What is the optimum rpm range for cruising around town or on the highway at 70 mph?

My tach is reading around 3k when I am rolling at 70mph

For what purpose? Engine longevity/cooling, momentum, acceleration, mpg, etc?

The one rule of thumb for I always see here for stock engines is 3,000 RPM and above to keep enough air going over the heads. Cruising in 5th gear at less than 60 mph is supposed to be a head-killer over time.

Having said all that, I think you give your engine what it wants at any given time. On my build (2270 w/9950 cam) I worry about my oil temps way more than head temps, so I don't mind cruising at somewhat lower RPM's, especially on hot days.

Chris Foley told me the same: use revs 3k or above, for airflow. Also avoids "lugging" the engine.

I may run down to 2k in town (less noise) but that's at very light throttle and basically idling around. Any significant load and I'll downshift.

My stock 2.0 definitely likes 3k. At 60 in fifth I am around 2800 but the heads keep cool in mine (although I do have an afr gauge, a rebuilt and home tuned mps so I know it is getting a good mix at that rpm and vacuum level). Head temps on a hot day stay between 280 and 310 (at 60). At 70 I am around 3100. The head temps stay around 310 -360. Climbs will bring it into that upper range and up to 375. Around town 260.
My oil stays nice and cool (210), never more than 230 by my gauge. Switching to the Mobil1 0w40 has really helped with oil temp. You lose a little pressure but I think the extra flow is worth it.

Fuel consumption is an interesting one. If I don't drive like an idiot it gets really good mileage. I have a vacuum gauge on the dash and it seems "cruising" is always in the range on 12 -15 lbs. There is probably some math or something that would allow us to figure out where the perfect spot would be for these engines. Lugging is bad on fuel but so is really high rpms. I suspect at 45, 4th gear is better in mpg than 5th, but I bet at 60 5th is better than 4th (on a flat).

My numbers for both oil and head temps with my 2056 are very similar to this, but with the caveat that i have an aux oil cooler, that has made a huge difference keeping my oil temps good. i run the DT , brad penn or Valvoline 20/50 but had thought about using a 10-40 of same/similar racing oil with high ZPPD to help that flow as you mention. i have read that helps with overall engine temps and better lube at start up.
Factory recommendations were for 30wt summer and 10wt winter that's what's in the manual and the Haynes, but i digress, i don't want to flood your thread with oil weight discussions - back on track for the O.P.- don't lug it, i cruise around 75mph and rpms will be in the 3500-3800 range i think, and around town or local highway always in 4th and above 3000rpm for better air flow for head cooling

there are other factors for the RPM level, like tire/rim diameter. i am very close to stock, but at one time i had smaller lower profile wheels and my speedo was way off and the rpms were probably 4-500 higher.

Phil

Yup. The rule of thumb is that 70mph at 3K rpm is the start of 5th gear for cooling purposes. Though I have been able to cruise at 65 in 5th without issues. Usually though I'm going between 70-80mph in the 914. I measure cooling with a CHT on the #3 spark plug.

External oil cooler keeps my oil cool - i've never seen it get over 225 since I installed the cooler - and it usually stays at 215 or below.

Range is really hard for me to judge. The odometer has never worked on my car, and my gas tank is wonky. It goes from full to 1/2 really fast. And then I can drive 100 miles (IE my drive back from NE Gathering last week) and it will barely move from 1/2 to 1/4.

Zach

Biggest factor on these little cars for cruising RPM is tire aspect ratio.

50 - 55 - 60 - 65 - 70


As the numbers go down RPM goes up and MPH goes down.

As the numbers go up RPM goes down and MPH goes up.


Tire revolutions per mile is determined by tire aspect ratio, more RPM’s per mile means quicker acceleration and lower gas mileage also the reverse is true.

Shoot... I'm pushing 4k in my 6 at 70-75ish (IIRC)

Wonder if I should go bigger tires...

If the gear ratios are the same in a /6 as they are in a /4 - then yes you need taller tires. Your speedo must be way off too. Guessing you are using a 50 series tire?

Zach

I think I am at about 2,600 RPM at 70 MPH; I am running 195x55x15 tires.

might be, what are you using?
i am running 205/55-16 thats with in an 1/8" of stock 914 tire diameter , its close between a 205/50 and 205/55 but /50 on a 15" are definitely smaller than what the german engineers had in mind when the set the gearing ratios.

