What is the rpm sweet spot for highway driving

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my5thmopar

my5thmopar

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I want to change gear sets because the take off is sluggish. I also do a fair amount of highway driving at 70-75 mph. I'm really trying to find a balance. My car has a 318, Edelbrock 650 and small XE256 cam. Trans is 904 and it is not the low 1st. I have 4 different gear sets to play with. Here are the RPM vs gear ratios.

I guess the basic question is the title...What is the rpm sweet spot for highway driving?

Currently running 215/70/14 the spec sheet says the tires are 25.8 calculated at 70 mph.

2.76-2583
2.94-2752
3.23-3023
3.91-3660

Thanks Craig
 
When I ran a 318 it fried tires with 276 rear. If your going 70-75 I think Ide keep them gears.
 
Depends on your cam. What are your cam specs.
 
1970Duster said:
Depends on your cam. What are your cam specs.
Click to expand...

Here ya go.

Duration @ 0.006": 256° / 268° Duration @ 0.050": 212° / 218°
Max Lift w/ 1.5RR: .447" / .454" Lobe Separation: 110°
Intake Centerline: 106°
 
You say a fair amount, but to me that means 40-50 miles everyday or every other day.

So how far you going? If you're only up to those speeds for 5-10 minutes, I say put in the fun gears.
 
I like the 3.23. Short stroke, small cubes, small cam. Needs RPM.
 
65_valiant said:
You say a fair amount, but to me that means 40-50 miles everyday or every other day.

So how far you going? If you're only up to those speeds for 5-10 minutes, I say put in the fun gears.
Click to expand...

Yes. 40-90 miles one way at a steady 70 mph.
 
what converter?

if you have a high stall converter you don't want your cruising RPM to drop below where the converter picks up or it will fry the convertor

(at least, that's how they used to be)
 
depends how much mileage in mpg/longevity your willing sacrifice for performance
 
-what 273 said.
Personally I like 2200ish. Motor is just cleaned up, out of reversion. Shes feeling content.Almost happy. The vibration and noise are down. Its smooth sailing. Your engine with a smaller cam, could easily cruise at a lesser rpm. You aleady have the sweetest commonly available gear, the 2.76.
-If you need a bit more oomph, look to the engine tune, or a lower first gear. The 2.74 low set will get you 13% more grunt. Thats the difference between a 318 and a 360. But, its only good for first gear. Second gear falls to 6% increase. Thats the difference between the 318 and a 340.
-The only drawback is the larger splits.The rpms will fall more between shifts. Your cam should be very torquey, and get you through that ok. And if you find a loc-up, your revs will fall another 5% from your posted data-sheet. So its win,win for the trasmishin.lol
 
I would say 3.2 is bit too much for 70 mph, i have that and its about 3000rpm.
I would personally want to get od someday 2000-2500 rpm would be good for cruising. Thats why i drive 55-60 mph most of the time.

If your cam is not too big engine should have plenty of torque to cruise at low rpm


New cars that have 200 hp can easily cruise at 1000rpm 70 mph, so nice 318 should easily do 2000 rpm
 
Even with tall rear gears the cruising RPMs for our old V-8 Mopars without overdrive are much higher than they need to be. A 318 with a small cam like yours would get best gas mileage in the 1800-2200 range and could probably go a tad lower; problem is you don't want to be under much load at that low speed which is why newer cars shift down from top gear with less than 1/4 throttle even though they cruise at 1000-1500 RPM.

If you say your car is sluggish off the line I'm guessing you already have 2.76 gears? Your cruising RPM with those gears is already high so going with any "shorter" gears will just take it higher. I think going to 3.23 gears from 2.76s would be nice, big enough jump that you will notice it wind up quicker but 3000 RPM cruise will still be reasonable. Personally I've found that gear changes don't seem to increase the torque as much as make the engine rev up quicker and easier to keep in the powerband, that's just based off feel though shorter gears DO give you more torque at the wheels.
 
I have 3.55's with a 28.5" tire and cruise 70 at 2600-2800 all day long.
 
3:23= Look Good Drive Good Sound Good. Upgrade the brakes and Stop Good. Run safely on the highway with the BMW's all day long!
 
3.23 has always been a good "compromise" gear. I think 90% of the "hi-po" Mopars of the '60s and '70s came with 3.23s as the standard gear. There is a reason for that.
 
Have you thought about swapping the trans. If your driving that much a day it may be worth going to a over drive. If I was in your situation I shoot for 2500 or 3000 but that may require more then a gear change.
 
For a mild performer like your mildly cammed engine has that see Hwy. driving of some distance, consider a O.D. Trans. The A-500 (904) has a .69 O.D. ratio.

3.23 X .69 = 2.22
4.10 X .69 = 2.82
4.30 X .69 = 2.96

Food for thought if the wallet can handle it.
 
Consider the speed at which you DRIVE. Do you drive 70 or is that just where you calculated? I couldn't imagine driving 70 even on the interstate. Just doesn't make good economic sense. When I had my Valiant, it had 4.30 gears. I drove that car everywhere, all the time. On the interstate with my big toe on the gas, just to keep it cruising at 55, it was turning a tick over 2700, that's ALL.
 
I say 2.94. 273's need a little RPM. I had 2.76 behind a 340 and it was just not matched (and that had more torque than a 273)
 
If your bottom end is soft you will need more gear which will create high rpm on the highway and hurt mileage.Seems the best option would be a quality converter built your specs that will give you a better launch but still cruise nicely.
A lot of guys will not spend $600 or $700 on a converter but it is the best money you can spend.I would recommend Dynamic 9 1/2"
 
Here's the easiest test in the world: Drive your next highway run in SECOND gear at 70MPH for 90 miles. Do not shift into drive!

You can figure 355's or 391s will give you about the same highway run.

You will get a first gear lower than your current one for burnouts and acceleration, but your current second gear will become your drive gear.

Numbers on a page are all speculative. When you listen to that engine whine in second gear for 90 miles, I think you'll want to keep what you've got or go with an overdrive.

My 2006 Dodge truck with a 6 speed overdrive goes 1900 at 70. I can't imagine running 3000 rpms for 100 miles!
 
You're talkin about two completely different vehicles. I get your point though. Your analysis is also a pretty good one.


gregpurcell said:
Here's the easiest test in the world: Drive your next highway run in SECOND gear at 70MPH for 90 miles. Do not shift into drive!

You can figure 355's or 391s will give you about the same highway run.

You will get a first gear lower than your current one for burnouts and acceleration, but your current second gear will become your drive gear.

Numbers on a page are all speculative. When you listen to that engine whine in second gear for 90 miles, I think you'll want to keep what you've got or go with an overdrive.

My 2006 Dodge truck with a 6 speed overdrive goes 1900 at 70. I can't imagine running 3000 rpms for 100 miles!
Click to expand...
 
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