DSM

iTrader: (1)

Background: 84 Z28 with a T-5 and 3.73 gears. Stock speedo gears are a 7 tooth purple drive gear and a 21 tooth red driven gear PN 3987921.

I had a leak from the seal that goes around the speedo gear, so I installed a new speedo gear sleeve PN 345215. It was dimensionally identical to the original although it looked a little bit different.

Later I installed 3.42 gears and I wanted to recalibrate the speedo. I needed a 19 tooth white driven gear to match with my 7 tooth drive gear. According to numerous posts on TGO, the part number for that gear was 14090592. However, when I received this gear, it was different from my original gear:
1) the new gear has a metal shaft, the old gear was all plastic
2) the new gear is 1/4" longer than the old gear
3) the "hub" on the back side of the new gear (the part that goes into the trans first) is 1/2" in diameter vs 1/4" on the old gear
4) the diameter of the new gear where it meets the seal in the sleeve is 0.312" vs 0.308" for the old gear

Despite all this, I installed the gear since it was supposedly the right one. The result was a leak from the speedo sleeve and my speedometer worked very erratically.

I removed the metal shaft gear and reinstalled my original gear. The speedo went back to working correctly (albeit not calibrated for the 3.42s), but the new gear apparently damaged the seal in the sleeve because there was still a small leak.

I then ordered a new sleeve and an all plastic 19 tooth white speedo gear PN 3987919. The speedo is now working correctly, calibrated for 3.42s and no leaks.

So the question is: what is the deal with the metal shaft T-5 speedo gears? Are they for later model T-5s with VSS senders? Or is there a different sleeve than the PN 345215 that works with the metal shaft gears?

For future reference, here are the PNs for the all plastic T-5 speedo gears:
Teeth PN Color
18 3987918 Brown
19 3987919 White
20 3987920 Blue
21 3987921 Red
22 3987922 Gray

The gears are about $5.50 each from the dealer, or half that from GM Parts Direct although they come in bags of 2. Beware of clowns on ebay selling these things for $15 each...no reason to pay that much for them.

Here is a picture of the metal shaft vs. all plastic gear along with the original speedo gear sleeve.

ken0hki

the metal shaft gear is not for a T5. it is for a TH350 transmission. a lot of people get confused, because the 350 gear will work in a T5, however you have to change the housing to the longer 350 one. it is a pretty common swap for T5s.

1986IROCZ355

So, considering you had the 7 tooth drive gear, and 21 tooth driven gear for 3.73 ratio. Does that mean, me having a 3.23 gear ratio factory, i should have the 7 tooth drive gear as well, and if im swapping to 3.73 ratio that i should change it to the 21 tooth?

kenbob

I'll be running 3.70's and 245/50/16 tires the calculator says 20.1 are you running 15 inch rims ? i think if i go with a 21 it will read like 3.90 gears why is my calculations so off

sofakingdom

iTrader: (1)

Speedo gear "calculations" are eeeeeeezy.

First thing you need to do is to ACTUALLY MEASURE your tire size. NOT attempt to "calculate" the diameter from the nominal "trade size".

With the car sitting on level ground and the tires that you actually own at the correct normal pressure that you actually run in them, pull a tape measure from the CENTER of a rear wheel, to the ground. This is the RADIUS of your tire, as it actually lives when on the car. It won't be "half the diameter", because the "diameter" fails to account for the part of the tire that's flattened out where it sits on the ground. YUUUUJJJJJJJJE source of error.

Take that number, multiply by 2 × pi. Use the Scientific function of a calculator such as the Windows one, that has pi out to at least 4 decimal places. If you don't have that, use 3.141593 as its value. The result is the # of inches the tire rolls per revolution. Divide by 12; this is how many feet per rev. Divide 5280 (the # of feet in a mile) by this #; this is how many times the tire rolls per mile. Multiply this by the teeth on the ring gear, and divide by the teeth on the pinion. (as opposed to using, say, 3.73 or whatever the colloquial value for the ratio is... 41 & 11 for 3.73 for example) This gives you how many times the drive shaft turns per mile. Divide this by 1000 (NOT 1001, as is often reported); this gives you the speedo gear ratio you need. Multiply this ratio by the # of drive gear teeth you have, or that you can actually buy, to get the # of driven teeth you will need.

Note that it starts out with MEASURING your tire radius. This is where most of the inaccuracy in "speedo gear calculators" comes from. The "calculators" might be accurate, but if you put in numbers that are GARBAGE, the result you get is GARBAGE. Just like you can go to Office Max and put 2+2 into any calculator on the shelf and get 4; but if the things you're REALLY trying to add up are REALLY 2 and 3, the answer you get when you punch in 2+2 will be "correct" but still GARBAGE. Garbage in, garbage out.

So as an example let's say you MEASURE your wheels and find that the center is 12¾" off the ground. That's 12.75". (I'd like to be more precise than that, but w/e, it's just an example... I think you can easily get as close as, to the nearest 1/8"; which of course might still be 12¾, but, get it as precise as you can). So:

• 12.75 × 2 × pi = 80.1106 (inches per wheel rev)
• 80.1106 ÷ 12 = 6.6759 (feet per wheel rev)
• 5280 ÷ 6.6759 = 790.9 (wheel revs per mile)
• 790.9 × 37 ÷ 10 = 2926.4 (drive shaft revs per mile... you could get away with using the ratio in this particular one since its divisor is 10, which makes the ratio a terminating decimal number, not a repeating one)
• 2926.4 ÷ 1000 = 2.9264 (speedo gear ratio)
• 2.9264 × 7 = 20.484 (# of teeth you need on the driven gear, if you have a 7-tooth drive gear)

Needless to say you can't buy a gear with that # of teeth. Either 20 or 21; whole #s only; no fractions of teeth. You would have to accept some degree of error. 20 teeth will make the speedo read high, 21 will make it read low; by about 2½% in each case. I.e. around 1½ mph off at 60, 2½ mph off at 100. If it was me I'd choose the gear combo that makes the speedo read higher than reality: the wheels will grow slightly at speed, and also when they get hot and the tire pressure goes up, which will make it read lower than sitting still cold. OTOH they will shrink as the tread wears. So take into account all those sources of error that will cause reality to deviate from "calculator" to make an informed decision.

midias

I made this a while ago. It is what I use for my T5 speedo calculation. Also has part numbers

jmd

iTrader: (4)

sofakingdom

iTrader: (1)

Yup, 3:1 is arbitrarily close to 2.9254:1 quod erat demonstratum super... at least as far as, real-world gears you can actually buy.

And you're also right jmd, "The Calculator" is usually WRONG. Not because it's "wrong" in some way inherently; but more so rather, because of the GIGO principle explained above, wherein if you type in abuncha crap that DOESN'T match what you ACTUALLY have (*cough cough* tireradius *cough*) you end up with GARBAGE. Absolutely "correct" GARBAGE, but GARBAGE nonetheless.