fuel economy

[Dale99]

I know driving a classic like this, economy is not something to be truly concerned about, its more about the style. But I was wondering how many gallons per mile everyone is getting? When stating, please post type engine, trans, gears, tire size, and any other factors?


I just figured up, I think I'm getting around 4mpg in town.
350ci, 650cfm double pumper, th350, unknown gears, 33" tires.

[72blazerblue]

Well I was under the impression I had the worst on here, everyone seems to get around 10-14 from what I've seen.
I am getting roughly 8, and I don't have enough miles on the highway to say a highway mileage. I am running a 350 SBC bored to a 355 with a th350 on 32x11.50in tiress and a 2 inch lift.

[72blazerblue]

Oh and I have the 3.08 gear (3.07?); missed that one!

[Dale99]

I figured I would get 8-10. So the 4mpg is unexpected. I am thinking I have some steep gears since this truck was originally a 6cyl.

I used just a hair under 1/8 of a 30gal tank, to go 12 miles.

[RANDAL]

I get 15mpg if I drive it nice or 8mpg hot roddin it. im runnin 350sbc sm465 with 3.73 gear 33x12.5x15 bfg all terrains. not bad I think for being a 4 speed and 42 years old

[in deep 72]

4 mpg here as well. Doesn't bother me too much except only getting 80 miles to a tank sucks!

468BB/350/205, 40x18 Boggers, 4.10 gear

Planning on swapping gears to 4.88....afraid what that's going to do to the mileage!

[SamMargritz00]

17 mpg! haha but I did a LS swap. mixed city/highway I get around 15. 2002 5.3L/4L60E. 3:73 gears, 33 in tires. A lot better than when it had the worn out 350 in it. Got about.......6-8 haha.

[69-72]

I have got as high as 17-18 last summer on the highway. Stock 350 with 130,000 miles on it, 4 spd manual new BFG 31" tires. No idea on the gears, probably the efficient kind since its as slow as a turtle off the line.

[Dale99]

indeep, atleast yours would be a fun 4mpg!!

[truckster]

350/350 with a Quadrajet, 3.73 gears and 33 inch tires. I average 11 or so.

I'm getting it ready for a Vortec, so I'm hoping to do at least a couple of MPG better. Of course, I just got a job for after I retire from the Army, so my commute will go from 40 miles/day to about 9 - that should help too.

[RED 1970]

1970 - 4" lift- 35" BFG AT's - 350/TH350 - 3.73 gears - 10 mpg all day long.
Edelbrock 1406 / Performer manifold / MSD ignition / Headman long tube headers


Dale99, when was the last time you went through your carb?

[Rufton]

As a minimum, I'd pull a plug following cruise shut down and make sure it's just off white.
Wideband O2 gauge plus carb tuning will pay for itself in short order in 4 mpg case.

[SACRAMENTO 71]

LT1 350 w/ fuel injection & a 700r4, unknown gears on 38x14.50. Avg 12 MPG

Could be getting 3 MPG, still love my old girl!!

[1972CST]

1972 K5: 350/ TH350/ 3.07 gears/ 33" mud tires= 12.9mpg (35-55mph)
I havnt checked mpg's after swapping to 4.10 gears.

[Dale99]

Originally Posted by RED 1970

Dale99, when was the last time you went through your carb?

Couple months ago when I bought this thing. I have not one single clue about carbs n how to adjust them. Id rather throw it is far as I can and bolt on some fuel injection. But I have other things to do before I go that route.

But this thread does show mine is below average which is what I was wanting to know. Tells me something is off. Now its a matter finding out if its tuning, tire alignment, friction in drive train, or my foot....

[Rufton]

Could be a combo deal.

I've seen alignment off pretty bad and not affect mpg that much.
If tires are snow plowing then the edges will wear fast.

If clutch is slipping you'd probably notice.

Late timing will impact mpg.

Carb is often person of interest. Some carbs are almost fun to tune.

