7.25 front differential gears

[4D55 Performance]

I want to change my differential gears from 3.90 to 3.55 on my 1983 Mitsubishi Pickup. I want to lower the RPM's at cruising speed on the interstate to maximize my mpg and driving comfort. I can easily obtain the 3.55 gears for the 8" rear end from a 2wd Mitsubishi Pickup but I'm wondering if the 3.545 gears from a Starion will fit in the front of a 7.25" diff on a 1983 Mitsubishi Pickup?

[pennyman1]

you can swap the whole diff with gears from a starion into the front diff on your 83 truck

[4D55 Performance]

you can swap the whole diff with gears from a starion into the front diff on your 83 truck

That's what I hear. It looks like my plan is a go. I have already located the Starion gears for the front. Now I just need some 3.54 gears for the rear from a 2wd Mitsubishi Pickup. This should really help lower my RPM's.

[mikewarme]

Spencer you are starting to Jerry rig my truck . Are you going the route of Kerby ? Or Eric ????? It will not have the same performance.

[pennyman1]

oh man - nightmares from the past. I would have to agree with Mike on this one - in fact it would make more sense to put in the 4.22s for even better performance with bigger tires.

[4D55 Performance]

Spencer you are starting to Jerry rig my truck . Are you going the route of Kerby ? Or Eric ????? It will not have the same performance.

There are some concerns I will need to cut the radiator support to remove the front 3rd member. I don't know, maybe it will be easiest to just cut front cross member out because it is a pain to unbolt it. :))

I hope I have my head on straight and that's why I am asking around. Here's the data Mike. What do you think?

The performance should not be affected much. From putting a few thousand miles on the truck it is obvious it is much happier at 2400rpms than 2600rpms. I want to decrease the tire size so I can lower the torsion bars as well. It will also look much better with the original tires and wheels. Currently the torsion bars are cranked all the way up to avoid rubbing issues with the oversized tires. I really do think it is the best solution to the problem. The truck is making 200 ft lbs torque. Originally it only made 125 ft lbs or torque so I am not worried about performance.



Current Setup

Tire Diameter(inches) 28.4" 225/70r16 Michilin, These tires rub due to the backspacing. They are also on 16" aftermarket wheels.
Gear Ratio 3.909
Miles Per Hour 65mph
RPM @ 65 MPH-In Drive 3006rpm
RPM @ 65 MPH-In Overdrive 2600rpm



Purposed Setup


Tire Diameter(inches) 27.7" 215/75r15 BFG AT, This is the original tire and tire size and they will be mounted on the original wheels
Gear Ratio 3.54
Miles Per Hour 65mph
RPM @ 65 MPH-In Drive 2794rpm
RPM @ 65 MPH-In Overdrive 2414rpm

[4D55 Performance]

oh man - nightmares from the past. I would have to agree with Mike on this one - in fact it would make more sense to put in the 4.22s for even better performance with bigger tires.


I am definitely staying away from bigger tires. I actually want to go with smaller tires but ultimately it is the backspacing of the wheels that is causing my rubbing issues. I want to return it back to stock tires and wheels. The truck is very drive-able but with all the power of the new engine I think it is the right thing to do.

This engine is a lot different than the original 4d55. The 4d55 made max torque at 2900rpm from my dyno experience. The D4BF/4d56 makes max at 2000rpm which is a huge difference. These 2 engines have a very different power band and the higher gearing would help put this engine more in it's sweet spot.

If I went with the 4.22 gears the truck would not be driveable in my opinion. It would be down right miserable.