8 MPG on a 302... Normal?

[hollowch]

Everything stock except 35s 4.11s and 5inch lift. Is there any way to adjust fuel mixture for daily driving?

 

[Explorer]

I'd say that's about right for that setup. On the stock Bronco's I've had, 12mpg was the average. Some get better, but that was the normal for three diff ones. Overall average not just interstate.

 

[Heus33]

How are you measuring your milage?

 

[Explorer]

Think I see where you're going. If speedo hasn't been geared to his setup, no way to tell correctly.

 

[GrillMaster]

check the spedo with a GPS and then go easy on the freeway. You could try to determine at what speed you get the best MPG at freeway speeds.m I bet you can get it up to 12. I get 7 around town when I floor it a lot.

 

[Viperwolf1]

Speedo error and odometer error are two different things. I'm pretty sure you're not using the speedo to determine how far you've driven. Check the accuracy of the odometer.

 

[bmc69]

I would agree that 12 is certainly within reach. My rig with a 408 and running on 39.5 IROKs gets exactly 10 mpg on the highway, running around 60 mph.

The odometer and speedo will be off by equal amounts if you did a gear change and did not change the speedo drive ger accordingly....

 

[Viperwolf1]

The odometer and speedo will be off by equal amounts if you did a gear change and did not change the speedo drive ger accordingly....

Should be, but I guaranty they are not.

 

[OUMX117]

Should be, but I guaranty they are not.

They are both driven off of the same (speedo) cable, so they should be off by the same amount. Otherwise there is a mechanical problem in your odometer.

 

[Explorer]

I'll take your word on this one Viperwolf. I too thought odometer and speedo errors would go together. You've been doing this longer than I have.

 

[Viperwolf1]

They are both driven off of the same (speedo) cable, so they should be off by the same amount. Otherwise there is a mechanical problem in your odometer.

First off the speedo is not mechanically connected to the cable. It's magnetically coupled. That adds a point of error to the system.

2nd, with a driven speedo gear that has only 17 teeth the best accuracy you can expect is going to be +/- 3%. That degree of accuracy is also stated in the factory service manual. Speedo came calibrated within 3% and odometer came cal'd within 3%. However, because of the speedo gear limits, the speedo could have been + 3% and the odometer could have been - 3%. All within factory specs. Now add bigger tires/lower gears and use the speedo to determine which new speedo gear you need. You get lucky and get one that gives you 0% speedo error. What does that do to the odo error? That's right, you increased it to 6%. I know this because my speedo is dead on with GPS and my odo is now 6% slow.

 

[NY-Bronco]

First off the speedo is not mechanically connected to the cable. It's magnetically coupled. That adds a point of error to the system.

2nd, with a driven speedo gear that has only 17 teeth the best accuracy you can expect is going to be +/- 3%. That degree of accuracy is also stated in the factory service manual. Speedo came calibrated within 3% and odometer came cal'd within 3%. However, because of the speedo gear limits, the speedo could have been + 3% and the odometer could have been - 3%. All within factory specs. Now add bigger tires/lower gears and use the speedo to determine which new speedo gear you need. You get lucky and get one that gives you 0% speedo error. What does that do to the odo error? That's right, you increased it to 6%.

Interesting. I always thought they were mechanically connected. Always learn something new. Thanks man!! ;D

 

[Kyle.malone]

Just go with a new electronic speedo/odomoeter from autometer. ;D Then you will never have to worry about anything if you swap gears, tire size, etc. You just have to recalibrate it if you change anything in a 2 mile drive..

 

[Rustytruck]

Since you post no details on what you have motor wise then we have to assume the factory 2 barrel 302.
If it were mine I would put a vacuum guage on the motor and look for a steady vacuum reading. I would check and replace vacuum hoses and the carb gaskits in order to get a steady reading. I would then adjust the Idle mixture screws equall amounts to get the highest steady vacuum reading.
Then I would change the timing at Idle to 12 - 15 degrees advanced. The more the better as long as you dont have pinging, starting issues or overheating. after you adjust the timing you will need to reset the carb all over again. I would not run regular gas especially if you are running fuel with alcohol in it. Make sure you have a clean air filter. If you truly have the stock engine you have the worse case to try to get milage. If you try to fly down the freeway after 55 mph your just dumping fuel on the ground. You dont have enough engine power and torque to pull the 35" tires and the 5 inches of lift and your gearing in the axles are not enough. The poor tired engine is trying to pull an elephant up hill wearing flipflops.
Most people here are trying to get better milage on the freeway but your worst milage is at idle through 3rd gear on the street. Make sure the engine is doing the best it can in those areas first and your poor gear ratio to tire size is a real problem there. Advance in timing will help there.

