I could just tell you what speedometer gears for what axle ratio, but this only works if everybody runs the same size rear tires. So, here's the only correct way to know exactly what speedometer gears you need. First, you need to gather the following information: (1) your rear tire size, such as 245/50R16. (2) your transmission, such as 700R-4, or TH350, or T5 or whatever. This is to determine what tooth counts of driven gears are available to work this formula. For most of you using a GM trans that isn't an overdrive automatic, you ca net drive gears with tooth counts of 6,7,8,9 and 10. For the 200-4R there's 10, 11, 12 and 13. For the 700R-4 there's 15, 17, 18 and 19. (3) your exact axle ratio, such as 2.7333333:1, 2.769230769:1, 2.92856142857:1, 3.07142857:1, 3.0769230769:1, 3.230769230769:1, 3.27272727:1, 3.4166667:1, 3.45454545:1, 3.70000:1, 3.72727:1, 3.90909:1, or 4.100000:1. So then, you need to know your rear tire size's diameter when new and not carrying the weight of the car. This is easiest found at TireRack.com, or it can be calculated. For 245/50R16s, most are 25.7" diameter. Now, the formula followed by an example: 20168.0672268(the constant that makes this work) divided by tire diameter, then that times the number of teeth on the speedometer drive gear, then that times the axle ratio, then divide that by 1000. Then repeat with every other drive gear for your trans, until you find the closest match. Now, the example: get your calculator and follow along: 20168.067 divided by 25.7(for 245/50R16s) is 784.74968 times 15(for a 700R-4) is 11771.245 times 3.7272727(for 3.73:1 axle) is 43874.64 divided by 1000 is 43.87464. So, you need a 44 tooth driven gear. This does exist, so you're good. But what if your closest result for your combo is something like 38.5? Then, to avoid a speeding ticket, round it down to 38.

a bump to keep this thread with it's companion thread.

This thing is perty cool. I always thought I was going faster than what my spedo said so tonight me and my dad clocked it and my car said i was doin 55 but he said I was doing around 67. Dose anybody know the rear gear for a stock 2.8 84 camaro, if not how do I find this out. I need to get this thing straightened out before I get a ticket. thanks

According to the tech info on this site it's 3.42 if it's a manual. and
3.23 if it's an auto.

There seems to be some demand for the more popular combos to be pre-figured, so I'll post the best. It seems the 2 most popular tire sizes are 245/50R16, and 275/40R17, which has the convenience to me of both being 25.7" diameter. That, with a 700R-4, works like this: 3.08:1 use 17:41. 3.23:1 use 15:38 or 17:43. 3.42:1 use 15:40. 3.73:1 use 15:44. This also works for 225/60R15 and 245/45R17 and 245/40R18 and 315/35R17.
I prefer a 275/40R18 because it better fills the fender openings, and with a 3.23:1 use 18:44. 3.42:1 needs 17:44. 3.08:1 takes 18:42. This also applies for 245/60R15 and 295/50R15 and 225/60R16 and 245/45R18.

Atilla, just wanted to give a quick thanks for this useful post. After doing the math and figuring out what I needed for my 4.10, speedo is now dead on.

For anyone interested, with a 4.10 ratio, I needed a 15 tooth drive and a 45 tooth driven gear.

While that is the closest possible combo for any tire shorter than 27.55" dia., it will be optimistic, so if you have a 15:45 with a 4.10:1 and tires shorter than 27.55", then you'll be going a wee bit slower than what your speedometer says. I'm glad to hear that someone took the time to try, and found it useful. If he or she can , and I can, then you can, too!

-

That seems like an awful lot of work...

I've had to do this so often that I just made a calculator in Excel, with handy part number and color references. All you do is enter your tire size, axle ratio, and the number of teeth on the drive gear. For a 700R4, there are only four options for drive gear, and they are listed. I would generally assume you already have one, so start with it. If not, keep trying until one works.

The file can be had at:
http://ul.to/vaw6ff

If anyone can host this more permanently, that would be awesome.

