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Freeway Revs


MarkB.

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Back in February, Aerodynamic posted a question about his highway revs.  I believe the final consensus was that he probably had a R180 with and 4.8 final drive ratio in his Z and that he shouldn't worry about it hurting the engine.

 

I have a similar situation.  My '72 510 with an L20B also has extremely high revs on the freeway.  Note that here in Texas the freeway speed is 75 and 80 is typical cruising speed.  My speedo doesn't work (it reads up to about 30 and then goes nuts bouncing up and down until I come to a stop).  But my newly installed Autometer tach reads 4100 at a speed which I assume, based on surrounding traffic, is somewhere around 75.  My tires are P195/70 R14 and I have a 4-speed.  I have no idea what gears are in there or in the rear differential. 

 

While I enjoy the glorious cacophony that is a Datsun engine at 4100 rpm, it does become a bit fatiguing after about 20 minutes or so.  This is an area I do not understand other than do "know" that if I had a smaller number final diff gear, I would have lower rpm at the same speed.

 

So my questions are:

 

1)  may I assume that I have the same diff as Aero?

2)  if I wanted to get a lower ratio, could I replace a gear in the diff or do I need a whole new diff?

3)  I have a late 70's Z five speed in the corner, would this help reduce rpm at speed in some way?

4)  any clues as to my speedo problem?

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Fourth gear isn't really a gear and effectively is straight through 1 to 1.

 

With P195/70 R14 tires and a stock 3.90? 510 differential at 4,100 RPMs you would be going 77.43MPH. Sounds about right to me.

 

Your speedo cable is rubbing the inside of the outer sheath and momentarily snagging. As the inner 'cable' is nothing but a tightly coiled wire spring it keeps turning at the transmission end storing up energy. When it slips loose it releases this coiled up energy and the needle bounces higher. Look for damage to the outer sheath that would let dirt and water in or sharp bends or kinks. The transmission end unscrews  and the inner cable can be pulled out. Have a care as it is often slippery with lube and is like a floppy long worm. Lube with speedometer 'grease' or dry graphite powder. Anything with molybdenum disulphide in it is good.

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The freeway speeds where i live are 70+ and i installed a longtail 5speed a few months ago and we drove to the wreckers the other day and traffic was only doing 60+ mph,,  man it was almost a relaxing drive...  4100 rpms aint that bad but since i don't have a tach i can only go on how it makes me feel an the 5speed made me feel warm and fuzzy.

 

:)

 

 

Rock auto still sells speedo cables pretty cheap.

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As long as you are sure it's a late '70s the overdrive is about 13.6% and this would reduce your 4,100 down to 3,542.

 

Now, it is possible to run so much over drive that it would drop the RPMs and thus the HP/torque and to such a lower level that you can't maintain that speed. It's all about wind resistance. In order to double your speed you need 4 times the HP, not twice. You will be able to go faster in 4th than 5th. This is called being speed limited. Over drive is for reducing normal cruise speed RPMs to gain a small amount of mileage. You may be able to go almost 80 at 3,500 (in 5th) but you may also need to have the carburetor into the secondaries to do it.

 

 

Some later 280zx 5 speeds are 25.5% over drive. That 3,054 RPMs @ almost 80. Do able perhaps for an L16 but not without a struggle and maybe not on a windy day.

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hey MARKB. if it helps you i swapped out my 3.89 diff gears for 3.55 and yes it does make a little difference. but im also running 205/50/15 wheels, so it kinda evens its self out for me, but in your case since you are using stock wheels im sure that it will drop your rpms maybe a few hundred

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sorry for bringing this back up from the dead, but I am curious to why my 620 has such high revs and wrong speedo readout.

78 620 with tire size 205/55/16. 3spd auto so I understand it would be at a high rpm on the freeway. 65mph is around 4k rpm. but my speedo is completely off. It reads 75 when my gps shows me 65.

Any idea why this is? I believe my tire size isn't far from OE size, so why is it off by so much? Thanks all

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I have a 1970 521, L-16, 4 speed, basically stock.  The engine is doing about 3,600 to 4,000, at 55 to 60.

If you own a Datsun truck, remember, it is a truck.  It had to be geared to be able to move a full load, from a deap stop, and also be able to accelerate that load, at 55 MPH, for passing.

It just so happens that the torque peak of the l-16 engine is about 3600, the legal speed on freeways in the USA in the 1970's.  That is OK, the redline on an L-16 around 7,000 RPM.

 

I am not sure about a 78 620, but the original tire size for a 521 was a 6.00-14.  They did not have aspect ratios on tires back then, but think it was somewhere between 80 and 85.  The tires were tall, and skinny.  It would not surprise me if a stock 6.00-14 tire. on a stock Datsun 620 wheel, has a larger diameter, or rolling radius than a 205/55/16 tire.

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sorry for bringing this back up from the dead, but I am curious to why my 620 has such high revs and wrong speedo readout.

78 620 with tire size 205/55/16. 3spd auto so I understand it would be at a high rpm on the freeway. 65mph is around 4k rpm. but my speedo is completely off. It reads 75 when my gps shows me 65.

Any idea why this is? I believe my tire size isn't far from OE size, so why is it off by so much? Thanks all

 

That tire, stock gears (4.375) and at a true 65MPH, the RPMs are 3,840

 

The original stock tire (6:50X14) at 65MPH the RPMs would be 3,860.

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It's not as bad as it sounds. It's just an illusion that it's revving too high. Lots of these trucks were made with this combo. The '73 automatic used an even lower 4.625 ratio and they were fine.

 

Swapping in a zx 5 speed, with flywheel, clutch and pressure plate, release bearing, pilot bushing, both pedals, clutch master cylinder, hard and flex line, clutch slave with push rod, clutch arm and dust boot, shifter and boot.... well it's a lot of time and bother with not much pay off. Unless looking for a reason to get rid of your automatic.

 

A much simpler, cheaper and easier way is to replace your 4.375 differential with a 4.11 or 3.889, even a 3.70.

 

At 65MPH on your tires the RPM would be...

 

4.625........ 4,059

4.374........ 3,840

4.11......... 3,607

3.90......... 3,413

3.70......... 3,247

 

Take note that what ever you pick is arbitrary and acceleration will get more and more sluggish as you go down. Mileage might increase slightly

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