77 rustrat Posted January 1, 2010 Report Share Posted January 1, 2010 hey i have a question. and yes i have searched and cant find my answer. but i was curious as to knowing if i put a newer model rearend in my truck if i would get more top end speed and better gearing. if anyone knows let me know thanx Quote Link to comment
datzenmike Posted January 1, 2010 Report Share Posted January 1, 2010 Just remember 'you can't get something for nothing' or 'if I give you this I get to take that', also known as the conservation of energy law. You have a '77 620 so it will have a 4.375 ratio rear end which in a truck, is geared for the truck's L20B engine power and for the maximum weight it carries, about 1,200 lb. Now most of the time you aren't carrying a load so the gearing is a bit low (high numerically) than you need. If you rarely carry a load you could raise rearing (lower it numerically) to 4.11, 3.889 or even 3.700. The '84-'86 720 with the Z20 option has a rear ratio of 3.364! This will have the effect of lowering the RPMs when compared to the same speed with the old gears OR increase your speed when compared to the same RPM using the old gears. Keep in mind that lower gears allows the motor to rev higher at the same speed and make more power through a shorter distance which is good for carrying or pulling heavy loads and climbing hills. It is not always best for economy. Higher gears reduce engine RPMs and power is spread over a longer distance which can improve economy but is bad for carrying, towing and hill climbing. The '79 620 standard transmission truck is equipped with a 4.11 rear end. To compensate for this when carrying a heavy load, the 1st gear is much lower ratio than previous standards. Some 720s 2wd also have the 4.11 (automatics usually) as well as the 3.889 and 3.700 rear ends. (Z24 standards usually) As speed increases so does wind resistance. To double your speed you need 4X the power, not 2X. Going too low numerically in the rear end can drop RPMs enough that the motor isn't making enough power to push through the air at highway cruise speeds. This is called power limiting and you can often go faster in 4th gear than 5th! Add a head wind and/or a slight hill and you are in trouble. Quote Link to comment
77 rustrat Posted January 1, 2010 Author Report Share Posted January 1, 2010 great thanx alot that was what i needed to know so now i just need to find a rearend that i can use that has lower gerars thanx Quote Link to comment
datzenmike Posted January 1, 2010 Report Share Posted January 1, 2010 There are lots of 2wd 720 out there. Look on the inner passenger side fender back below the hood hinge. There is a small aluminum engine tag attached there with this info stamped on it. Here's a 720 tag. Look down near the bottom left where it says TRANS, AXLE.... FS5W71B.... HF38. The HF38 is the axle type/size and ratio. 38 means 3.889. An HF37 would be 3.70, an HF41 would be 4.11 and so on. Quote Link to comment
sycloned916 Posted January 1, 2010 Report Share Posted January 1, 2010 Time to hunt the junk yards.I have the same problem too high of a gear,the 4.8? I have blows so I thnik this week I'll hit afew PnP's. Quote Link to comment
77 rustrat Posted January 1, 2010 Author Report Share Posted January 1, 2010 thanx man Quote Link to comment
datzenmike Posted January 1, 2010 Report Share Posted January 1, 2010 Time to hunt the junk yards.I have the same problem too high of a gear,the 4.8? I have blows so I thnik this week I'll hit afew PnP's. A 4.375 or 4.11 would work even with a turbo? Quote Link to comment
sycloned916 Posted January 1, 2010 Report Share Posted January 1, 2010 A 4.375 or 4.11 would work even with a turbo? That's what I was thinking more towards the 4.11,and having a 23.5in dia tire doesn't help either. Thanks for that good info. Quote Link to comment
DanielC Posted January 1, 2010 Report Share Posted January 1, 2010 Changing the rear axle ratio will also change the speedometer. for example, if you have the 4.857 because your truck originally had a 1300 motor, putting a 4.375, or 4.11 rear end in will cause your speedometer to read way low. You might be able to bluff your way out od a speeding ticket once, but I would not count on it. Datsun used different pinions on the transmission end of the speedometer cable to correct this. If you are at the junkyard pulling a rear end, grab the speedometer pinion out of the transmission, if you can. Quote Link to comment
sycloned916 Posted January 2, 2010 Report Share Posted January 2, 2010 Changing the rear axle ratio will also change the speedometer. for example, if you have the 4.857 because your truck originally had a 1300 motor, putting a 4.375, or 4.11 rear end in will cause your speedometer to read way low. You might be able to bluff your way out od a speeding ticket once, but I would not count on it. Datsun used different pinions on the transmission end of the speedometer cable to correct this. If you are at the junkyard pulling a rear end, grab the speedometer pinion out of the transmission, if you can. The Sr tranny has a elec sensor and my speedo is an elec programable one,so I can reprogram it. I might get afew diff's 4.11,4.375 and a 3.90. Quote Link to comment
datzenmike Posted January 2, 2010 Report Share Posted January 2, 2010 Has to be an H-190 from a truck. 510 and 810/Maxima goons have different axle splines. Quote Link to comment
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