 

[hollowch]

How are you measuring your milage?

Off the odometer reading. The gear in the tranny was changed but the speedo still reads about 5mph slower @ 45mph. I don't know if thats a good thing or a bad thing in the case of gas mileage?

 

[hollowch]

Since you post no details on what you have motor wise then we have to assume the factory 2 barrel 302.
If it were mine I would put a vacuum guage on the motor and look for a steady vacuum reading. I would check and replace vacuum hoses and the carb gaskits in order to get a steady reading. I would then adjust the Idle mixture screws equall amounts to get the highest steady vacuum reading.
Then I would change the timing at Idle to 12 - 15 degrees advanced. The more the better as long as you dont have pinging, starting issues or overheating. after you adjust the timing you will need to reset the carb all over again. I would not run regular gas especially if you are running fuel with alcohol in it. Make sure you have a clean air filter. If you truly have the stock engine you have the worse case to try to get milage. If you try to fly down the freeway after 55 mph your just dumping fuel on the ground. You dont have enough engine power and torque to pull the 35" tires and the 5 inches of lift and your gearing in the axles are not enough. The poor tired engine is trying to pull an elephant up hill wearing flipflops.
Most people here are trying to get better milage on the freeway but your worst milage is at idle through 3rd gear on the street. Make sure the engine is doing the best it can in those areas first and your poor gear ratio to tire size is a real problem there. Advance in timing will help there.

Alright well what would you suggest changing the ratio to? I was under the impression that 4.10's were ideal for 35s. Here check it out for yourself http://www.4lo.com/calc/geartable.htm

 

[DirtDonk]

Add 9-10% to your number of miles and re-calculate.
Sounds like you're closer to 9 or 10 than you think. But verify your percentage with a known mile compared to your odometer reading. With luck, you should have a mileage marker somewhere nearby. Or, if you're living in one of those great states where every road marker has a mile number on it, you're good to go.

Paul

(edit: I corrected the previous "11" to the current 9 or 10 because I was factoring the 10% from 10 mpg, not the 8 he's showing.)

 

[fetorino]

That chart doesn't tell the whole story

Alright well what would you suggest changing the ratio to? I was under the impression that 4.10's were ideal for 35s. Here check it out for yourself http://www.4lo.com/calc/geartable.htm

a brand new stock 302 2bbl was never designed to power your bronco with that much lift (frontal area) and tires that large.

You choose gearing based on cruising at the torque peak. You may be at the torque peak with that gearing but your torque peak may not really be enough to overcome the load you are asking it to push.

All that being said proper carb adjustment and ignition timing as mentioned are a big deal in milage. Do you have a vacuum advance on your distributor? If not get a distributor with one. I would add initial timing but be aware of your total timing. With my FE I hold the total timing to 40 but I don't think your 2bbl 302 will spin as much rpm so the total may need to be lower. (maybe a real 302 expert will chime in here).

Find find out how much initial timing the motor wants with some experimentation. Probably 10+ deg. From there dial in the distributor to be all in by 3000. The mechanical advance should equal the total (provided by someone here) minus whatever you arrive at for initial. Make sure you have a properly functioning vacuum advance and that it is hooked up.

No matter what physics will keep you from getting much more than 10 mpg.

 

[DirtDonk]

That 10+ degrees is right on. Factory specs were in the 6° to 8° range over the years if I remember, but these things love about 12° initial in many cases. Some don't, but most do.
And welcome to CB fatorino. Thanks for chiming in.

Paul

 

[fetorino]

Thanks for the welcome on my first post

That 10+ degrees is right on. Factory specs were in the 6° to 8° range over the years if I remember, but these things love about 12° initial in many cases. Some don't, but most do.
And welcome to CB fatorino. Thanks for chiming in.

Paul

First post here anyway.

As reference for the original poster I found this article on SBF timing.

http://www.carbdford.com/viewtopic.php?t=5543