Just like our engines, the diesel-specific transmissions behind them have come a long way over the years. From the three- and four-speed, mechanically controlled automatics of the ’80s and ’90s to the modern six-speed, electronically controlled units currently on the market, things have changed—drastically. With factory horsepower and torque ratings constantly on the rise, transmissions have been the bene- ficiaries of necessities such as stronger, multi-disc lockup torque converters, larger transmission coolers, improved fluid flow, internal and external filters, solenoid bodies (as opposed to valvebodies), and added clutch capacities for more holding power.
Also similar to diesel engines, not all transmissions are created equal. Some feature shorter gear ratios than others, some transmissions are void of a direct (1:1) gear, and some handle added horsepower while others slip. And, many of the early diesel application automatics came without an overdrive gear. While most transmissions are still built in-house these days (Ford, Chrysler), others are outsourced (Allison and Aisin, respectively). Read on to find out which transmission dwells under your truck, which gear ratios it utilizes, and the approximate maximum horsepower level it can sustain in stock form.
GM TH700R4
Found On: ’82 to ’91 Chevy C10, C20; GMC K10, K20; and ’88 to ’98 Chevy and GMC 1500
Gear Ratios
First: 3.06
Second: 1.63
Third: 1.00
Fourth: 0.070
Reverse: 2.29
Stock Safe Zone: 200 to 300 rwhp
| Gm Th700r4 Transmission
GM TH400
Found On: ’82 to ’91 Chevy C30; GMC K30; and ’88 to ’90 2500 and 3500
Gear Ratios
First: 2.48
Second: 1.48
Third: 1.00
Reverse: 2.07
Stock Safe Zone: 200 to 300 rwhp
| Gm Th400 Transmission
GM 4L80E
Found On: ’91 to ’98 Chevy and GMC 1500, 2500, and 3500
Gear Ratios
First: 2.48
Second: 1.48
Third: 1.00
Fourth: 0.75
Reverse: 2.07
Stock Safe Zone: 200 to 300 rwhp
| Gm 4l80e Transmission
Allison 1000
Found On: ’01 to ’13 Chevy and GMC 2500 and 3500
Gear Ratios
First: 3.10
Second: 1.81
Third: 1.41
Fourth: 1.00
Fifth: 0.71
*Sixth: 0.61
Reverse: 4.49
Stock Safe Zone: 400 to 500 rwhp
* = ’06 model year and newer
| Allison 1000 Transmission
Ford C6 Found On: ’83 to ’88 Ford F-250 and F-350
Gear Ratios First: 2.46
Second: 1.46
Third: 1.00
Reverse: 2.00
Stock Safe Zone: 200 to 250 rwhp
| Ford C6 Transmission
Ford 4R100
Found On: ’99 to ’03 Ford F-250, F-350, and F-450
Gear Ratios
First: 2.71
Second: 1.54
Third: 1.00
Fourth: 0.71
Reverse: 2.88
Stock Safe Zone: 350 to 450 rwhp
| Ford 4r100 Transmission
Ford E4OD
Found On: ’89 to ’97 Ford F-250 and F-350
Gear Ratios
First: 2.71
Second: 1.54
Third: 1.00
Fourth: 0.71
Reverse: 2.18
Stock Safe Zone: 200 to 300 rwhp
| Ford E40d Transmission
Ford 5R110
Found On: ’03 to ’10 Ford F-250, F-350, and F-450
Gear Ratios
First: 3.11
Second: 2.22
Third: 1.55
Fourth: 1.00
Fifth: 0.71
Reverse: 2.88
Stock Safe Zone: 450 to 550 rwhp
*In cold weather, an alternate Fourth gear with a 1.10:1 ratio is used.
| Ford 5r110 Transmission
Ford 6R140
Found On: ’11 to ’13 Ford F-250, F-350, and F-450
Gear Ratios
First: 3.97
Second: 2.32
Third: 1.52
Fourth: 1.15
Fifth: 0.86
Sixth: 0.67
Reverse: 3.13
Stock Safe Zone: 450 to 550 rwhp
| Ford 6r140 Transmission
Chrysler TF727
Found On: ’89 to ’91½ Dodge W and D 250 and 350
Gear Ratios
First: 2.45
Second: 1.45
Third: 1.00
Reverse: 2.21
Stock Safe Zone: 250 to 350 rwhp
| Chrysler Tf727 Transmission
Chrysler 46RH, 47RH, 47RE, and 48RE Found On: ’91½ to ’93 Dodge W and D 250 and D 350 (46RH); ’94 to ’95 Dodge Ram 2500 and 3500 (47RH); ’96 to ’02 Dodge Ram 2500 and 3500 (47RE); and ’03 to ’07 Dodge Ram 2500 and 3500 (48RE)
Gear Ratios First: 2.45
Second: 1.45
Third: 1.00
Fourth: 0.69
Reverse: 2.35 (46RH), 2.21 (47RH, 47RE, and 48RE)
Stock Safe Zone: 300 to 400 rwhp
| Chrysler 46 Rh 47rh 47 Re 46 Re Transmission
Aisin Seiki AS68RC
Found On: ’07½ to ’13 Dodge chassis cab
Gear Ratios
First: 3.74
Second: 2.00
Third: 1.34
Fourth: 1.00
Fifth: 0.77
Sixth: 0.63
Reverse: 3.54
Stock Safe Zone: N/A
| Aisin Seiki As68rc Transmission
Chrysler 68RFE
Found On: ’07½ to ’13 Dodge Ram 2500 and 3500
Gear Ratios
First: 3.23
Second: 1.84
Third: 1.41
Fourth: 1.00
Fifth: 0.82
Sixth: 0.63
Reverse: 4.44
Stock Safe Zone: 350 to 400 rwhp
| Chrysler 68rfe Transmission
