Earlier this month I drove my '64 Galaxie, with my rebuit engine, to Georgia and back for a total of about 1200 miles. Gas mileage averaged 12.3, which I thought was not too bad for this new setup. I now have slightly less than 1500 miles on it.

I think next summer I'm going to finally drive it to California (where it was born) and back (hopefully!), but think perhaps I should consider changing the rear. I'm running a 3.70 posi now. Is there a formula that I can use to determine mileage if using other gears, like 3.50 maybe? Would an open rear make any difference? Don't want to go to high, but better mileage might be nice on a 6000 mile trip. I'm running a toploader.

I could use the savings to buy more beverages on a beautiful California beach while fighting off the beach babes I hear abound there!

thats a lot of coin back in the pocket vs 12.3mpg. 3.50 you would probally see little differnce. gears and mileage you usally cant have both unless you have overdrive. but with 3.00 the car will probally be soggy in the take off buy strong on the highway.

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1963 thunderbird 416 tri power c6 3.89
1979 f150 466 mostly stock' my grandpa's old truck
1992 lincoln t-car
2003 dodge ram 1500

Here are some handy calculators:

http://wahiduddin.net/calc/calc_speed_rpm.htm

http://www.wallaceracing.com/gear-speed.php

Have never seen though a mileage versus gear ratio calculator as likely too many other variables come into play. For the effort, a switch to 3:50's is only a 6% reduction (3.50/3.70). A 3.0 gear would IMHO be better as that would reduce rpms by approx. 19%. IIRC, many auto equipped '63's and '64's came with 3.0's from the factory...my '63 352 2-door Galaxie did at least.

Also, an open versus posi (locker, Trac Loc) would make no difference in mileage nor rpms in high gear. It would be cheaper too to swap in a 3.0 open chunk too.....given that you would switch back to your current 3.7 gear chunk once you go home....assuming too you wouldn't fall into the clutches of one of those CA beach honeys and never come home! LOL.

3.00 was a common rear and I've had many 64's with them. I didn't want to go with that ratio, but then it's a long trip and would definitely make me stay outta the throttle. With the current rear, I'm running 3200 at 70 mph. Tires are 28" tall (2.75 x 15's) on the rear. Nice to have more calculator links...thanks.

put 300miles on it changed the oil and filter and went on a 150 mi one way trip to a car show. I averaged 14-15 mpg at 65 miles an hr. The car has 3.50 gears & toploader trans & 2-4s. Suprized me as the best I got before the rebuild was 11-12 mpg.

Try here
http://www.grimmjeeper.com/gears.html

If your combo needs to have a higher RPM to effectively fill the cylinders then going to a numerically lower gear could actually worsen your gas mileage.

If your combo is relatively stock then you would probably be happy with 2.75-1 or 3.00-1 gears, an open diff would not hurt either.


53 F100, Volare clip, 4 wheel disc 390-wide ratio toploader

390, but definitely not anywhere near stock. Cam range is 2600 to 6400. Good point you make. I'm sure I'll go with the open rear because I already have one and it's an easy swap. More things to save and consider for my planning book...thanks.

Planning, changing things, etc, can be a fun winter project.

Too high a gear will lug the engine and reduce mileage. Or you can do like me and just drive faster. I think 3.70 is a bit too low for optimum mileage and 3.00 or 3.25 would help quite a bit. It also depends on the tranny. If you have a close ratio 2.32 low Toploader first gear the 3.00's aren't going to impress the beach bunny when she says "I like guys that can peel out".

Better use 4.56's and just drive on the shoulder with the four ways on!

It is not as well wrung out as the hp/et Gonkulator but it is not too bad, it matches everything from my 8mpg-in-the-city cars to my old 38mpg highway Pinto. So here is some input based on real world data plus this new "MPGonkulator".

Our 3.00 geared c-6 390 64 Gal gets about 16mpg on 60mph road trips, and the MPGonk says 16.04mpg. At 70mph, the MPGonk says 14.55mpg.

With a 3.70 gear, the MPGonk numbers are 14.46mpg at 60mph and 13.03mpg at 70mph cruise.

Now, this is stock converter and Ford 390-2v cam so it depends somewhat on the cam - but for this car, slowing down to 60mph would be as helpful as putting in the tall 3.00 gear, which will NOT be as impressive after you reach the beach action....

Then again, it is not as safe to drive slower than the flow of traffic, so cruising at 60mph could add risk.

Similarly, I once had a 429 Torino set up for high mpg, 3.00 geared toploader. On a certain half hour road trip, it would get 21mpg in 4th gear (I had one of those old MPG Meters, wish they still existed). Running the same trip in the 1.29 3rd gear (3.87 gears or so) I got just about 18.5mpg, and the MPGonk concurs with this real data.

So I'd say, without knowing the details of your car so I am just guessing at them, if you swap from 3.70 to 3.00 you would get 13.3mpg instead of 12.3mpg.

Worth it? I guess too, that adds a risk of generating a seal leak during the swap, and not discovering it until it gets the brake linings wet halfway through the trip......

