Was up people I have another question..... What is a good cruising rpm. I have a 69 cougar with a 351w and a three speed FMX. My rearend is 3.00 ratio and I have late mustang wheels 17 inches. What is the I deal rpm at what speed. Also what is the best wheel for this ratio to get good mileage.    

There are several factors that go into the ideal rpm at a given speed.  The cam profile is the first and biggest.  A cam with a lot of overlap will run better and more efficiently at a higher rpm a cam with little overlap will do better at lower rpm.  Also how torquey is the engine down low?  You want a speed fast enough to not lug the engine but not so fast to be really revving it.  Lugging the engine too low will create an incomplete burn and consequently waste fuel.

My Firebird had 2:73 gears and a 700r4 trans, it cruised at 80mph at 2,000rpm. It wasn't too bad because the Pontiac's torque, but at 60mph it was a dog in OD.  I always felt that with a ~3:23 rear end I'd probably have gotten better mileage and it would have brought the rpm up to around 2,500.

Over all in a broad general statement I think 2200-2500rpm on the highway would be the best cruising rpm for noise and mileage.  I run my Centurion at 3,000rpm and my T-bird slightly higher on the highway and I know I would get better mileage if I just slowed down to about 2,500rpm because of both wind resistance and it's a more comfortable range for the engine.  

Ak Miller proved about twenty years ago in Hot Rod (with three Mercurys) that turning the engine as slow as possible at cruise (assuming it has enough torque to propel the vehicle) is the ticket to maximum mileage.

Miller ran cruise manifold vacuum as low as 5" hg on a 3800 pound, 255 cid Cougar (barely ticking over 1000 r.p.m. at the then top legal speed of 55 m.p.h.)  

To oversimplify, fewer r.p.m. means less air/fuel consumed.  Fewer r.p.m. also reduces internal friction losses.

The problems are throttle response and "big" intake tracts (cam, heads, intake, exhaust) that won't build enough low speed torque because of reversion to pull a tall gear/deep overdrive combination. (Additional problems may be fuel metering and mixture distribution at low intake air velocities)

In factory set ups, such as the GM LS-1s that Jeff Smith keeps ramming down our throats, final drive ratios (in overdrive) are commonly in the 1.7:1 range.

Given the facts that volumetric efficiency is extremely low at cruise (remember, manifold vacuum is usually between 10" to 15" h.g. in most street cars) and the power required for most cars to maintain a level cruise at speed is less than 25 h.p., raising the r.p.m. to any speed above what the engine will maintain without stumbling (incomplete burn because of excessive reversion and incomplete scavenging) trades extra fuel for driveability.    

I have yet to see any instrumented testing that contradicts Miller's data. Nor have I noticed any contrary results in my own testing.  Thus, I strongly suspect the "efficiency range" folk wisdom is a baseless canard.  

Call it as you will, I tended to get better mileage at 80mph than I did at 65-70mph on my trips to Las Vegas from So. Cal.  26-28mpg vs. 22-24mpg on repeated trips.  I always ran a vacuum gauge (still do on road trips) and I could pull more vacuum at 80mph (12-14 inHg) than at 65mph (8-10inHg) which told me I was getting more bang for my buck in efficiency.

Engine wise it was a Pontiac 400 with what I believe to be an RV cam in it so down low grunt was plenty and idle vacuum under good atmospheric conditions would pull 20 inches at 600rpm. Q-jet carb and stock points distributor.

My assumptions are that I could get 15+inHg of vacuum at 80mph with better gears (like 3.00+) and given it only take a fixed amount of HP to make my Firebird cruise at 80mph regardless of rpm (more Hp would speed the car up, less would slow it down) if I got the vacuum up with a slight increase in rpm it would just mean the engine was running leaner (increased vacuum) with a better mechanical advantage in the rear end gears at the same speed and Hp. It's balancing rpm vs. mechanical advantage numerically higher gears. Now there is a point at which this assumption fails, and I think that is the point in which the cruise vacuum equals the idle vacuum in gear (engine loaded). As with this whole last paragraph this was a theory I was unable to test but will eventually with another car once I can generate another statistically relevant mpg sample with one of my cars and want to change the gears.  

dr511scj_1:
Ak Miller proved about twenty years ago in Hot Rod (with three Mercurys) that turning the engine as slow as possible at cruise (assuming it has enough torque to propel the vehicle) is the ticket to maximum mileage.

