So i've done a bunch of searches to figure how to calibrate my speedo after my 3:08-3:73 swap. The typical answer has been to go to GM and ask them what speedo gears i need. So i did that today and the guy told me that because of the different speed sensors on the vehicle the speedo can't be calibrated without reprograming the computer. Then he told me my gears were a bad idea anyway. I thought the speedo gears where what the sensors were looking at when they gave the indicated speed. Did i totally misunderstand or is the chevy parts guy off? So if someone who has done this swap can tell me for sure i will be really greatful and a lot less frustrated. Thanks a lot for any help.

 

Speedometers are easy. I don't understand how people can get all wound up over trying to figure them out. Well, maybe I do... they're only humans, after all; and humans aren't very good with logic or numbers.

Here's the Speedometer Magic Universal Constant: (SMUC for short)

• 1 mile = 1000 revolutions

So, all you have to do, is figure out your tire diameter and multiply that by pi, which will tell you how far the tires roll in one full rotation; divide the number of inches in a mile, which is 12 x 5280, by that tire diameter; now you know how many times the tire turns in a mile. Then multiply that by the rear gear ratio, now you know how many times the drive shaft turns in a mile. Now divide that by the speedo gear ratio, that will tell you how many times the cable or VSS will rotate in a mile. Pick a speedo gear ratio using gears you can actually obtain that comes the closest to giving you the magic 1000 revolutions per mile. Done.

Example:

Let's assume your tires are 26.5" diameter, and you have a 3.73 gear. So your tire circumference, the distance the tire rolls in one revolution, is about 82.25". There's 12 x 5280 inches per mile, or 63360. Divide that by 82.25. That tells you that your tires turn about 770.3 times per mile. Multiply that by your rear gear, which is 3.7272727272 etc, you will discover that your drive shaft turns 2871 times per mile. So, you need speedo gears with a ratio of 2.871:1 to put your speedo perfect. Now I just happen to know (just as the parts counter guy ought to but probably doesn't), since you have a T-5, that you can get a 7-tooth drive gear; the gear that comes closest to the ideal ratio is 20, since the perfect number is 20.09 (2.871 * 7 = 20.09), but you can't buy gears with fractions of teeth (and wouldn't want them even if you could).

So all you have to do is go to the dealer and tell him you want a 7-tooth drive gear and a 20-tooth driven gear, for a T-5; ignore whatever lip he gives you about him being a g0d and you being an idiot and it won't work, just tell him to shut up and give you the gears and take your money, and go put them in. They will be within a quarter of a mile an hour at 100 mph.

If your tire diameter is different, your results may vary. But I can tell you from many experiences that the drive gear you want is 7-tooth blue. There's no point in over-calculating this, since you can only get 19, or 20, or 21, or 22 etc. tooth driven gears, so you'll never get it perfect, but you should always be able to get it within 2.5% (1.5 mph at 60).

 

Thanks for that explanation. That helps a lot. I was getting really pissed at that parts guy. Especially since he told me he could get me the right parts if he had my vin number. so i went outside coppied it then he told me i couldn't do it. I think the experience was still better than dealing with Ford. Thanks again. You rule.

Sam

 

Yeah, he actually has to do hard work, like think, and look stuff up.... some of those guys just hate that.

Your VIN won't help because if you buy gears for that, you'll end up with the right ones for 3.08.... not real helpful.

Don't forget, yours are the ones for electronic speedo; the drive gear is the same, but the driven gear is different from the older cable style. A cable-type gear has a shaft that the cable plugs into, but the electronic type is just a little round gear with a D-shaped hole in the middle that fits onto the VSS. I don't know the GM part #s offhand, but on the tech data pages on this site, there's some of that stuff; the #s you need might be there.

 

Drive gears.....

9--red------(14053347)
7--purple---(14071731)

Driven..........


19 White----(14090592)
20 Blue-----(14090593)
22 Grey-----(14090595)

 

why dont you get an electronic speedo recalibration box? they work great

 

the gears are way cheaper

 

Maybe the guy's knuckles dragging on the parts counter wasn't clue enough for ya

I happen to be really fortunate to have a good parts manager at my small town chevy dealer. He's gone out of his way to get parts for my PONTIAC that were specific to the Firebird, and my numerous Fiero projects.

 

ya but there is not a gear combination that can accurately calibrate for every gear ratio, not even close