Speedo gears
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,469
Likes: 6
From: Winchester Ontario
Speedo gears
I have an 89 with a zf6 that was initially equipped with a 3.33 rear gear.
I checked online and it should be a 15 tooth drive gear and a 40 tooth driven gear.
I have since upgraded tires from 275/40/17 to 285/35/19 which now shows the speedo going 120 kmh (75 mph) when I'm GPS'd at 125 kmh (78 mph).
I am also changing rear gear to a 3.73 from the initial 3.33.
Does anyone know of a calculator that allows tire change and gear change to find the correct speedo gearing?
TL;DR I have 3.73 gear with 285/35/19 tires. What speedo gear do I need now?
I checked online and it should be a 15 tooth drive gear and a 40 tooth driven gear.
I have since upgraded tires from 275/40/17 to 285/35/19 which now shows the speedo going 120 kmh (75 mph) when I'm GPS'd at 125 kmh (78 mph).
I am also changing rear gear to a 3.73 from the initial 3.33.
Does anyone know of a calculator that allows tire change and gear change to find the correct speedo gearing?
TL;DR I have 3.73 gear with 285/35/19 tires. What speedo gear do I need now?
Team Owner
Joined: Sep 2003
Posts: 25,394
Likes: 2,741
If you actually want to do it correctly you need to do it using "loaded radius/diameter/circumference" and not advertised specifications for tire size. Is it important? Maybe! Your 42 should do well I'd think
Last edited by WVZR-1; Jul 29, 2017 at 10:59 AM.
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,469
Likes: 6
From: Winchester Ontario
What would be the cost to simply swap out a speedo gear later? I'm actually planning on going 305/30/19 and the calculator shows a 43 tooth (purple or dark blue) gear. I wonder if I should just get the 43 now or if it's not too much, just swap the gears when I get the tires.
Team Owner
Joined: Sep 2003
Posts: 25,394
Likes: 2,741
What would be the cost to simply swap out a speedo gear later? I'm actually planning on going 305/30/19 and the calculator shows a 43 tooth (purple or dark blue) gear. I wonder if I should just get the 43 now or if it's not too much, just swap the gears when I get the tires.
The swap is easy so long as you're dealing with a numerical higher number than 40. If you by chance wanted/needed 39 or less tooth count then a VSS is needed.
3.33 for '89 I thought only done in production later than maybe February? Yours is a February or later build?
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,469
Likes: 6
From: Winchester Ontario
Mine came with 3.33 so it must be post Feb.
Cheapest place for the speedo gears I've found was amazon. Cheaper than ebay.
I don't see the ZF6 under listed applications though. Would it matter?
Cheapest place for the speedo gears I've found was amazon. Cheaper than ebay.
I don't see the ZF6 under listed applications though. Would it matter?
Team Owner
Joined: Sep 2003
Posts: 25,394
Likes: 2,741
Go ahead and just delete your post #8 & 9 I'd think.
If I were in Canada I might try to use that as an advantage. Take 25513049 (42) & 25513050 (43) to a GM dealer there. They've been NLA for a very long time but I'd think GM Canada has dealer network where inventory is shared and a dealer could maybe locate them for you. I don't know that they do but I'd think so.
If I were in Canada I might try to use that as an advantage. Take 25513049 (42) & 25513050 (43) to a GM dealer there. They've been NLA for a very long time but I'd think GM Canada has dealer network where inventory is shared and a dealer could maybe locate them for you. I don't know that they do but I'd think so.
Last edited by WVZR-1; Jul 29, 2017 at 12:31 PM.
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