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Testing: Tire / Speedo / Differential Calculator - try it out! Drive/Driven gears

2M views 58 replies 21 participants last post by  Funkycheeze 
#1 · (Edited)
Well, in a fit of boredom I rebuilt a spreadsheet I made a couple years ago and inadvertently deleted. Using data gleaned from the forum I have compiled a one-stop calculator for changing tire sizes, speedometer gears, and differentials.

The excel sheet will tell you how far off your speedo is based on tire, differential, and speedometer gear combination. It will calculate and recommend the best speedo gear combination to use for your given tire/wheel/differential setup. While we aren't all changing tires and differentials all the time, it should be a nice thing to have around.

Instructions are included in the spreadsheet, although it's pretty straightforward.

Part numbers are good to go!

I am looking for you guys to test it out and see if there are any questions or suggestions you guys have for improving the sheet. Have fun!

Nov 7, 2016.
Literally 10 years later, this thread has over 900,000 views. I should probably make something I'm a little more proud of.

Now the file will live forever:
dead link

April 9, 2022.
Apparently forever isn't forever when it comes to google. New link!
Let's see if that works.
 
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#8 ·
Yep, very cool. Super handy for anyone whos gone to a different tire diameter and rear end. It works too, it produces the right result for my 225/50-15 and 4.1 rear end combo, which is the stock 4.3 speedo gears.

Dean forgot to mention that all of the part numbers are listed and up to date now too (Dean, edit your first post to fix that).
 
#10 ·
charles q said:
Could somebody post the results for my 245/50-16 and 3.73 diff? Don't have excel
The best combination for you is 11 tooth and 32 tooth. This puts you at 99.17% accuracy. Rigt now you are at ~89% accurate, meaning at 60mph your speed reads 53mph.

11 33481-22020
32 33403-19255

Assuming you've got a 4.30 gear-set in their ('84 5-speeds came with 4.30's, right?) you'd have to change both gears out. From what I know, the 11-tooth gear requres pulling the tailshaft, but the 32-tooth does not.
 
#12 ·
charles q said:
Thanks for the help. No not 4.30. Changed out to 3.73 for the 7MGTE
Gah, what I wrote was confusing. I meant to say that your car came with a 4.30 stock, meaning that you'd have a 10/35 combination. This kind of sucks because you have to change both gears as opposed to just the easy one.
 
#14 ·
Donn29 said:
What is the other gear, there is one at the end of the cable, are you talking about the one in the Trans?
There are two gears. One is located on the output shaft of the transmission, requiring you to remove the tailshaft to change it. This is the 10 or 11 tooth gear. The other (30-35 tooth) is just inside the tailshaft behind the 90 degree fitting that the speedo-cable threads onto. I believe this one is replacable be removing the 90 degree adapter and accessing it there. This is what I have been led to believe. Until I have messed around in there more, I won't say this information is totally correct. I plan on correcting my speedometer in the coming weeks, as I only have to replace the 30-35 tooth gear in mine to correct my ratio.
 
#32 ·
Yes.

SOOOOOOO, has anyone else actually used this thing to try and correct their speedo yet? I've just gone through all of this, used the calculator to find the right ratio for my shorter then stock tires and my new 3.72 diff (car was originally 4.3), and I must say, we have a bit of work left to do here.

Ok, first off, all of these speedo side shaft gears are stamped with 10 or 11 as well as the number of teeth on the gear (30-35). Those numbers listed in the excell sheet are NOT foriegn part numbers, we just don't understand how this all works yet. My first conclusion was that you could not match a gear stamped with 10-3X with an 11 tooth output shaft gear and vice-a-versa, and that most of the side shaft gears were available in 10 and 11 tooth versions and thats what those secondary part numbers are for. Now that could still be true, but I'm afraid the confusion doesn't end there. Turns out theres 2 different diameters of 10 tooth gears in use just in the w58s in our mk2s! Maybe in 11 tooth too. Theres side shaft gears in 10-3X and 11-3X gears in two different diameters, and you can't mix and match them! I'm probably confusing you all so here's the numbers from my findings so far and maybe some of you can help me figure this out (I still have some more research to do on my own). Luckily I have a plethora of trannies sitting around in my shop, here's what I've found so far...

w58 from what I thought was an 83 5spd mk2 (might not be), should be a 4.1 rear end: Has a 28.8mm OD 10-31 side shaft gear.
w58 from an 84 Celica GTS with a 3.42 rear end: 11-31 with a 28.8mm OD
w58 from an 84 Supra and another from an 85, both 4.3 cars: 10-35 with a 31.4mm OD
r154 from an 88 Supra Turbo with a 3.9 rear end: 10-32 with a 28.8mm OD

In addition there appears to be 2 different styles of the actual side shaft in use though they seem to be directly interchangable, even the r154 could take a mk2 w58 side shaft and 28.8mm gear. Of course the car I'm trying to adjust the speedo on has the larger 31.4mm gears and none of my 28.8 actually contact the 10 tooth gear on my output shaft. Before when I was under the assumption that you had to have a side gear marked 10 if you had a 10 tooth output shaft gear, I ordered the 10-32 gear with the pn 33403-29125 since the 33403-19255 32 gear was more common in the EPC so I figured it was the 11-32 one. Well I ended up with a 28.8 OD 10-32 gear, exactly the same one as was in my r154 and then I discovered the diameter difference and now I can't use the damn thing (and probably can't return or exchange it, special order from CA through the dealer). So now I'm thinking\hoping the 19255 number is for a 31.4 diameter 10-32 gear. I did stick my 11-31 gear into a tranny with a 10 tooth gear and it seemed to rotate just fine.

Anyways, I can't find any rhyme or reason to why theres 2 different diameters in use here as I have early and later styles in the small diameters in both 10 and 11 teeth variants. If anyone has any insights or has another size of gear in a tranny kicking around that they can measure, please pipe up! I will get back to you all after some more digging in the epc and hopefully exchanging this speedo gear I bought for the other number.
 
#18 ·
Very Nice.

Works great if you are swapping to a larger gear (numerically lower).

What if I go from a 3.73 to a 4.30 on a n/a car?

I will take a look at the formulas on the spreedsheet too.

Thanks!
 
#20 ·
I'm not very good at reading your spread sheet. I have a 5 speed out of a 82 supra which came with a 3.73. I put the transmission in my 84 after auto swap but the rear end is still the 4.10. Could you tell me what I would need to do to get the speed right. Or would I need to get either a 3.73 rear end or a 4.10 transmission?
 
#23 ·
Just for clarification if I wanted to go with the 4.10 I would have to change both gears in my trans that came with a 3.73 rear end or could I just change the gears out on the side of the transmission?
 
#24 ·
The plastic outer gear will have two numbers printed on it. ...36 and 10... or something to that effect. So long as the small number is the same between the two outer gears, you're set. (The OD's will be the same as well.) Otherwise, welcome to splitting a tranny. (hold pressure on the output shaft while you pop ONLY the rear portion off and you will halve your chances of leaks once you put it together again.

--billyM
 
#28 ·
For those who don't have Microsoft Office (or don't want it), look into OpenOffice.org. It's a free open source office suite available for Microsoft Windows and most Unix-like operating systems. It will read and write to Microsoft's proprietary Office formats and the ISO approved open document format.

Sorry if that sounds like an infomercial/shameless plug. Just figured I'd try and help the people who aren't able to check out the spreadsheet you made.

Thanks for that, by the way!
 
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