Referencing the T-5 article (Pull It Yourself Ford Five-Speeds) in the September 06 issue, Jeff Smith stated that anything over 9.5 (1st gear X rear gear) is overkill for the street. Does this apply to automatics as well? Given a 4000 lb car with a 2.84 1st gear, would 3.73s be too steep (10.6 overall)? Or would 3.55s (10. overall) be the better setup?

The engine is a warmed up 5.0 and O/D is .7

Thanks.

Plenty of folks run quick on the street with deep gears and  4-speed automatics.  So don't worry so much about first gear ratios.

Gear for maximum RPM at the trap speed your engine will pull.  For example, 500 h.p. in a 4,000 lb car ought to hit 118 mph in the traps.  Thus, the correct gear and tire size will have the engine hitting the peak power rpm in third gear at 118 mph in the traps.   

A deep first gear in an street-oriented AT isn't as much of a liabliity because shifing is easier and often quicker.  And it makes the car easier to drive at part throttle on the street.  However, the popularity of two-speed Powerglide automatics in high-torque drag cars suggests that reductions in potential torque multiplication are sometimes necessary.  A Powerglide is basically a three-speed automatic that's missing first gear (a miserable transmission on the street,  unless you've got 500+ lbs/ft of torque to make up its lack of "Low" gear) .

Relatively low torque "small block" street engines in heavy vehicles generally  need all the mechanical help they can get from gearing.

http://speedzzter.blogspot.com

given weight of car the 3.73s would be a better choice.....

nosnerd

It depends on how much power the car is making, the rpm at which it's making peak power,  and how tall the tires are.

Given what little we know about the combination, 3.73s are probably in the ball park.   But he might need as much as a 4.11 or as little as a 3.55 to hit the traps at the power peak.

BTW, don't forget that with gasoline at $3.00 a gallon,  deep gears kill mileage--even with a .7 overdrive.   During the last energy crisis, C.J. Baker once proved in his old BBC/TH400 Chevelle that using as little as a 2.73 and going through the traps at the power-peak in second, instead of third,  only cost a few tenths of E.T. but yielded significant mileage improvements.

Former PHR/Super Chevy editor Doug Marion achieved similar results with his 1970 Monte Carlo "Econoperformer" project car back in the day.

Perhaps CC ought to update this research . . . .

http://speedzzter.blogspot.com

Now why does eveyone think that lower rpm's = better gas mileage?

I have two testimonies.

Story A

- I have an 81' chevy c10....4 on the floor manual OD(3+1) with a 305 4bbl and 2.65 rear gears.

The truck gets better gas mileage at 85mph than at 65mph in 4th gear....yes....I have managed to knock down 18-19 MPG at 85mph in a v8 powered 4000lb truck.

Story B

- I also have a 1968 caprice with a 355....mild cam, headers, performer intake, and performer 600 carb....4.10 gears

While spinning 3600rpms at 60 mph I have gotten around 18-20 MPG with this set-ups as well. ...although overall top speed was slower.

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I dont think that just because you are turning a high rpm means that you are getting bad gas mileage or that you will get good gas mileage at a lower rpm.

I think it depends on the combo of cam and gears as well as state of tune.

God bless
-Shred

I can let you know for sure here in the next coming days...I just finished swapping a set of 3.73's into my 68 Cutlass.  Earlier this summer it knocked down 20 on the highway with 3.08's out back.  I was at 1700 rpm at 65 with the 3.08's...I'll be at 2050 rpm at 65 with the 3.73's (I guess I didn't mention it has an overdrive in it).  We'll see if there is any improvement.  Its an ideal test...same car, same tires, same engine, same trans, different gears.  

Getting good MPG can come from at alot of things. My 65 got 10-11 mpg with 3.36 gears and TH350. Then it got 14 mpg when I traded to a overdrive stick. Here's the confusing part. When I switched from Holley 650 double pumper to a Proform 750 with jets 4 and 6 sizes larger and a second power valve my car gets 17 mpg. I averaged close to 17 on the Anti-tour including the night of drag racing.

I guess the 750 allowed my engine to run more effeciently than the 650.

19 m.p.g. from a 305 with "Bonneville-tall" gearing really isn't much to brag about.

Of course, at a steady-state 85 m.p.h. cruise, aerodynamic drag is a far more significant contributor to fuel use than weight.  Weight becomes a major factor in acceleration, but once all that road-hugging mass is moving at speed, cutting through the air requires most of the fuel burn.

Such anecdotes are ok, but the late Ak Miller took a more scientific approach to this question years ago when consulting for Ford Motor Company's Lincoln-Mercury Division.   Miller slowed the engine speed at cruise on three different cars (a Capri, a Zephyr and a Cougar) and performed careful before and after testing.   Slowing down the engine at cruise yielded significant mileage gains every time.

I think it was Pat Ganahl who took issue with this theory in Hot Rod.  He built an SBC-powered street rod that was supposedly "optimized" to cruise at 3000 r.p.m.   But his car never made more than about 19 m.p.g., if memory serves.

The reason overdrive works is that slower engine speeds tend to reduce frictional, rotational and "pumping" losses. 

An engine obviously flows less air the slower you turn it.  Because of practical and mechanical limitations on how lean a conventional Otto-cycle engine can be run (especially with carburetion), reducing the amount of air pumped by the engine for a given amount of work performed through slowing it down will generally reduce the amount of fuel consumed.  Given that throttled engines run at abysmal volumetric efficiency at cruise (just cover 3/4 of your nose and see how easily you breath), reduction of rpm for a given work output reduces "pumping" losses.  

