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Discussion Starter #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:
https://drive.google.com/file/d/0B7IsroguAts_MS1oN3ZvSHAtb3M/view?usp=sharing
 

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Discussion Starter #6
charles q said:
For those of us without Excel (just the viewer) what do we do?
The google spreadsheets are still pretty primitive, so I'd say you're just about out of luck. I'll take a look into it tomorrow and see what can be done.
 

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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).
 

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Discussion Starter #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.
 

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Discussion Starter #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.
 

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Discussion Starter #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.
 

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Discussion Starter #16
swank said:
Does the R154 use the same part numbers?
That's a very good question. If someone can provide an answer... and the requisite part numbers if necessary, i'll include them (and the R154 final drive calculation) in the sheet.
 

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I'll look that up in my epc some time soon, but the comments for the 30-35 gears I looked up in the epc didn't list the MA70s so I'm guessing the answer is no.
 

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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!
 

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Discussion Starter #19
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!
I've noticed it gets finicky when you get speedometer ratios on the high or low side of the available range. I'll take a look into it and see what I can do.
 

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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?
 
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