I just did a 2.77 to 3.27 rear end swap and now I need speedo gears as my speedo is nowhere near accurate. Does anyone still carry the correct ones? how much different would it be if I had to resort to 3.23 for example

Will

 

About 1%, which is a little over ½ mph at 60.... less than the error that will occur because of the difference between one mfr's tires of a given size, or from changes in air pressure, or even from the change in tire diameter from wear.

Don't outsmart yourslef about this. Speedo gears are only available in whole numbers of teeth. On a T-5 for example, you have a 7 or 8 tooth drive gear (big choice there) or 18, 19, 20, 21, or 22 tooth driven gear. That's all there are to choose from, no matter what brand of rear they go with. So there willl almost always be some error, in addition to the uncertainy in things like tire properties.

The only people I know of that "still carry" them at all, or ever did for that matter, is GM. A few others such as SLP buy them in bulk and resell them, and I've been seeing them on ebay, the same way. But I think you'll find they came from GM.

 

Never found an 8 tooth drive gear; just 9. Is there an 8?

 

So I hear. But those bigger numbers are for grocery cart gears, and I don't have any cars that use those, and I sure as hell have never bought one, so I could be wrong about those. Mine uses a 7 tooth (3.73 gears) and every core I've ever bought has come from a similar application so 7s are all I have even just laying around. The driven gears are from about 18 to 22 teeth.

The 700s use drive gears in the neighborhood of 15-19 teeth and driven from around 38 to 46. So the steps are a little finer. However not all driven numbers are available to mate with all drive numbers. So it's a little more iffy to get just exactly the set you want with those.

Do a search on my userID for the phrase "SMUC", find one of the ones where I tell how a speedometer/odometer works, what they are actually measuring, and how they are calibrated. Everything you always wanted to know about speedos is in there.

 

Understood. My first T5 was an 84 L69 TA box; had the 7. My second T5 was an 88 TBI unit; had the 9 (until I changed it out) and the one a friend just acquired w/ the VSS in it has a 9/22 combo. I'll grab the service # off it when I change out the gears on it (it's going into a 3.73 rear car.)

I've done that search.

IIRC, it doesn't always match the proven GM combinations of speedo gears, so I don't go by it. Do the 3.73 math for stock 84 Z28 tyres and a 700-R4 or T5 or 200-4R, see what it comes up with...

 

The reason it doesn't always match the GM combos, is because GM used the actual measured tire radius of the OE tires with the car sitting on them at rated tire pressure, not some "calculated" radius based on the section width, aspect ratio, and wheel diameter. Mostly, the GM combos turn out to be one tooth less on the driven gear than one might get by "calculating", because the actual radius is always smaller than the "calculated" radius, from the flattened contact patch. But if you do what you're supposed to do, and measure your car with a measuring tape from the center of the wheel to the ground, it will give you the right gears every time for your car, with no possibility of error; regardless of what the "official" combos are. Because, by measuring your car, you incorporate your tires into the measurement, and is therefore more accurate for your car than GM's measurement based on some other tires.

Think about it..... speedo shops calibrate speedometers. You take your speedo out, send it off, and they calibrate it, and send it back. How do they do that without your car? What's their standard? It's real simple..... 1000 revolutions equals one mile. All you have to do, is find the right gears that rotate your cable (or VSS) 1000 times per mile, and you're golden.