Horrible Fuel Mileage

[my302]

So the other day I realized I was getting about 7 mpg out of my 75 bronco with a stock 302. WTF!! I was wondering what I can do because there is obviously something wrong. I really don't know where to start though. I was thinking of spark plugs and what not. A few things to keep in mind, I am running 4:88 gears, 33" tires and a NP435. I also have a 470 Holley Truck Avenger which is too small for the 302. Would the 470 TA be causing some or a lot of this? How about the gears? When I run on the highway I run about 2700 RPM tops. The idle sounds good and nothing is indicating vacuum leaks. Any ideas?? Or money, if people would just give me LOTS of money I wouldn't have to worry about this.

[landshark]

That's pretty high gearing for 33's and a np 435 but have a feeling that's not the whole problem maybe time for a tune up. Plug wires rotor. Carb rebuid.

[Crazyone]

Holley says that the 470 is too small for a 302. Ask Zen on here, he had the same problem with an Avenger 470. I believe he solved it by trading it for a 670. I know that his 470 would run really rich.

[my302]

I've been talking to Holley and I'm going to be sending the 470 in with $75 for a 670 Truck Avenger. The tech did say the 470's do run rich, but surely not that bad. Also, I'm going to do the whole spark plug, wires and distributor thing. I'm wondering if its just everything together (gears, carb, plugs) or if there's something really out of wack. Everything seems to be running fine until I get to the pump. Ouch!

[Jesus_man]

What are some of you other carb guys around here running for MPG's? My fear is you won't see night/day improvement over 7. But hey, even 8 is a 13% improvement!

If you want fuel mileage, you'll have to look at alternative fuels - like diesel. But EFI will help a fair amount as well.

[Crazyone]

I have a 670 Street Avenger that I get about 9-10 mpg in town and about 13 hwy. I know that the truck avengers are jetted differently and I believe that the secondary jets are a larger step up from the primaries versus the street carbs. If you are into the secondaries at all the mpg goes down considerably. That being said, you should still do a little better with the 670 over the 470.

[Kinder]

7 is bad in my experience, but not the worst there is. I believe the best I got out of my 302 w/ motorcraft 2100 2bbl was 12-13 mpg, Bubba Smurf is getting 10 mpg with a 351 / 2100 combo last time we checked. The 351 is a little tired, so I'm not too worried about it, keeping my eye out for a modern roller to swap out.

EFI guys can post their mileage, but I don't think many folks are seeing anything above 15mpg with mixed driving unless their running big gears and skinny tires.

I'd do a complete tune up and look for problems in the drivetrain.

[Justin]

I'll second kinder's response. I'd say that 7 is low but not unheard of, especially if you've got a heavy foot. I suspect that the carb change along with tuning and some tune-up work should make a difference.

[Colorado75bronc]

With a Holley 600, 302, np435, 35's and 4.10's I average 10-11 in town, 12-13 hwy running 75-80, but it has reached 15 three times on the way to Denver running 65

[ZOSO]

when we went to moab in may Viperwolf got 17mpg goin out there running 70mph.

[Digger]

A 470 carb seems small for the application. I just bought my Bronco, so I have been tinkering with the carb a lot lately. (PO admitted it was running rich and engine has dieseling problems at shut down as a result thanks to carbon buildup)

Carb tuning methodology is similar to EFI strategy. At light throttle, the air/fuel mix is fairly lean for economy, at mid throttle, you are near 14.7:1, and at heavy throttle you are running rich, but you are trying to make power. Being the 470 is a small carb, you are probably opening the secondaries on the highway and they are going to be jetted slightly richer for power and therefore waste more fuel.

I'm running a mildly built 351W with a Holley 650 Double Pumper with a truck avenger vent tube, doing 65 mph, I'm barely in the throttle, meaning I'm only using the primaries. (you can hear when the secondaries open up)

Holley jets their carbs from the factory for an average engine at sea level. Mine 650DP for example came stock with 67 primary and 73 secondary. I've re-jetted it to 64/70 for the Denver area. Holley recommends dropping one jet size for every 2000ft of elevation gain. Look at your carb # on the choke tower and visit this site:
http://www.allcarbs.com/tech.php?art=21
That will tell you what your 470's stock jetting was, then you can check to see if it was rejetted for Colorado.

