Gas mileage in older vehicles

Blaze

Blaze

The Jeeper Reaper
Joined
Aug 9, 2005
Location
Wake Forest, NC
OK, been thinking in my head here, because I love building off the wall crap.

Say I was going to take a 64 Impala and build it.

If you take a Late-model Pontiac G8 and look at it, and I may be looking at this all wrong but hear me out here.

G8 Gt:
Curb weight: 3995lbs
Rear gears (auto): 2.92
MPG: 15/24

64 Impala:
Curb Weight: 3525lbs
Rear gears: 3.08 (probably)
MPG: like 10 from what I can find.

Now, I would think if I took a 65 Impala and put a late model engine in it with a modern overdrive transmission, maybe bumped the gearing down to a 2.73 or something, I could easily get what a G8 does, probably better since I'd be shaving off like 400lbs.

Am I thinking about this all wrong? Not like it really matters to me, but it sure does to my wife. :lol:

Probably won't be doing anything for a long time since I have a reliable daily driver right now, but for future reference.....
 
My wife has a 1999 Chrysler Concorde Lxi that we bought with 199,000 miles on it. It currently has over 240,000 miles on it and on the highway, it gets 27-29 mpg. Around town it drops to about 15, but as a highway driver, it rocks expecially since we only paid $1200 for it and we have MAYBE put $500 into repairs since we've owned it.
 
I'm talking about OLD stuff, like vintage vehicles. I know some of the 90s stuff got good mileage, my 95 Voyager got pretty decent mileage for a powered brick. They don't weigh as much and have all the modern transmissions and fuel injection and all.
 
aerodynamics (lack thereoff) of the impala might negate some of the expected fuel mileage gains for highway driving. I wouldnt think that much though. Sounds like a good plan.
I used to drive a stock 70 Chevelle/300hp 350/4-bbl/4spd on regular leaded gas, got around 9 mpg IIRC. I was 16, more concerned with burnouts, etc than fuel economy, but I did measure it occasionally for kicks.
 
64 Impala not nearly as aerodynamic as the G8, but that probably doesn't matter much until you get into the upper mph range (ie running 70+mph on the interstate).
 
Right, definitely less aerodynamic, but I wonder if the aerodynamics would equal the extra weight of the newer car.

Hell, my little Subie only gets 24/27 right now. If I was able to pull 16/24 or so in something a lot more pimp, that would be awesome. :lol:
 
Remember MPGs are affected by lots of things, not just weight and engine efficiency (although those are big).
E.g. aerodynamics can make a big difference. Just adding a roof rack knocked 3 mpgs off of my wife's car.

EDIT - took too long to type this ;-)
 
RatLabGuy said:
Just adding a roof rack knocked 3 mpgs off of my wife's car.

EDIT - took too long to type this ;-)
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Wow, I wouldn't have suspected that much of a decrease. That is crazy.
 
I had a 80's C4 beater Corvette, pulled the 4+3 trans out and put a T56 in it with a bunch of other mods, it knocked down 30 plus mpg all day long on the highway, I put 180,000 miles on it. Sold it when i had kids, would love to do it again but with a 4cyl turbo diesel up front, I bet it would do over 50 mpg easy on the highway.
 
I had a 68 Galaxie fast back. 390 2.95 gears and c-6 auto. I got 15-16 around town a little better on the interstate. It had so much torque and with the high, I could just about idle every where.
 
drkelly said:
Wow, I wouldn't have suspected that much of a decrease. That is crazy.
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I was shocked as well. Took the rack off, MPGs went right back.

This is a Mazda5 mini-minivan.
 
Blaze said:
You guys might think I am totally crazy, but I was thinking about trying to find my grandmother's old 75 Valiant Brougham.

Wonder what kind of drivetrain options I'd have there.....
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4.7 setup out of a 2wd ram pick-up or oohhoohhhoohh a 5.7 hemi to keep it brand loyal and be economical and then when wifey not looking it would still haul A$$ :driver:
 
That's kind of what I was thinking.

I have a few feelers out there to see if I can find it. My grandfather is trying to see if he can dig up the VIN for me so I can try and find the owner. My grandmother died in 1993, so I would love to have her car back.

Definitely something you would NEVER see.
 
Weight is your main factor for city MPG, aero is your main factor for highway MPG, and the transition zone is about 45-65mph, but will really depend on a ton of incalculable factors.

Another big driver for both is rotational mass and rolling resistance. Smaller, lighter wheels and tires combined with disc brakes instead of drums will help. Newer cars all use ball bearings instead of tapered roller bearings. Transaxles eliminate driveshafts and standard axles with ring and pinion.
 
Opposite end.....

My 94 4cyl yj got about 14mpg. I'm installing a new 351m and expect to get better mileage with bigger tires.
 
GotWood said:
Opposite end.....

My 94 4cyl yj got about 14mpg. I'm installing a new 351m and expect to get better mileage with bigger tires.
Click to expand...

About ten yrs ago I pulled the 4cyl out of my mostly stock 93 yj and swapped in a TBI sbc/700r4 with 3/4 ton full widths and 35s (was my DD). I went from 14 mpg to 17.5 mpg
 
I'm knocking back a consistent (and easily obtainable) 20 mpg in my K30 on 39s with the Cummins in it :D I'm a huge fan of Cummins diesels and LSx motors. I just swapped a 6.0 in my Silverado. While I don't expect it to be amazing on mileage, I'm sure I could build a 4.8/5.3 that would be. Gearing, tire size, weight, and aerodynamics are big factors. Gearing is probably the most overlooked, honestly. People figure they can just stuff a 2.73 gear in it and get amazing mileage, but if you're under the optimum operating range for whatever motor you're using it's going to lug it down and suck some fuel. For example, my dad 85 K10. 3.08 gears, 700R4, and 31x10.50s. 10 mpg! I bet with 3.73s/4.10s it would be much better and feel way more peppy. In addition to that, just remember that a motor is just an air pump. Easy in, easy out usually means good power and efficiency :driver:

BTW, my next project is putting the 4.8 from my Silverado in my 85 S10 with a T56 or T5. Going to go for good mileage and a sleeper/street truck.
 
NC-V said:
About ten yrs ago I pulled the 4cyl out of my mostly stock 93 yj and swapped in a TBI sbc/700r4 with 3/4 ton full widths and 35s (was my DD). I went from 14 mpg to 17.5 mpg
Click to expand...

It's hard to truely varify MPG on lifted Jeeps, etc unless you can be sure that your gearing stayed in ratio to the old gearing. There's a lot of variables there, but one thing is for sure, 4 cyl Jeeps do NOT get very good gas milage because they are overworked. Same thing goes for my mini-van. When I worked for Ecolab, I had a company vehicle and my first one was a 6 cyl Town & Countryminivan that was nearly identical (same color even!) than my wife's Dodge Caravan. Main difference was the Caravan was a 4 cyl. I would consitantly get 2-3 mpg BETTER than hers. We even tested it out on a long trip when we took both. We loade them as close to identical as we could (Mythbusters style) accounting for the various weights of passengers and cargo. The 6 cyl won by almost 5 mpg on that trip. She had to fill her tank almost 70 miles sooner than mine as well.


Power to weight to gear ratio is key with MPG. You can stick a 100 mpg scooter motor in a Suburban and guess what... you will have the world's crappiest gas milage in an SUV.
 
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