Dave

Okay, I've had enough whining from the lazy and the weak-minded.
So I decided to just start on a cheat sheet. Not all combinations
can be exactly calibrated, but here's the best possible calibration
for each combination given.
This first batch is for anyone using a 700R-4 with any of these tire
sizes on the rear axle of their car:
205/60R15
225/55R16
245/50R16
245/45R17
275/40R17
315/35R17
245/40R18
If your tire size isn't here, don't use these results.
2.73:1 use 41 tooth driven with 19 tooth drive
2.77:1 39 18
2.86:1 38 17
2.93:1 39 17
3.00:1 40 17
3.07:1 41 17
3.07:1 36 15
3.08:1 41 17
3.08:1 36 15
3.23:1 38 15
3.23:1 43 17
3.25:1 38 15
3.27:1 44 17
3.31:1 44 17
3.31:1 39 15
3.42:1 40 15
3.45:1 41 15
3.50:1 41 15
3.54:1 42 15
3.55:1 42 15
3.70:1 44 15
3.73:1 44 15
more no cal.

Cheat sheet 2. Again, it's for 700R-4 cars. But with a new list of tire sizes,
and a new set of calibrations. If your rear tires are any of these sizes;
255/60R15
255/55R16
255/50R17
255/45R18
285/40R18
255/40R19
255/35R20
NO other sizes for these calibrations:
2.73:1 no cal
2.77:1 35 17
2.86:1 36 17
2.93:1 37 17
3.00:1 38 17
3.07:1 39 17
3.08:1 39 17
3.23:1 41 17
3.27:1 44 18
3.31:1 37 15
3.42:1 38 15
3.45:1 no cal
3.50:1 39 15
3.55:1 45 17
more no cal.

I have a 1985 camaro that no matter what driven gear you put in, it strips within a few miles. Any ideas on what may do this. Seems like even the wrong gear should still work but would be off on mph. Thanks for any help with this.

have you tried changing the driven gear carrier? If so, which versions have you tried? If not, which one do you have?

I have tried both types of gear housings. had a 38T driven and a 15T Drive. 43T/15T.

Tire diam is 26.10 with a 3.23 gear. have seen where a 18/45 is compatable with a 15/37 but if that is true I do not know why it still strips the gears reguardless. I also have replace the speedo cable with a brand new cable nad housing. Also purchased another speedo cluster to eliminate that chance.

this is the site that I found the compatability for the gears.

http://www.gtsparkplugs.com/SpeedoGearCalc.html

the 18 / 45 is correct, and they are compatible. I'm currently doing it. I ordered a new 45 tooth gear, and it came with the correct carrier in plastic. I don't trust the old aluminum carriers unless you're using them with a driven gear having a tooth count right in the exact middle of the range marked on the carrier. I'm using the earlier style, mine is an '84 after all.
What I suggest next is replacing your tailshaft housing. All indications are that yours has some machining defect, or has become bent, or something.

is the 15 /37 compatible with that or does it have to be the 18/45

sorry just reread that and saw it was.

any idea what the original was? I would think it was the 18/45. If so I would change over to that when replacing the housing.

I just bought a 90 camaro with the 305 tbi (I know its tbi so im geussing 305) and the t5 trans. The trans is a borg-warner with a merchandising option(?) and 3.08 gears according to the sticker in the center console. So I tryed to check the spedo using a another car that is allways right on with the signs that detect your speed. According to him I was doing 50 when he was doing 55 and I was doing 12 when he was doing 15. I want to know what my actuall speed is but im not sure how. When it says im going 55 I am running about 2350 rpms. Can anyone help? The guy who I bought it from did alot of stuff to the car himself but I dont know what?

So I finnaly got the nerve to crawl under the front of the car completly and look around and I noticed something that made me very happy (not). When you look at the front of the trans you can see the bottom of the flywheel. I am asmuing now that either the engine or the tranny is not stock and the guy forgot to put the plate back on the bottom.