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Chevy Gear-Splitting Unit - Rev It Up And Go

Underdrive And Overdrive Your Factory Transmission With Some Help From Gear Vendors

By Mike Harrington, Photography by Mike Harrington

No, this little gizmo is not part of the Gear Vendors unit. It's a decibel meter reader. We took this along for a ride on the highway. When cruising at about 65-70 mph with the overdrive unit off, the rear gears were screaming and crying to the tune of about 87 dB in the cockpit. When the overdrive was turned on and the rpm dropped 22 percent, the dB reader measured in at 78 dB, almost a 10-point drop. Of course, with a drop in rpm comes a gain in fuel economy.

As a side note, might we also add that installing the Gear Vendors unit was really a piece of cake. It only took a couple of hours from start to finish. Perhaps the hardest part was finding a place to shorten the driveshaft, and that wasn't difficult either. The Gear Vendors unit also comes with a two-year unlimited mileage and 1,000- plus-horsepower warranty. The rub is the price. At about $2,300, it can put a ding in your wallet. But once it's on the vehicle, its performance lasts a lifetime.

TH350 Three-Speed Automatic
  Axle Ratio 4.56 4.10 3.73 3.55 3.42 3.08
  Trans Ratio Final Drive Ratio
1st 2.52 11.49 10.33 9.40 8.95 8.62 7.76
Over 1.97 8.96 8.06 7.33 6.98 6.72 6.05
2nd 1.52 6.93 6.23 5.67 5.40 5.20 4.68
Over 1.19 5.41 4.86 4.42 4.20 4.06 3.65
3rd 1.00 4.56 4.10 3.73 3.55 3.42 3.08
Over 0.78 3.56 3.20 2.91 2.77 2.67 2.40
TH375/400/475 Three-Speed Automatic
  Axle Ratio 4.88 4.56 4.10 3.73 3.55 3.42
  Trans Ratio Final Drive Ratio
1st 2.48 12.10 11.31 10.17 9.25 8.80 8.48
Over 1.93 9.44 8.82 7.93 7.22 6.85 6.60
2nd 1.48 7.22 6.75 6.07 5.52 5.25 5.06
Over 1.15 5.63 5.27 4.73 4.31 4.09 3.95
3rd 1.00 4.88 4.56 4.10 3.73 3.55 3.42
Over 0.78 3.81 3.56 3.20 2.91 2.77 2.67
700-R4/4L60E Four-Speed Automatic Overdrive
  Axle Ratio 4.88 4.56 4.10 3.73 3.42 3.23
  Trans Ratio Final Drive Ratio
1st 3.06 14.93 13.95 12.55 11.41 10.47 9.88
Over 2.39 11.64 10.88 9.79 8.90 8.17 7.71
2nd 1.63 7.95 7.43 6.68 6.08 5.57 5.26
Over 1.27 6.20 5.80 5.21 4.74 4.34 4.10
3rd 1.00 4.88 4.56 4.10 3.73 3.42 3.23
Over 0.78 3.81 3.56 3.20 2.91 2.67 2.52
4th GM OD 0.70 3.42 3.19 2.87 2.61 2.39 2.26
Double OD 0.55 2.67 2.49 2.24 2.04 1.86 1.76
M21 Four-Speed Close Ratio - Showing Clutchless Options
  Axle Ratio 4.88 4.56 4.10 3.73 3.55 3.42
  Trans Ratio Final Drive Ratio
1st 2.20 10.74 10.03 9.02 8.21 7.81 7.52
Over 1.71 8.35 7.81 7.02 6.38 6.08 5.85
2nd 1.64 8.01 7.48 6.72 6.12 5.82 5.61
Over 1.28 6.23 5.82 5.23 4.76 4.53 4.36
3rd 1.28 6.25 5.84 5.25 4.77 4.54 4.38
Over 0.99 4.86 4.54 4.08 3.71 3.54 3.41
4th 1.00 4.88 4.56 4.11 3.73 3.55 3.42
Over 0.78 3.79 3.55 3.19 2.91 2.77 2.66
SOURCES
Gear Vendors
8-00/-999-9555
gearvendors.com
Harrisons Restorations
By Mike Harrington
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