2.77 speedo gears
11-19-2003, 05:50 AM
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Car: 1990 Formula
Engine: 305 TPI
Transmission: WC T5
here is info I have aquired
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).
Drive gears.....
9--red------(14053347)
7--purple---(14071731)
Driven..........
19 White----(14090592)
20 Blue-----(14090593)
22 Grey-----(14090595)
• 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).
Drive gears.....
9--red------(14053347)
7--purple---(14071731)
Driven..........
19 White----(14090592)
20 Blue-----(14090593)
22 Grey-----(14090595)
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