The current 12.3mpg seems low though, unless it was at 75mph or above. Are you trying to run a Holley double pumper and get good gas mileage or something like that? What color is inside the tailpipes after a good long cruise, it is it tuned for best MPG now?

Some interesting figures you post there. I don't mind too much cruising at 70 (although it's hard to stay at that nuber)and 3200 rpm. The rpm's go up tooo much for hourly runs involving 75+. With my combo, I'm looking for pit stops at 250 miles of before.

You Gonk'ed this setup for me in the early stages of engine planning, and I sorta built it around several of your estimates.

It's a +.040 390, forged pistons, 10.7 comp, ported/polished/milled heads, Hooker super comps, 2 1/2" mandrel exh sys with Dynomax turbo mufflers, tri-power, solid cam is: 240/240, 570/570, 108*, installed 2* adv, solids set at .021/.021 with the proper rocker assy., blueprinted mech adv dist, engine is balanced/blueprinted, timing is 18/38 I believe, 8 qt pan with dual remote filter and other stuff too that I can't think of at the moment. Plugs look very good running Autolite 32's, but the exh is blacker than I like, so I suppose I should rejet the carbs this winter. Hate to downsize the jets because it never loads up, idels great at 1k and really, really does well in mid to top end and never hesitates a second. Hope this info helps some... Thanks Werby.

Those are what the MPGonk says for a 70mph trip (this includes short stops every couple hours for gas and rest, etc.)

Given your setup, I sure wouldn't bother with the doggy gear, the MPGonk basically can't tell one gear from the other in this case.

It looks like there's more MPG in there.
Do you have vac advance too?
I think 38 total at cruise is not quite enough, that might add 1mpg, about 45 total (mech+vac) is usually about right at cruise.
The other is probably due to the jets, Ford made a big deal with the 390/401, stating that the jets were the same as the full-power runs. Probably too rich for MPG, maybe lean out the center carb 2 jet sizes just for the trip. It will be nice and warm cruising those Southern Cal beaches anyway so the leaner jets will be ok.

If it likes that, I'd drill the power valve holes out bigger and leave the leaner jets in for good, but drilling is kinda permanent.
Or get one of those fancy/pricey metering blocks for the center carb so you can tune it - - -

I'm a bit surprised that the Gonk didn't show more of a variation in mileage. The numbers you give would convince me not to bother changing the rear. Too much work and complete loss of performance for the miniscule savings, not to mention starting of in 2nd gear probably. I thought about this last night and decided I'd really hate 3.0 gears, regardless of gas savings. I didn't build this for 3.0's.

I have mech adv dist. I'll have to look at my notes from Faron to see what my total adv is; it may be over 40. I only checked it once this summer and forget what the actual reading was. I'll lower the center carb jet size a few numbers and see how that works. I do know that the performance was very noticeable after having the dist blueprinted by him, and that no vac advance on the street is absolutely no problem at all; just like he promised.

I did the math and compared the CR vs WR if I recall the close ratio was nearer to starting off in the wide ratios 2nd gear. I think it carried out to changing 3.50s to 2.75s. FoMoCo royally screwed up offering nothing but close ratios in 427,428 and 429s. The last thing these big block torque monster cars needed was tall first gears when trying to get out of the hole on street tires.
I asked because swapping in a wide ratio big in/out trans and dropping in a 3.00 or so rear would let you lower that rear end ratio and still have room to play LOL thats if you believe a FE has an adequately wide power band and that it does not act like a Boss 302!

Samll in/out

If you have a stout 3,-6,000 rpm cam that wants RPM to make power vs a 2,-5,000 mild cam what happens when you force that high RPM engine to struggle below the powerband as far as efficiency/fuel mileage?

..if so..a hi rpm cam would be inefficient down low......

I would think turning an engine at a lower rpm is the best way to get better mileage...which would mean a mild cam ????

My 65 F-100 with worn out 352, Autolite 4100 on a 76 cast iron truck intake, dual exhaust, 3 on the column, 3.25 9", 390 2V cam (198-201 dur...428-431 list) averages qround 15 mpg.


Richard.

I think it all has to do with throttle opening and engine vacuum. Anything that increases torque can potentially help mileage at highway speeds.

I've also found slower speeds are not the answer. You get exactly ZERO MPG when stationary. An engine uses so many pounds of fuel per hour at a given load. The faster you can go for that same load the more miles you travel for that amount of fuel.

For what it's worth i've gotten 20mpg from a 390 4 speed (always have a stick if you want mileage) 67 Ranchero at 4500 rpm with 4.11 gears.(Imperial gallons here) That's with a decently big cam and at that rpm hills don't even move the vacuum guage needle off 17.

If so about what speed? I know it's a loose number but I gather MPG info wherever I can.

If I had to give a rule of thumb I'd say that while peak efficiency, and likely lowest BSFC (brake specific fuel consumption) is at peak torq, the peak MPG is probably about 2/3 of that RPM. I'd bet on that 2/3 for anything from a vanilla cam to about the roughest thing that most of us would run out on the street. It's also close to the way the factory gears things for best MPG. Not a magic number but a rough estimate.