I prefer 3K rpm or slightly more for general responsivity. I can hit 105 mph in 4th at about 5.5 K rpm IIRC. 1.7 with stock wheel/tire size. IMO, gas mileage is right on in general.

I run 225/50r16s. My speedo is about 4 mph fast at 55. Can't believe those are bigger diameter than stock but must be...

near enough around 3K at 70mph in 5.
correct profile 165 XAS tyres.
checked against GPS on phone by passenger.
68.5 mph.

not a pedant - in cop paranoid mode on hume highway.
speed limit = 110 kph. or 68.35 mph.
tolerance is down to 1 kph officially.
"dictatorship" is on an insane post covid revenue drive.


never get out of 3 around town.
always revving. a lot.
its a 1.8 and can't pull the skin off custard.

amusingly the 914 has the most accurate speedo of all my cars.
i never would have thought that.
but then the other cars are french.
can't speak for the tacho. assume its as good as the german speedo?

Click to view attachment

Has anyone come up with an electric fan arrangement to push or pull outside air over the cooling fins, in addition to the mechanical fan?

I doubt an electric would be useful. The actual mechanical fan moves some real air at even low rpms. It's just a crazy amount of air at high rpms.

I tend to stay in 3rd in town but I do go 4th occasionally.

Now, 3rd out in the hills around 40 - 60 equals fun and pure driving enjoyment.

true.

800 litres per minute @ 4K revs.
around about 14 cubic feet.

blowing a gale i reckon.

------

and while its warming up and the flaps are closed its forcing all those litres out those crazy heater valve things with dust caps on them. i would not want to be the dumkopf who screwed around with that with electric fans. it was all worked out when it comes to volkswagens.

cruising speed in an early beetle was pedal to the metal. 60 mph. perverse reverse engineering contained the whole set up to an autobahn formula.

they dropped the ball with type 3 engines but picked it up again and kicked goals with the type 4. the golden rule is don't lug them at low revs and load. it used to be drummed into you in the 1960s and 70s.

Running the same size rear tires.

Speedo is also about 5 mph high from around 50 - 90 per GPS.

what you are saying is your speedo is reading 5mph higher than your actual speed?

that would be correct if that is what is going on.
speedo will be reading higher.

225/50r16 = about 632 mm diam.

165 XAS 15 = about 646mm diam. which is pretty much stock.
and the speedo is almost dead on, at least on mine.

ie your tyres are not bigger than stock and the speed should be faster on the speedo as a result.

If any time applying load, I down shift so RPM is always above 3,000/3,500. This is for a 2.2 S six, power band is from 4,000 to 7,000.

Nope. The gearing in a stock /6 is shorter.

4 - A, F, N, V, ZD
6 - A, GA, O, V, ZA

5th is shorter so higher RPM at freeway speeds. 2 and 3 are actually taller, 4th is the same. Changing wheels and tires is not gonna be a good fix as the result may make a weird looking car with a speedo that lies. Easy enough to change the gearing. The 914 really does have the best all around modern gearing for all around use.

I'm running 205/60-15 (627mm diameter)

I've only ever used a GPS speedometer, but I may be misremembering the speed/revs.

trans is a side shift from a 4-cyl, I don't believe it's been opened.

You would think so... But it is definitely reading faster on the speedo.

You have to have different gearing as Mike noted above (or be mis-remembering your speed vs revs as you said). Your tires are really close to mine (205/55 r15) and I'm right at 3K when doing 70. With a taller tire you should be revving less, not more. Also, is you car flared? 205/55r15 is the largest tire I could get under mine without rubbing. Did you need to roll your fenders or something to get the 205/60 to fit?

Zach

With my 235-55 ZR17's (27"OD) I'm running 3100 RPM at 70 mph.
Interesting that the speedo reads consistent with my GPS even though these tires are O/S compared to stock OEM . Don't know what's in my tranny however.

I've been running Michelin 205-65-15's on stock steel 5 1/2"rims for several years, and have cross referenced my speedo vs. GPS and have about a 2 MPH variation at 70MPH. I've wondered how accurate the GPS readings are, as my tire diameter is very close to stock tire diameter.

Guessing the error is in my memory, I'll check next time I have it at highway speeds.

Not flared, but I did roll the inner lips flat in the rear.