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Super High Revs on the Highway


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Hi everyone,


I have a 74 260Z that has a swapped L28 and a 5 speed transmission (not sure the exact model).


Anyway, my gearing is extremely short. Too short for the kind of driving I have to do every day.  I'm slightly worried that running the car 5 days a week for an extended time at high rpms will eventually lead to problems.  In order to even keep up with highway traffic around me I am running the engine upwards of 4000 rpm at times.  As much as I don't like doing this, I am still barely keeping up with commuter traffic at about 65-70ish mph.


It's also really loud and is murdering hurting my fuel mileage haha.


Anyway, after searching around it seems that even Zs with 5 speeds and gears like 4.11 and such only run at around 3000 rpms at the same speed I am going.  It seems crazy to me that I should be so high up in the revs.  


I'm not even sure if I have a valid question coming out of all of this or not, but I was wondering if anyone had any suggestions on gearing.  I genuinely have no idea what I'm talking about.  Am I the only one who spins this high on the freeway?






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My Honda runs at 4000k to do about 110kmh and close to 5000k for 140kmh. 

My 620 is only a 4spd and I dont have a tach so I can only assume its 3000k+ to do highway speeds, and even then it struggles, my speedo is waaaay off to so I actually have no idea how fast I am going. 


These are older cars and maybe driving at 60-70mph for extended time was just not thought of.


However, yours being a Z, I can only guess its fine driving like that as its a "performance" car?

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First of all, you are not driving a car with a domestic V8.  The L-engine in your car, in fact all L-engines are designed to rev.

A mild camshaft L-16, in a 521 has a torque peak around 3600 RPM, a horsepower peak at 5600 RPM, a "yellow zone" starting at 6500 RPM and a 7000 redline.

I would think a Z car engine would have similar RPM limits, but have higher RPMs for peak horsepower, but I could be wrong, especially about newer Z cars. 


A common misconception is that if you lower the engine RPM 10 % at highway speeds, you will reduce the fuel consumption by 10 %, driving at the same speed.  This is not true.  The changing of engine RPM has absolutely no effect on the parasitic air drag on the car, at the same speed.  The same is true of the wheel bearings.    There is a slight gain possible, due to the slower engine rpm having very slightly less parasitic drag, but remember it takes the same amount of power to move the car at the same speed, and if the engine is geared higher, to run slower, you have to open the throttle more to produce the power, so any gains made by having slower engine RPM are partially given up by having more throttle, to make the same amount of power.


By changing the rear axle to a lower numerically gear, (4.11 to a 3.90) you can gain a very slight increase in economy.  You could also accomplish the same thing by finding a transmission with a different fifth gear, say going from a .86 overdrive to a .79 overdrive.  but witht eh change in rear axle ratios, you will spend more time in lower gears, in in the case of the lower numerically fifth gear, you will have to shift into fifth later, and may have to downshift into fourth to pass other vehicles, or climb hills, go against headwinds.


Long story short, you could spend a lot of money, very slightly increasing you fuel economy, and take a long time to make it economically worthwhile.   But if it makes the car more enjoyable to you, that is the reason to do it.


A far cheaper way to increase the fuel economy is to simply slow down.  Doubling your speed takes four times the amount of power to move the car.  What this means is slowing slightly, say for example from 75 to 70 will increase the fuel way more than a gearing change.

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What about your tire size? If smaller than stock then thats the issue. But i agree gearin change that too much well effect performance more than save fuel or engine life. My truck with 83 720 trans and 4.11 gears is around 3400 @ 70. but 60-65 engine starts to bog down in 5th and rev more than i like in 4th.

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Not understanding this.

Assuming tach is accurate...
Stock tires (195/70R14) 4000 RPMs using a 4.11 ratio rear end and an early 5 speed overdrive of 0.864 you would be going almost 83 MPH. If you have the later overdrive (0.745) you would be going 96 MPH!!!!!!!!

The 260Z came with a 3.364 axle ratio so @ 4000 you would be going 101 MPH!!!!!!! or 117 MPH!!!!

Assuming the speedo and other cars are going 70 MPH....
70 MPH with 195/70R14 tires 4.11 rear end in overdrive (0.864) you should be @ 3374 RPMs
With a 3.364 axle this would be 2762 RPMs

I'm betting your tach is out to lunch... and never came back.



Assuming both tach and speedometer are correct and the clutch not slipping...

You have a  4.875 R-180 rear end.

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get in the slow lane, i do in my goon and i enjoy 55-65 and i may get somewhere 10min later than if i was pushing 70-75, but i get way more thumbs up driving because people can admire my car longer.!



there is a reason there is a red or orangeish line at 55 on the spedo. those were freeway speeds in the 70's  and i find i get better gas mileage right around that line?

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I agree. Here most of the speed limits are 80k or 50 MPH. Never a need to go faster, I still get there. Who the hell would want to hurry to work???I just don't get that. Fuck everyone else if they don't like going the limit +3km over. They can go around me or leave earlier tomorrow. I very rarely drive to work and take the bike path. At work I drive a van and it seems stupid to hurry or get a ticket so the whole days pay goes to the province. So I lay back and enjoy.


It takes at least 4 times as much power to push a car along at 80 MPH than it does at 40MPH just from the air resistance. Four times!!!

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I agree with Mike that something isnt adding up with the numbers. I have a zx 5 speed and 4.11 rear in my 240z and I do about 3-3.5k ont he highway cruising at 70mph. Even if you have the more rare close ratio zx 5 speed it wouldnt be 4k to do 70mph.

4k is like a light jog for a L series so long as its properly maintained. It certainly does help gas mileage but if your driving a 40yo sports car for gas mileage Idk what to tell you. The easiest thing to do to pussify the car and get your mileage is to pick up a stock 3.36 dif. I had 3.56 in mine and was doing like under 3k on the highway. Really boring and fuel efficient. 

I would start confirming ratios. Find out what 5 speed, what speedo cog your running, what final your running etc... Then you'll have your answers. I would put money on your rear being super high and the wrong speedo cog as well. 

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Looks like I need to get on it and find out what I've got.


Datzenmike, I appreciate you mythbusting my claim.  I've developed a mistrust for both my speedo and tach.  It really doesn't add up now that it's laid out.  


72240z,  I totally forgot about my speedo cable replacement.  That's probably causing problems as I replaced it with another cable from a Z without thinking.


Like others recommended, I've just settled with driving slower and that's fine with me.  I guess I'm off to find my diff ratio next...


Thanks for the input everyone.

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A speedo cable is a speedo cable.... one turn in for one out. However the little plastic drive cog inside the transmission is a different story. The tranny swapped in could very well be for a much different axle ratio and/or tire size.

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