...then again it is almost 40 C.I. smaller than a 390....

I also drive like a granny..the last time I raced on a street
that was deserted I hit a huge light pole in my 67 notchback Barracuda going 110 or so...needless to say it messed the car up and cracked the block (340).

I had amnesia for 3 months and didnt even know I had owned the Barracuda till I saw it in a junkyard close to my house.
I had no idea how it got there...

I built that car up from a shell.....sniff....sad..


Richard.

Long ago (30 years now) an auto engineer told me that the best mileage (auto trans) is in high gear at the lowest rpm (where the trans stays in high). Hadn't paid a of of attention to it until the last year of so with our recently high gas prices. So, I played around this summer with my daily driver car (Nissan Altima 2.5, sorry!). Most cars today with lock-up converters shift into high, with lockup, at about 45 mph. Wow, was he right as normal mileage (32 or so) went to 42-43 mpg at a steady, cruise controlled speed of about 45 mph. Admittedly, not very practical for our poster's trip to CA but it does prove a point.



This also may not work well with a heavily modified engine (rpm's may be too low for efficient cylinder filling as stated above) but if the rpms at cruise (say 65 mph for the heck of it) are kept right at the low end of the cam's range (say 2,500 in a cam set for 2,500-6,500 rpm) methinks this would produce the best mileage.



Someone else mentioned a moto-meter (sp?) vacuum gauge which really was merely a dash installed vacuum gauge. Having an easily read meter today would help to keep manifold vac. at the highest level thereby giving the best mileage. Too bad it takes a lot of effort to avoid nailing that FE powered beast from time to time!

http://www.canadiandriver.com/articles/jk/080507.htm

from home---Livonia---to Sebring, FL for the 12 Hours. Swapped in a 3.50 instead of the factory 4.11. Put on 8.20-15s. I-75 was not completed. Ran 100+ as much as possible, with best top just over 150. Trip average=11.7 MPG. One tank best 16.1 MPG. Fun factor---at least a zillion. (Sunoco 260= thirty five cents)
KS

Ken,
Thanks for some good old data.
The MPGonk got 11.3mpg for an "average 100mph cruise" road trip in your 427/425hp.
For the 150mph portion [wow] the MPGonk said 6.8mpg.
Best was a cruise loafing along at 50mph, for 15.6mpg.
Slower than that, the mpg started dropping off.
Maybe that explains the 16.1mpg tankful? Or a downhill portion?

I think Holley's are not too good on gas, BUT - the more load they see, the better they get. So the MPG disadvantage of a Holley would mostly go away cruising at 100mph plus. It might explain Ford's use of the Holley 4v in a lot of trucks - if they are run mostly under load, any MPG disadvantage disappears. Haven't really quantified it yet, but if your car was cruising on just a pair of barrels from one of the 550 Holley's, it might explain your actually pretty impressive MPG.

Maybe Ford is reading our posts on this, it should not be that hard to get a 40mpg Ford into the showroom.

When your pulling 12" of vacuum that means your engines a 8cyl _?_Cu In vacuum pump thats sucking like hell to produce that vaccum. Where as a Diesel with no vacuum to contend with is controlled by simply adding fuel. When your cruising at 70MPH with 18" your MPGs burdened by a big air pump.
I recall a 924 Porsche Ad that claimed because of the cars low drag Coeffcient it took only 5Hp to maintain 60 MPH on the Hwy. Always dreamed a WOT tiny engine of just enough power to push the car down the Hwy @ 70mph would be the way to get mileage, to get it up to speed you add a Turbo and maybe even NOS to give it that additional power when needed. 400Hp is a MPG joke if it takes only 25Hp to maintain 70MPH in an old Ford

Closed course, professional driver, don't try this at home... besides the statute of limitations has limitations... right?

Bakersfield to Las Vegas in two hours (including stop at In&Out Burger), 31.5 miles per imperial gallon (20% bigger than US gallon). At RPM's above 4000 and as high as 5500 in a .67 5th gear with 3.55 rear end ratio and 26" tall tires. Fuel injected 1988 5.0L Mustang with E303 cam and shorty headers, no cats.

Car got it's best mileage on that segment of the trip and it was uphill!

I did a run to LA and back, from Vegas. I got 13 and change. This was a 434 inch 427, Offy P-sonic, 1000 Holley, Isky EE-390 250@.050, 108 LSA on 105 and a Toploader with a 3.70 rear

I did the same run with the same setup and a 3.00 gear not long after.

Guess what? Same mileage, but MUCH quieter LOL

Now mich was clearly outside of an efficient RPM range with the taller gears, but it was also soooo much more comfortable

With that being said, my 489 now has a 2.36, yes TWO.POINT THREE SIX, final drive (in 5th) and with the added stroke sure does love highway driving


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- 70 Fastback Mustang, 489 cid FE, TKO-600 5 speed, 3.70 9 inch
- 71 F100 shortbed 4x4, 397 cid FE, headers, Street Dominator, 280H, 5 lug Dana 60, 4 speed