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Gas Mileage Test 5 speed vs 6 speed

10K views 42 replies 20 participants last post by  427L88 
#1 ·
Over the weekend a buddy of mine and I tested our OD's on the highway to see what kind of fuel economy our cars could do.

Mine is the 71 Chevelle SS454 with T56 Magnum and 3.91 gears.

His is the 68 Chevelle SS396 with LS6 454 crate engine, TKO600, and 4.11 gears.

Both having EFI, similar weight and power, a lot of the comparison came down to the extra overdrive gear since it was highway only.

What kind of mileage do you guys get?

 
#8 ·
1969 350" in 1957 4 door 3650 with driver then add a 20 foot pontoon boat.
Yukon OK to Texoma on Texas side 190 miles.
Averaged 11.5 MPG.
I August of this year with AC on and 1969 350" Ported 601 heads 280H magnum flat tappet RPM intake and 750 Edelbrock 1407. Recurved 1978 HEI.
3.70 gears NO overdrive TH 350.

Yea not a BBC.
Drove a Dually tow truck I stuck a 454 into, had granny 4 speed and 4.11 gears .. pulling a car for 1/2 the trip.. mostly city I tuned it to 12 MPG dropping the APT adjustment on the Quadrajet and Recurved the HEI.
Before APT adjustment and recurve it would get 6 MPG.

I feel the Andrews 68 Chevelle could have got better MPG if he was a little leaner in the cruise.
My part throttle cruise from 2200-3200 rpm cruise is in the 14.5 area and 18 MPG is the normal on highway.

But maybe his car does not like that..I am sure he has thought about that or tried it though.
 
#12 ·
Recently, I went for a nice little drive for a couple of hours. A total of 72 miles were driven. About 15 miles were interstate. The remainder of the miles were "stop and go city traffic" and some back road driving. Interstate was driven at 70-75 mph (6th gear) and the back roads around 50-ish mph (5th gear). Hammered on it pretty good getting onto the interstate, the rest of the time was pretty normal driving, but not "babying" it. Topped the tank off and it took right at 6 gallons to fill it up to the spout. 12 mpg. I don't think that's too bad considering my combo.......496, 10.5 cr, Brodix rect port heads, solid roller (256/264, .660/.660, 108lsa), Vic Jr, Pro Systems Venom-VX 1050 carb, Tremec 6 speed, 4.56 rear gears with 30" tires. Last year did a short "all interstate" drive and was able to squeak out a little over 15mpg. With this drive I was really watching how I drove and "babied it" quiet abit. :yes: :hurray:
 
#16 ·
Thanks for the info, this is helpful. I've started looking into changing out my 4 speed for a 5 or 6 speed. A lot of people are steering me towards the 6 speed, did not realize it would make a 6 mpg difference. For me it's mainly the RPM's. My longest highway trips now are 20 minutes, just don't want to listen to the 427 @3200 rpms for longer than that.

Just wanted to add, one of my son's drives a car with a 5 speed and one drives a car with a 6 speed. I drive these cars regularly. When I drive the Chevelle and start going thru the gears, I always keep shifting as if I had at least a 5 speed. It seams natural, especially when your in 4th at 40 mph.
 
#18 ·
My 66 gets 12mpg (my setup is in my signature below). When I calculated it, there was no highway driving, just stoplight-to-stoplight driving around town. I hardly ever drive it on the highway, but with 3.42 gears and no OD I don't expect it would be any better on the highway.
 
#25 ·
It was a cool video, but not a fair comparison. Five speed, six speed, it doesn't matter. It comes down to RPM and final drive ratio. I'm not surprised the 68 did worse. It had what, 800 RPM disadvantage with taller rear gears and barely any overdrive. I'd bet it would do a lot better mileage wise with a TKO600 with the .64 5th gear instead of .82. A good baseline would have been a road trip with both cars never going past 4th gear at 1:1, just to see how close they really are. Still cool as hell seeing you get out on the road and have fun though.
 
#38 ·
20mpg with 500 plus cubes? Thats better than our almost new 5.0 F150.
If thats real more power to the owner seems way out there. Is that some type of avg instant mpg or the whole tank?
How is this being calulated?


Id bet my sb barely gets double digits hwy if I was real careful next to zip around town it seems. HArd to track with old gauges really
 
#40 ·
My '65 Bel Air Wagon will get 18.2 on the highway with the AC on high, cruising at 68-70mph. Cruises at 2600 rpm. 350", 305 'HO' heads, Holley 650 spreadbore, EB performer, 1 5/8" long tubes with an unknown cam that sounds a step or two above stock. HEI built & tuned by me, with a stock vac can on full manifold vacuum and a Crane vac can travel limiter (10* vac advance) & springs (blue/yellow). TH350, 3.08's 28" rear tires. I recently drove it from Little Rock, AR to Pittsburgh, PA and that's what it got over the 900 mile trip, all interstate on 89 octane gas. Lots of time spent tuning to get it running clean from start/idle to full throttle. Runs great.

Once upon a time, my '69 C10 had a stock '78 454 out of a 3/4 ton GMC, an 'RV cam' and a TKO-500 (.68 OD) with 3.73's. It also had a little 650 Holley spreadbore with a cast iron GM intake & exh manifolds on it and would get 15mpg on the highway without any real tuning.

Now that same C10 has a 532" engine and a TREMEC Magnum (2.66 1st/.6 OD) and I can't keep my foot out of it long enough to find out what it gets. It's got a 1050cfm carburetor, so....

My '61 Bel Air has a ZZ454, M21 Muncie & 4.10's. Not very good hwy mileage on this one either. Around 10mpg as 70mph is 3600.
 
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#41 ·
You guys are killin' me!! MPG really inst a huge issue with these cars, but DANG man, I dont want to rev my new engine like I did the 427 for 3250 rpms at hours at a time. DAG NAB IT! Here comes a Gear Vendors. ( no 5 or 6 speed, sorry).

PLUS, the new 600 hp bullet can hang at 1700 rpms cruise w/o issue, so it needs a gear reduction @ 75 mph.
 
#42 ·

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#43 ·
heck man, we got 15 + from a tripower. To each his own Jackie, but I've pulled Chevelles a few time with buds, and it seems too much like work to be of any fun to me. Most of my fun is the driving. And the folks at the rendevous, wherever it may be.
 
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