Calculating Optimal Highway Cruising RPM

MPG is one factor in the determination of efficency. Efficency is a way of evaluating peak performance. Ignoring any factor in detrmining performance is like pissing in the wind, it'll come back to you. For crying out loud.

 

I've owned two cars with low gears. One had a 4:11 the other a 4:30. I'd never own those gear sets again. I just don't need the neck snapping and RPM.

3:50 is the lowest I go. It's plenty responsive. We are talking about 400 RPM here. I'd leave the damned thing alone. Regardless of what someone says in Hollywood. 1800 RPM on any car not going uphill is a fine long distance cruising RPM.

 

Speaking of Vacuum Gauges, does anyone know of a source of good quality ones that can be mounted inside the car, not the cheapies made in, You know Where.
The Milage Minder type were advertised in magazines years ago, but you never see them anymore.
HemiDeuce.

 

People who actually drive their hot rods across the country.

Just because you trailer your car doesn't mean everyone has to.

When I drive on the freeway I get 23 MPG in 4th gear (700R4). Around town I get about 10, and I hardly ever get out of third. It's hot rod enough for me in the first two gears using a 3:50 rear end.

 

Ok, but does that happen at a useful rpm?

Sounds like your reversion bit happens when the engine is just barely above lugging under a particular condition.

Interesting too, that most cars seem to be geared right around the peak torque figure during cruise.

 

Not completely sure about where they're made today, but over the years I've used Stewart-Warners 2 5/8" vacuum gauge.
Think I have three of them now.

One of the reasons is, I've always used S-Ws, but the main thing is the bigger gauge is very visible, easy to read etc. and best of all it has a needle sweep of about 250* or so.
Lots easier to follow than the 2 1/8" gauges, S-W or not that have a 120* needle sweep.

If you do a little digging, S-W makes a "Marine" 2 1/8" vacuum gauge with 250* sweep that has a brass case and the case is a little deeper than the standard 2 1/8" 120* sweep vacuum gauge.

I see a lot of S-W gauges that are USA made at the car oriented swaps and the ones that aren't antiquey don't sell for a whole lot.
Couple years back I got a good assortment of the little gauges for $5. each.

Kinda funny that most of them you find are ammeters and fuel level gauges.


Speedway is a good place to get the S-W 2 5/8" gauge.
Last couple of them I bought came from there.

 

3500?

No, and thats the reason for the swap to a higher geared diff.

I note that the factory supplied figures for the torque peak on the 455 Buick engines I run are listed at 2400-2800 rpm.

As well as, so far it looks like there will be an mpg improvement, but I need some more time and miles to see just where it settles out at.

Most of the guys I talk to have very similar experiences with mpg/torque peak etc.

Real world stuff and not just laboratory figures.

 

Real world stuff and not just laboratory figures.

Just laboratory figures... like NASCAR uses.

Right - the "real world', where the laws of physics are suspended based on what Bubba did with his Shivolay.

 

You're either a retired auto engineer or fresh out of college auto engineer.

Either way, you're gonna have to talk slow for some of us....

 

A good rule of thumb I have found:

If the rear gear and the tire size are the same, you will get about 30 mph per 1000 engine rpm. That gives 2000 at 60, 2500 at 75, and 3000 at 90.

I ran this combination in my roadster (3.00 gear, 30 inch tire) for a long time, and it worked well, although for a car this light (2450 with my fat ass in the seat), I just recently went to an AOD and 3.90 gears. Digs harder from the git-go, and cruises even easier (2.60 overall ratio in high; 2200 or so at 75).

My '40 coupe uses a similar combination. The rear gear is 4.57, but with the AOD transmission, high works out to 3.04. With 30 inch tires, it rolls down the highway pretty well (around 25-2600 at 75). This car pushes a lot more air than the roadster, so I don't think I will gain much by going higher on the gearing, except, of course, reduced noise, vibration, and wear.

 

Knowing cool stuff is no fun unless you share it......

 

One "modern" thing that I have found to be indispensable is an Oxygen sensor and an air-fuel gauge. Not traditional, I know, but it does give you a complete picture what is happening when used in conjunction with a vacuum gauge. It does not have to live in the car.

It is how I get 26 mpg out of a 305. Of course, a deep rear end ratio and 6 gears helps out too. It is at the torque peak at 65, in 6th.

I'd swap that Q-Jet for a carburetor you can tune, and then tune the hell out of it. Just remember, as with so many other things in life, bigger is not often better.

Be realistic about your average freeway top speed. I data-logged a month of freeway driving in my DD, and I was shocked to find that the average speed was 57. I feel like I am at 65-70 all of the time. Yeah Bay Area traffic!

 

I'd swap that Q-Jet for a carburetor you can tune

hahhahahahaha

 

I can't tune a Q-Jet, and don't know anybody who claims they can. I know it can be done, but it is currently beyond me. If you can, more power to you! How about a tech article?

 

Sorry, that was rude of me.
Started learning QJ to improve the mileage on a friend's new 1970 Buick Grand Sport - very easy, 10 minute job.
The only difficult part is that you can't be as precise as Holley as to the cut-in point of the secondary steps on the metering rods - you have to try a different spring, cut off a coil etc. Carb stays assembled, stays on the engine - only the air horn comes off. Holley you just replace the PV, but you have to get wet to do it.
Doug Roe's book is excellent. Many, many sites with good DIY stuff, even some for specific engines (mostly GM V8, duh).

 

i have a 28" tire with a 4.10 rear gear, and i got an overdrive A833 transmission, so i can cruise on the freeway at 70 mph at a reasonable 2400 rpm. i checked out those diff ratio calculators, but this one is my favorite. you might have to go to 'custom' to add your tranny ratios (i do).

http://vexer.com/68rt/speed.html

 

As far as finding the "sweet spot," which gear ratio to use etc., i don't have a clear idea. And i don't have time to find out either. i can,however, give you my numbers:

i turn 1800 at 55mph and 2100 at 65mph.

i get between 15.8 and 16.1 mpg if i keep it at or below 65mph.

The setup:
goodwrench crate 350 completely stock
stock quadrajet
TH-350C (lockup converter) no overdrive. This means i have 1:1 direct drive.
NOTE: i am not nessesarily touting direct drive vs. overdrive, i'm just stating what i got and what works for me. Incidentally, i feel that direct drive takes a little less energy to turn vs. overdrive, but that's maybe for another thread.

2.56 diff gear.

This is in a 1/2 pickup and weight is at least 3800 lbs. possibly up to 4000. Also, aerodynamics are not optimal.

Tires, which i feel make all the difference in the world are presently off brand mismatch of pepboys and junkyard 235/75 15's. Switching to michelin X's or equivalent could probably bring an additional 1 mpg increase.

2) Another thing to remember is i feel that optimal mpg, all other things being equal, is going to be achieved with the stock, snorkle type air cleaner vs. an open element air cleaner. There is a thermostat in the snorkle to control intake air temp. Warmer=leaner, leaner=less fuel burned.