RPM for highway cruising

I knew my speedometer was off, so today, when I went out for a drive, I brought along my GSP. Looks like my speedometer is off by approximately 9%.
At a corrected 70 mph my tach shows 3800 rpm, and at 75 mph it shows 4000 rpm. Is this considered too high of a cruising speed for our engines? My IM will do a lot of cruising and most will be in the 70-75 mph range.

That's about right for a stock 1971 and up transaxle; we always pretty much doubled the tach for the mph. It's terrible for a true Freeway Flyer unless your tire radius is considerably less than stock.

The engine, even a good stocker, will work well in that rev range, but air cooled engines need to have the rpm varied frequently during long drives, don't keep a constant speed and rpm for long periods.

A well built engine will rev freely past 7,000 and can live well at 6,000. We have an engined designed by AJ, with some of his parts and pieces installed, that takes 8,000 on nearly every shift and is running strong after three years.

Luck,

TC

Good thing cruise control isnt a standard option on these cars.

Ron, This has been talked about a lot here---you can see more if you will do a search.

You are seeing 4,000 at 75 MPH . At 75 I'm doing 3,200.
At 4,000 RPMs I'm doing 88 MPH.

What are your specs? Mine are:

Tire height, 25"
R/P 4.120
4th gear .821

Mine was set up for long highway drives but it's still plenty peppy around town. I don't drive around in town in 3rd as 4th is plenty strong and doesn't lug the engine at all.

I wonder what your specs are. Also, maybe you aren't getting a real accurate speed reading?

What is your highway MPG?

Jack, are you running a 195/65/15 rear tire?

Larry, my tires are just the 165 X 15s. I don't have the clearance at the rear fenders to run larger tires.

I presently have 185/65-15s (24.5" diameter).

When I first owned the car I had some work done on the tranny, but I've forgotten what I had done. I do know that first through to third are on the short side and there is a big gap between third and fourth.

My speeds were checked with a GPS, not the speedometer.

I haven't driven the car enough to do a mpg check.

Ron--I envy you being able to run the larger tires. My space between the edge of the fender and the tire is so small that I can't get my hand in there to was the rear tires--I actually have to roll the car a little bit to wash the top part of the tire. It never rubs though.

That's not a big complaint though---everything is really a-ok.

I'm running the same size that Ron is, but I have less than 0.25" between the tire and the fender. My speedo reads high because of the lower profile, but I like the stickiness.

--"but air cooled engines need to have the rpm varied frequently during long drives, don't keep a constant speed and rpm for long periods."

Um, why?

Yeah, that's a new one on me, too.

A few, maybe more than a few really, I became friends with one of the head mechanics at Kurtzman's VW and Cutters. He had a lot of 'ol skool and pre-skool knowledge and always cautioned me not to drive my Ghia at constant speed for long periods of time. Didn't make sense to me, but he explained that the VW aircooled engine made maximum horsepower, reached maximum cooling, got maximum gas mileage, created maximum oil pressure, and maintained maximum operating temp all at different RPMs. And the engine was very suceptable to temp changes which could alter everything about the engine including the actual size of it.

He said that by occasionally varying the speed that I drove, I would have a better chance of hitting one or more of these optimum RPMs for a certain length of time and temp, mileage, oil pressure, etc. would improve.

If the constant speed was wrong for one or more of the crucial readings, say for cooling,the the engine would heat up, the oil pressure would then suffer, the engine size would change, the engine may start running leaner, making it hotter still, lowering the oil pressure even more, and so on.

Might be bullshit, might be cautionary or just folktale, but it's what he swore by.

I still do what he said and still get REALLY great service out of my stock/mild engines. I mean CRAZY good service, considering.

Anyway, just saying.

Oh......

OK.

I guess......

Who am I to be a disbeliever of an Old Skool kinda guy.

But wait a minute....

I'm an Old Skool kinda guy, too, and ya know what?

I don't believe it!!

Yeah, the engine does all that stuff at different RPMs, but so does every other engine out there, air and water cooled, so....BALONEY!

I'm with Cory: Drive it like you stole it!

Oh, and Ron O....an answer to your original question:

I tend to run between 70 and 80 on turnpikes (if you're on I95 in the South and not doing at least 80, you get run over).

Running 16 inch rims and I think I have about 24.5" diameter on the outside of the tires (at least it sounds familiar for a 225R16X50).

The tach usually runs between 3,250 (about 68) and 4K (about 78), and most often between 3,500 and 4K (sometimes blipping to 4,200 if I'm on the New Hampshire Turnpike on a Friday afternoon and just trying to keep up with traffic).

On highways it seems to like running above 3,250 but with the external cooler it never goes above 200F no matter what, unless I stop for a light or something and then climbs to 205F and stops until I move again and then returns to 200. All checked with a Dipstick thermometer.

This help?

gn

Gordo,

If you're ever up this way again, I'll introduce you to Jim, you'll honestly like him. You can tell him yourself that you think he's bullshitting.

I'd love to be there for that . . .

Does Jim have any views on adding MMO now and then ? :-)

At 125 MPH @ 6700 rpm

My front end feels light at anything over 80 mph. At 125 mph my IM would join the speedster air force. :-)

Jack, my 185/65 tire is VERY close to the body on my passenger side. We're talking millimeters here. There's a fair bit of room on the driver's side.

"At 125 MPH AT 6700 rpm"

Really?

4900 @ 100