Miller ran cruise manifold vacuum as low as 5" hg on a 3800 pound, 255 cid Cougar (barely ticking over 1000 r.p.m. at the then top legal speed of 55 m.p.h.)  

To oversimplify, fewer r.p.m. means less air/fuel consumed.  Fewer r.p.m. also reduces internal friction losses.

The problems are throttle response and "big" intake tracts (cam, heads, intake, exhaust) that won't build enough low speed torque because of reversion to pull a tall gear/deep overdrive combination. (Additional problems may be fuel metering and mixture distribution at low intake air velocities)

In factory set ups, such as the GM LS-1s that Jeff Smith keeps ramming down our throats, final drive ratios (in overdrive) are commonly in the 1.7:1 range.

Given the facts that volumetric efficiency is extremely low at cruise (remember, manifold vacuum is usually between 10" to 15" h.g. in most street cars) and the power required for most cars to maintain a level cruise at speed is less than 25 h.p., raising the r.p.m. to any speed above what the engine will maintain without stumbling (incomplete burn because of excessive reversion and incomplete scavenging) trades extra fuel for driveability.    

I have yet to see any instrumented testing that contradicts Miller's data. Nor have I noticed any contrary results in my own testing.  Thus, I strongly suspect the "efficiency range" folk wisdom is a baseless canard.

This will assume you are on perfectly flat ground and do not need any changes in speed.  Once hills are thrown into the equation, a lower gear set is needed to prevent downshifting at every hill you come across (or dumping fuel down the intake into an engine that isn't changing its speed).

I did not lose any mileage when I switched from a 3.08 to a 3.73.  Actually my best recorded mileage was with the 3.73's...25 mpg, 100 mile trip (normally its 20 mpg...cool temps from rain, windows being up, and little traffic really helped).  But I travel through an area with quite a few hills.

So basically what you've said is correct...but the lowest amount of power to keep the car moving has changed...because of the addition of gravity acting against the car.  So now you must move higher in the RPM range to obtain maximum mileage.  

From Southern California to Las Vegas is mostly uphill to reference my experience.  

On my way to work today I checked what RPM I did at what speed. I did 2,500 rpm at about 70mph. 80mph would probably 2800 to 3000 RPM. So I am assuming that kinda alright isnt it? Also How can I measure or find out the MPG's. Also so the wheel size is not that big of a factor?  

Sounds like you probably have the 3.00:1 rear end if your tires are taller than original.  My T-bird has the 3.00:1 and stock rims and tires and runs a hair over 3000 at 80.  But unless you plan on changing rearend gears I wouldn't worry about it being a variable in your mileage, it's going to be a constant ratio.  

As for calculating your MPG the mathmatics are simple enough, take the miles you've driven and divide it by the amount of fuel you use.  Miles driven/Gallons. The most accurate measurements are made from complete fill up to complete fill up.  Error is introduced in the accuracy of your speedometer and Odometer as well as consistancy of filling up to the same level. For long trips I usually use a GPS for mileage and after the pump kicks off the first time I slowly pump the gas in until it shuts off again.

Wheel size matters in two ways.  Changing the height (diameter) of the tire is similar to changing the rear end gears but on a smaller scale.  A taller tire will result in lower RPM's.  The other way it matters is that a tire/wheel size change effects the accuracy of the speedometer/odometer so it can create the illusion of good or bad gas mileage depending on tire size.  In either case keep the proper amount of air in all the tires to maximize fuel mileage.