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I own an 83 280zxt 5spd with stock gears and I'm trying to put a wider tire on my car. From what I've learned through the archives what I'm looking for is a 225 55 15. Sounds great except I don't want to pay $194 per tire on my car...I'm just not that rich. Anyways, I've got an R200 diff with 3.9 gears siting in my garage that I'd been planning on putting in my car. My question is this, is there any way to set up a configuration between a speedo cog, 3.9 diff and a certain size tire to keep my speedo right...without paying a fortune for tires? I don't care if my tire is lets say a 60 or 65 or 70, just as long as I can get atleast a 225 width and still have my speedometer fairly accurate. Anyone been in this situation before?
 

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Links to solve your kinks

Hey Spork,

I can't give you direct data, but look up these folks/links below....you may find the RPM calculator and Kurt especially useful. Also try http://geocities.com/zgarage2001/engine.html This guy has data on stock cogs, motorsports has custom cogs and is one of the many vendors on the last link.

Kurt is cool, races and knows Panasport rims and tire combos. Will chat at length and cut a deal on rims, caps and lugs.
http://www.kspeed.com

Great prices on performance tires http://www.tirerack.com/index.jsp

Transmission rear end and tire size speed vs. RPM calculator (you can enter in custom gear ratios instead of using the pull-down menus) http://www.geocities.com/z_design_studio/transmission.html

Z-Barn, Roger is knowledgeable and helpful. Quick shipment of used parts http://www.zbarn.com/index.html

Lots of links for Datsun/Nissan http://www.performancemarket.com/nissan.htm
 

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Maybe solution - long!

Spork - Easiest way is to keep your tire overall diameter close to stock and use the speedo cog for the 3.9, which is the WHITE one. I think a 225/55/15 is the closest overall diameter to the original but you should check up on that. www.tirerack.com has some good info on this.

BUT WAIT - you want to mix/match tires/speedo cog so you don't have to pay $$$ for 225/55/15? You might be able to do this but you may find out that you won't get any closer than just using 225/60s with the correct speedo cog. Here's my thoughts on how to calculate it:

Assume: 1 driveshaft rotation, 25 inch *stock* tire diameter, 19 teeth on the speedo cog (I think the 3.9 cog has 19). Double-check those assumptions for the real calculations.

for 1 d.s. rotation, you go 3.9*25 inches, or 97.5 inches. The ratio b/w 97.5 inches and 19 teeth is 97.5/19 = 5.13. If you keep this ratio of cog teeth to distance travelled, your speedo shouldn't change (my reasoning, at least)

SO - let's say you look at a tire with 26 inch diameter. Calculate backwards to get (3.9*26)/5.13 = 19.77, so a cog with 20 teeth would be closer.

Does this make sense or do I need more gin?
 
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