One other thing that probably doesn't apply to you, but when I got my truck, it has an aggressive cam grind and only make 9-10 inches of vacuum at idle, which meant the stock #65 power valve was dumping fuel in the engine at unnecessary times, therefore enriching the A/F ratio and wasting gas. I has since stepped down to a #45.

[Digger]

Since I haven't had my Bronco very long, I thought I would update my previous post with a little more information regarding mileage and carb tuning.

My last jet change, by Holley's recommendation, should have left my truck jetted for 6000 ft ASL. Turns out they were really close. Under certain conditions, especially on these 90+ degree days, I would get occasional pinging from the engine. I drove it into the mountains today, listening and watching altitude. At 6500 ft, I could still get it to ping. All pinging was gone by 7000 ft. Armed with this knowledge, I will step back up one jet size, which is two sizes below stock. (4000 ft ASL)

Ironically I had the exact same experience with the carb on my Yamaha ATV. Recommended jetting for 5000-6000 ft was too lean and I ended up closer to jetting the factory recommended for 4000 ft.

Performance:
Going back and forth to work is not very impressive. With mixed stop-and-go and highway the truck is getting 12-13 mpg. However pure highway performance was better than expected. Using the same gas pump every time and a GPS to record mileage, the truck is getting 15.8 mpg. This is with 351W with 9:1 compression, a NP435, 4.10s and 35's. I kept speed between 55-60mph the whole time and drove it trying to keep the secondaries close. This wasn't too hard with a 650cfm carb. With overdrive and EFI, I hope to push that to 17-19 mpg.

[Justin]

Given what you're driving, 15.8 is darn impressive. I'm not sure, but I could be getting single digits.

[landshark]

I'm getting mid 13's around town on my new EFI rebuild, mild 302 with stock 3.50 gears on 30" BFG's C4 auto. I haven't had it on the highway but expect somewhere in the neighborhood of 15 -16 MPG.

Viper got close to 17 going to Moab on the highway...

[HIX]

Currently running stock original 302, 2bbl carb, zf5 tranny, np205 with 32x11.5 tires and stock gearing. Around town I get 14ish and on the highway @ 65ish I can consistently get 16-17 as long as I am not romping on it. Best tank ever was my shake down drive on 70 east from Denver. I drove 172 miles (according to the gps) on the front aux tank before it went dry. Keep in mind that I did the hypermiling thing...steady 60mph on flats in 5th gear and when I came to a hill I would keep the pedal in the same position so I would accelerate downhill and slow going uphill so it was an exercise in patience. I did this same run with the stock 3spd and d20 before I pulled the tranny out and got 148 miles before running dry so on the surface it would appear that I added 3-4mpg by swapping in the zf5 but it could also have been due to a tailwind, better gas, air temp/humidity or something like that.

[Kinder]

I finally did a quick mpg check, 19.2 mpg mixed driving (around town, interstate & cruising up into the mtns to Idaho Springs) and loaded with camping gear for four. I will have to run a few tanks through it and not smash the skinny pedal to find out what I'm really getting. I'm looking for anything over 20 mpg.

[Digger]

I finally did a quick mpg check, 19.2 mpg mixed driving (around town, interstate & cruising up into the mtns to Idaho Springs) and loaded with camping gear for four. I will have to run a few tanks through it and not smash the skinny pedal to find out what I'm really getting. I'm looking for anything over 20 mpg.

Wow, what is your setup? I would be happy if I could hit 20. That is what I was getting with my last truck. I'm thinking the 351W can achieve good mileage as long as I'm light on the throttle. It's easy to burn gas zipping up to speed.

[Justin]

He's got some whimpy little 4 banger (that is awesome).

[Jesus_man]

He's got some whimpy little 4 banger (that is awesome).

Burning alternative fuels dang hippies!

[Kinder]

I am cheating a tiny bit, 4BTA w/ 5 spd & yes I do plan on burning some alt fuels.

[Digger]

Nice. I wanted to do that in the Ranger I had. Maybe someday, but I'm a little addicted to the V8 rumble at the moment. (Been driving V6's for 16 years now)