[Mike C]

70 mph KILLS fuel economy. Auto trans with no lockup converter KILLS fuel economy. When I was going to school in Lubbock I drove back and forth to Austin at least once a month. With my .030 327 (331) and SM465 with 35" tires (Mickey Thompson Baja Belteds at the time) and 4.10 gears I got 14-15 mpg. But that was when speed limit was 55... Truck still gets about same at those speeds, but falls off dramatically as speed goes over 65. Going from a Holley 750 to a Q-jet helped a lot in driveability and a tiny bit in fuel economy.

But you guys getting 8-10 (except the near 500 ci big block guys....) need to look into tuning/tweaking your combo. Should be able to squeak 12 out of it, and from 8, that's a 50% increase!!!!! Help buy a ton of new parts.

[Dale99]

I checked alignment a little bit ago, appears fairly good.
No clutch
Timing could be an issue. I have no timing tab. Also, the drivetrain is in at an angle and the distrib rubs on the firewall constantly changing. The last part to correct all this is on its way now.

Tried checking rear axle ratio. Keep coming up with about a 1.73'ish. I know thats not right, lol. I will likely work on this while fixing my drivetrain angle issue.

[Rufton]

Originally Posted by Mike C

70 mph KILLS fuel economy. Auto trans with no lockup converter KILLS fuel economy.

I get about 18 mpg at 70 mph: V8 car, 2.79 ratio, no lockup auto, no OD, 24" tires. I have 40 data points and most of them are 70 mph hwy.

[Rufton]

Originally Posted by Dale99

I checked alignment a little bit ago, appears fairly good.
No clutch
Timing could be an issue. I have no timing tab. Also, the drivetrain is in at an angle and the distrib rubs on the firewall constantly changing. The last part to correct all this is on its way now.

Tried checking rear axle ratio. Keep coming up with about a 1.73'ish. I know thats not right, lol. I will likely work on this while fixing my drivetrain angle issue.

Defective auto trans can slip too much also.
Unknown timing is a big suspect.

[Mike C]

Originally Posted by Rufton

I get about 18 mpg at 70 mph: V8 car, 2.79 ratio, no lockup auto, no OD, 24" tires. I have 40 data points and most of them are 70 mph hwy.

This was specifically about Blazer/Jimmy, but if you want to look at cars, my BMW 325i gets 36 mpg at 60 mph, it gets 33 at 65 mph, it gets 30 at 75 and 28 at 80. This has been documented over 70k miles that we have driven it. That's with a 2.5l 6 cylinder 3.15 gears and a 1:1 top gear 5 speed. 3500 rpm at 80 mph. Oh, and it is a GPS verified speedo. But obviously every vehicle is different and getting it to operate at peak efficiency at a speed you drive is really important. And the aerodynamics of a Blazer with a 4" lift and 35" tires sucks no matter how you look at it.

Here is an interesting calculator that pretty much mirrors my experience.
http://www.mpgforspeed.com/

A 550 hp C6 'vette with a 6 speed can get nearly 30 mpg at 75 mph because it is amazingly aerodynamic and the motor is just idling in 6th gear. But around town it gets 16.

[Rufton]

Originally Posted by Mike C

This was specifically about Blazer/Jimmy, but if you want to look at cars, my BMW 325i gets 36 mpg at 60 mph, it gets 33 at 65 mph, it gets 30 at 75 and 28 at 80. .

I know. Just presenting an old school carb 60's car 70 mph data point with drive train technology similar to blazer.

[IwantaK5]

Originally Posted by Mike C

This was specifically about Blazer/Jimmy, but if you want to look at cars, my BMW 325i gets 36 mpg at 60 mph, it gets 33 at 65 mph, it gets 30 at 75 and 28 at 80.

Does your bmw do the annoying "turn the engine off every time you come to a complete stop" thing?

[jthomass]

I believe that the holley double pumper will be dumping fuel with the primaries and secondaries everytime you give it any throttle. Where the vacuum secondary carbs will only dump fuel from the secondaries at wide open throttle.
Seems like switching from a mechanical sec carb to a vacuum sec carb could help quite a bit unless you are at WOT a lot. Just a thought?

[Dale99]

jthomass. I will take your word for it. I know nothing about carbs.