manual trans with a W660 km/h speedo out there. I have photos on a manual trans GT produced in May of '73 with a W1062 MPH speedo, so it seepms that W1020 and W1062 both were used in '73. I have photos of a manual trans GT produced in May of '70 with a W897 MPH speedo. The 107 mile blue '73 GT that was bought by Suselbeek had a W1062 speedo.

Has anyone else seen a W1020 speedo? I'm wondering if this is a rare speedo.
The W1062 is a match with the ( later stock 4 speeds and Getrags ) with the 3.44 rear axle
The W897 is a match with the early stock 4 speed with the 3.44 rear axle.
 

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Pedal Smasher
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Discussion Starter #11
I've been reading through this thread some more. It does appear a W1020 speedo was used for 1.1L GT's, making it a pretty rare speedo. Only 3,573 GT's were made with a 1.1L engine. It's possible Opel had leftover supply of these and started putting them in random GT's bound for the US.

What I don't understand, is how the transmission mattered within the calculation. Why a W897 vs a W1062 for a US spec GT, if a 3.44 was used both times in the diff? If both had a 7 tooth gearbox screw wheel, same tires, same speedo drive... it doesn't make sense why Opel would have made a W897. Unless there was some other difference. I've heard of a brown speedo drive gear, but I don't know how many teeth this gear would have. If the brown speedo drive gear had a lower tooth count like 17 and a 5 tooth gearbox screw wheel, then a lower W number would make sense. That would put it at 872, which is a lot closer to W897.

I'm thinking the W1020 speedo uses a 5 tooth gearbox screw wheel and a brown speedo drive gear at 17 teeth. Combined with the 3.89 diff that was used in 1.1L GT's, this gives a calculated 986, which is pretty close for a stock tire of 165/80R13.

Can anyone confirm how many teeth the brown speedo drive gear had? Or the metal one that seems pretty rare?
 

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Pedal Smasher
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Discussion Starter #12
I'm almost done with a complete overhaul of my gear calculator. One of the many changes, will be the addition of the metric unit system to the whole thing. I'm also adding reverse... and I've found something pretty funny. Apparently, a stock GT with a TH180 automatic trans and a 3.89 in the diff, can hit 55 MPH at 6,000 RPM in reverse. It beats a stock 1.9L GT with a 4 speed, which will hit 36 MPH. I wonder if anyone has been crazy enough to try it? :ugh:
 

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Pedal Smasher
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Discussion Starter #13
Finally finished with the new gear calc excel file... with a bunch of supporting info on additional sheets. Some things worth noting...

- Gear calculator now has Metric units, aside from Imperial.
- Gear calculator now has the following transmissions built-in: 1.1L 4 Spd, 1.9L 4 Spd, TH180, ZF S 5-18/3, and Getrag 240. Model 94 has the ZF according to FIA homologation papers and the GT/J has the TH180, also due to FIA homologation papers.
- A possible tooth count for the hard to find brown speedo drive gear is included.
- A list of what seems to be factory GT speedo W numbers is included, along with the best speedo drive color matched to various diff gear ratios that seems to show up under the GT according to the source for the info.
- A list of differential gear ratios and what transmission they were found with, on GT's, Manta's, and Kadett's. There were a lot of iterations of the Kadett...

There are two possible stock tire sizes for the GT, 165/80R13 and 185/70R13, depending on who you ask. A common size for 7"x13" rims is 205/60R13.

Have fun playing with it... there is a crap ton of info and capability in this excel sheet.

https://drive.google.com/file/d/1wRxWRZmsX4ksRdh3b0CmnSjfVUh8ZeTZ/view?usp=sharing
 

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It appears to be very useful, thanks for posting.

However, I don't seem to be able to edit anything.

Maybe there is some permission you have to enable to share it?
 

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Pedal Smasher
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Discussion Starter #15 (Edited)
You need to download it. I can't upload an excel workbook to the forum. I actually forgot to protect the document, which would have required a password to edit any of the cells with formulas. So if you click on a any of the white cells where formulas are, push escape to not mess it up.

I don't know for sure if '69 and '70 GT's had a 5 tooth gearbox screw wheel and used a brown, 17 tooth speedo drive gear. This combo however would explain a W897 MPH speedo.
 

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You need to download it. I can't upload an excel workbook to the forum. I actually forgot to protect the document, which would have required a password to edit any of the cells with formulas. So if you click on a any of the white cells where formulas are, push escape to not mess it up.

I don't know for sure if '69 and '70 GT's had a 5 tooth gearbox screw wheel and used a brown, 17 tooth speedo drive gear. This combo however would explain a W897 MPH speedo.
There we go, downloading it worked. Thanks.
 

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Pardon the ignorance...

Suppose you have a mismatched tranny and speedo. Is there an adjustment you can make, or, is the adjustment "Remember that 60 means 52"?
 

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Pardon the ignorance...

Suppose you have a mismatched tranny and speedo. Is there an adjustment you can make, or, is the adjustment "Remember that 60 means 52"?
You can swap speedo gears, but only if the difference isn't too big. Larger or smaller diameter tires can sometimes help too, with or without swapping the speedo gear. If not, it's time to find a speedo with a more suitable W number.
 

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Pedal Smasher
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Discussion Starter #20
I've used your calculator Dieter. I decided to incorporate the speedo W number caculations into my gear calculator. The other thread never really covered what speedometers were made for the GT and that is something I'd like to try and document.

Related to gauges, has anyone else seen a tachometer for a GT with a built in clock?

Off topic, Dieter have you had the chance to work on the defroster vent scanning?
 

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