Speedo drive question

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tr6racer21
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Speedo drive question

Postby tr6racer21 » 06 Mar 2016 13:54

I have recently completed a VG30 conversion in my 4dr 510. Very happy with everything, but would like to get the speedometer working more accurately. The transmission is out of a z31, stock rear 3.9, and the tires are 195/50 15s. The speedp is indicating the vehicle speed at about 10-12 mph over the actual speed measured by a GPS phone app. I haven't pulled the current drive gear to count the teeth yet, but believe that can be determined with some surfing. After I put some more miles on the build I was thinking about swapping a 3.7 Subi LSD I have on the shelf. That's a change of about 5%...not nearly enough to make up for the current 20% difference. So I need some recommendations for drive gears that others have used when doing this swap to get the speedo closer to accurate. Thanks in advance.

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SteveEdmonton
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Re: Speedo drive question

Postby SteveEdmonton » 06 Mar 2016 14:40

There are several threads that talk about replacing these pinions. Obviously ,though, without knowing what gear you currently have, it's impossible to know how many teeth you ought to have in order to fix the problem.

Once you figure out your starting-place though it shouldn't be hard to figure out what you need. To make your speedo read lower by 20%, you'll need to get a gear with 20% more teeth than the one you have.

Here's a list I cribbed from somewhere else on this site (can't remember where)-- very helpful in knowing what to order from your local Nissan dealer once you know how many teeth you need. G'luck!

16 Tooth Cog (Yellow) - 3.36 diff ratio PN 32703-78100
17 Tooth Cog (Black) - 3.54 diff ratio PN 32703-78101
18 Tooth Cog (Blue) - 3.70 diff ratio PN 32703-78102
19 Tooth Cog (White) - 3.90 diff ratio PN 32703-78103
20 Tooth Cog (Red) - 4.11 diff ratio PN 32703-78104
21 Tooth Cog (Purple) - 4.375 diff ratio PN 32703-86401
22 Tooth Cog - 4.625 diff ratio PN 32703-86402
23 Tooth Cog - 4.875 diff ratio PN 32703-86403
'71 4-door
'74 MGB-GT
'04 Miata

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bertvorgon
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Re: Speedo drive question

Postby bertvorgon » 06 Mar 2016 14:46

Don't forget that at a point...you have to change the pinion gear holder, as the gear count gets bigger, then it has to offset some.

I can check in my Nissan Comp catalogue if you like, with the part number.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

tr6racer21
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Re: Speedo drive question

Postby tr6racer21 » 06 Mar 2016 15:01

thanks guys, I'll pull it out tomorrow and report back with a tooth count. Also am thinking about taking it out for a drive this evening and getting several reference points. Recording speed differences at say 20,40 60 and maybe 80 mph and taking an average % difference.

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Re: Speedo drive question

Postby datzenmike » 06 Mar 2016 17:50

The 22 tooth wheel is Orange.

That chart is approximate and only applies to perhaps one vehicle. An 80 long bed 720 has a 4.11 diff but uses a 19 tooth speedo gear because of the larger tires. Other models do the same. Each tooth change varies between 6.25% (16 to 17 tooth) and 4.5% from 22 to 23) Very roughly each tooth is near a 5% change in speed. It's not practical to go 100 MPH to know how much you are out, so go 50 and double the difference. Five out at 50 is 10% at one hundred so two tooth change is needed.

The 10-12 over the limit is meaningless unless the actual speed limit is noted It could be 10 out at 40 or 60MPH. GPS reading and the speedometer reading is the only accurate way to know what you need. Speedometers are not 10 out at all speeds. Ten out at 50 is only 5 out at 25 and so on. Doesn't matter what the number of teeth on your pinion is or is supposed to be, doesn't even matter what gear ratio you have. If reading fast by 10 % add two teeth to what ever your pinion tooth count is to slow the speedometer down 10%.


Keith, exactly. As the number of teeth increase so does the diameter of the wheel. The sleeve that holds the pinion gear must offset it farther away from the mainshaft or it will grind it away.. I'm running a 20 tooth in my 710 with 4.11. Would love a 22 tooth and matching sleeve.
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toylet
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Re: Speedo drive question

Postby toylet » 06 Mar 2016 20:48

might be cheaper and easier to just have your speedo calibrated 11 mph less

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bertvorgon
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Re: Speedo drive question

Postby bertvorgon » 07 Mar 2016 07:00

16 - 19 teeth require a different "sleeve assembly" than the 20 - 23 tooth pinion.

I'm not familiar of course with your Z31 transmission, so if Mike cam pipe in, I can look at the appropriate sleeve in the catalogue. They list as F4W71A, that sort of thing.

As Mike said also, in all my years and transmissions I have tried, I have never been able to get a speedo to read correctly through the range. It drifts more at the top end. remember our speedometers were designed back in the 60's, where speed accuracy and milage, were not an issue. Even with what would be the correct or close gear in my trans, I end up 10 MPH FAST at the top end.....80 MPH+. When the speedo is BURIED at the bottom of the gauge,,,last thing I'm looking at is how fast I'm going....... :lol:

I use my Garmin GPS as my speedometer. I did go with my Totoyta AE86 and get the speedo calibrated, as I changed the transmission in that thing. Again, they calibrate at 60 MPH, but even it drifted at the top end of the range. In my case the put a small box with brass gears in it, that goes between the input of the speedo drive on the transmission, then the cable went into that. Clearance could be and issue, etc.. I used a company that used to do all the taxi's in Vancouver, but, with today's digital dash cars, I wonder if anyone is left that does that.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

tr6racer21
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Re: Speedo drive question

Postby tr6racer21 » 07 Mar 2016 07:32

Thanks Keith,

I'll follow up with my adjustments and results


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