That's not to say that if a combination has a gigantic cam with lots of overlap and resulting exhaust reversion (reversion is what causes rough idle and low vacuum) or a fuel system which is excessively rich at part-throttle cruise, or perhaps a hogged-out induction tract with insufficient charge velocity, then an engine may not efficiently make the 15-30 h.p. necessary to "pull" tall gears at ordinary highway speeds with satisfactory driveability.

The average late model V8 F-body,  Corvette or Viper run effective factory final drive ratios (in overdrive) of 2.02:1 or less.  If cruise RPM didn't really matter, why would GM, Ford and DCX have spent millions developing overdrive technology.

http://speedzzter.blogspot.com

A double-pumper is hardly a mileage carb due to the fact that the accelerator pump shots tend to be overkill at less than full throttle openings, but the larger carb may have less "pressure drop" and thus air pressure through the bowl vents pushes less fuel through the jets.

(Background:  A carb works because of the "Bernoulli effect" which creates a pressure differential between the air flowing through the venturi and the static atmosphere pressurizing the fuel bowl* (an increase in air velocity decreases static pressure at the square of velocity).  The higher atmospheric pressure forces fuel through the metering system into the lower pressure venturi nozzle area.  It works just like when you create a low-pressure area in your mouth by sucking on a drink straw down at the drive-in--it's atmospheric pressure which is actually doing the bulk of the work (this would not apply to suction on fluids NOT pressurized by the atmosphere . . . . ))

*don't forget that even as you read this, you are under atmospheric pressure . . . .

http://speedzzter.blogspot.com

I would think tht cruise rpm would be lowered more commonly for  reliability than MPG's.

The bad thing about the 68' with 4.10 gears is that cruising at 4000rpms all day long lead to a rod knock and weak springs.

The truck turns 2000rpm's at 80mph all day long and will never break a sweat.

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Although I do understand that lower rpm's = less consumption. i still tink that just because an engine turns a high rpom doesnt mean that it will get or has to get poor mileage.

I also have an old volkswagen Golf GTI and its geared so close that at 80mph in overdrive(5th) I am turning 4100 rpms and I have done that for 3 1/2 hours on end and still got like 28mpg....albeit it only feeds 1.8 liters worth of engine.

It just runs very efficiently at that high of an RPM.
Im not really arguing...just conversing.

God bless
-Shred

I didn't go to the double pumper looking for great mpg. I went to it because of the power it produced under full throttle conditions. I rebuilt carbs for a speed shop and since we didn't have a test engine my car became the test engine. I had somewhere between 600 to 700 carbs on my car in a two and half year period. When I came across the 650 double pumper, I really liked the power. So, I kept the carb. The edelbrock 600 that I used to have was the best for mileage and reliability, but the car nosed over at 4800 rpm.

Now that I live in LA, I drive about 1100 miles round trip to see my family and friends in NorCal two, three times a year. Gas prices as they are, I started keeping track of mpg. I often thought about putting another Edelbrock on it for trips, but haven't done anything about it, yet.

Thanks for the replies.

The car is a 90 Lincoln Mark VII. 306 ci (supposedly..cheap rebuild several years ago) with AFR 165s, Trick Flow intake, 1.7 rr, conversion to mass air and currently 3.55s in the rear. I have a project going that will give me the steeper 2.84 1st gear. It hits the strip -maybe- twice a year and will make it's 5th Power Tour Long Haul next June. So I am looking to balance off the line with mpg.

Thanks again

"I also have an old volkswagen Golf GTI and its geared so close that at 80mph in overdrive(5th) I am turning 4100 rpms and I have done that for 3 1/2 hours on end and still got like 28mpg....albeit it only feeds 1.8 liters worth of engine."

The cruise airflow (and corresponding fuel flow at lean AFRs) and frictional and pumping losses of a 1.8-liter engine (even one cruising at 4100 r.p.m.) are substantially less than a 5.0 or larger engine. 

An archaic five-port BMC A-series four cylinder of 1.2 liters or less gets even more spectacular mileage buzzing at 4,000 r.p.m.  in a fly-weight original Mini Cooper or an MG Midget (despite the engine's horrible volumetric efficiency and low factory compression ratio), but that's not to say mileage won't substantially improve by lowering cruise engine speed.  (Of course driveability would suffer greatly).

Service life does tend to improve with slower speeds, but the real driving force behind modern overdrive gearing is maintaining Corporate Average Fuel Economy standards while allowing for more performance and the weight of optional equipment.  (the average 2006-model vehicle is over 500 pounds heavier than the average 1996 vehicle)

http://speedzzter.blogspot.com

To avoid the math of gear selection, you should play around with your specs at this site: http://www.kabamus.com/garage/gears.html

Assuming a 275/60-15 drag radial and a 6,000 r.p.m. peak engine speed,  3.55s will permit a theoretical maximum of 140 mph in the traps.  3.73s would lower it to 133 mph and 4.11s are only good for 121.   But if your power peak and desired trap RPM  is only 5,000 rpm, the 4.11s would limit trap speed to 101 mph.   3.73s would limit speed to 111 mph and 3.55s to 117 mph.   

Assuming your combination makes 400-425 h.p.,  (109-111 mph trap speed target)  the 3.73s might be the best compromise, putting you through the traps at around 5,000 rpm. 

Assuming a .67 overdrive (as is common to Ford AODs) 2,000 rpm with the 3.55 should be around 70 mph;  around 66 mph with the 3.73s and only 60 with the 4.11s. (that's about a 15% spread from best to worst--and that WILL show up at the gas pump).

The overall first gear ratio with a low 2.84 first gear doesn't vary much (from 11.6:1 with the 4.11s to 10.0:1 with the 3.55s).

http://speedzzter.blogspot.com

My rule has always been when you're picking up scraps of your connecting rods out of the field next to the 1000-ft mark, you've probably got too much gear . . . .