All Small Six Fuel economy - Update

This relates to all small sixes

John Ha

Well-known member
Supporter 2022
Supporter 2023
After I pulled the 351 and installed the 250 I6 I expected my fuel economy to increase. It hasn't and I can't figure out why it's so low. I've been getting between 11 (worst) and 14 (best) mpg, mostly in town but some highway driving. I actually got better mpg with the 351 than I do with this 6. I don't drive it hard at all - more like a grampa.

I have the 250 from a 73 Maverick, a T5, a 3:1 open 8" differential. The engine was rebuilt by a reputable local shop as a stock. It's overbored slightly. I'm running a stock cam, stock lifters and the rebuilt RBS carb that came with the engine.

I have found and fixed a couple of very small fuel leaks but the mileage has not improved. I have also confirmed that the choke is opening fully when the engine has warmed.

I've checked the drag on the wheels and did find the right front caliper was stuck. I fixed that and the mpg went up from 10 to between 11 and 14. There's no unreasonable drag from the brakes, front or rear.

I was wondering if the fuel pump might be leaking internally but the oil level hasn't changed so I'm thinking that's OK, but may change the pump anyway just for giggles.

I really like the car this way but would probably stick the 351 back in it if I can't figure this out and improve the fuel economy.

Any thoughts? I'm out of ideas.

Thank you!
 
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Sorry my response was not helpful- didn’t know you had the handbook and were not interested in upping the performance of your engine. Best of luck, there are some good suggestions below that sound more in line with what you are looking for.
 
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Not sure what year your 351 is, but the ‘73 250 in stock form is a low compression, low performance engine. Compression ratio starts at 8.0:1 and is made worse (maybe down to 7.7-7.8 ) by a modern head gasket which your shop probably used. Combine that with a cam that is 4 degrees retarded from earlier years and a lazy emissions-favoring timing advance curve and you’ve got a sluggish engine that’s rated 89 hp @ 3200, and 196 ft/lbs @ 1600 rpm in stock form, and probably slightly less in as-rebuilt form. I believe this is why you have to keep your foot in it more, impacting mileage.

All is not lost though, by utilizing some of the methods discussed in the Falcon Performance Handbook and all over this site, you can pretty easily up that performance to 1970 stock specs of 155 hp and 240 ft/lbs (9.0:1 compression, better cam) or higher. Many on this board easily get more than 200 hp out of a 250 and the cool part is your mileage will greatly improve too (as well as your fun factor). So look around, ask for help, get the handbook, if and you will learn all the specifics necessary.
I have the handbook and this is not a helpful response to my question. I have no interest in putting more money into this engine or upping the performance - it's adequately peppy in my 3200 pound convertible as it is, and my foot isn't in it. I have lots of pedal left at pretty much any speed I've driven it - up to 65 mph. It should do better than 12-13 mpg though.
 
Hi John, yes you are right your Mustang should and can do much better than 12 to 13 MPG especially since you also have the T5 trans with your 250. Here is my 250 six example my 1977 Maverick a 4 Dr. that I bough used back in 1977 was very low miles and all Calif. Emissions it also weighted in at close to 3200 pounds or maybe even a little more. Was mostly a factory stock 250 six just a couple small hidden mods and C4 trans also had a 3.00 rear axle ratio, stock size tires, Power Steering & Brakes and AC. Factory rating was 86 (SAE Net) so is harder to compare with the earlier engines used before 1972, these ratings are with full exhaust and engine accessories just as is installed in the car so more like a Rear Wheel HP rating than the older flywheel HP ratings where. They were also were run without a Cooling Fan, or any other engine driven accessories plus no Exhaust system. Yet mine was a very good performer and could leave on a stock SBF 302 or Brand X 305 / 307 and even some bigger V8's from a stop and be out in front at least for awhile. Though I didn't really drive it easy for a Max MPG I still regularly got 17's to about a best of 19.2 on a longer trip this is close to its factory rated specs.

There are a number of potential causes for this major drop in MPG ranging from the engine tune up and condition of the external parts used to chassis issues. so I will start first with the front end alignment settings since you have checked and fixed your brake drag issue. Have you done any timed coast down tests? On flat level ground how easy is it to push your car by hand?

Here are the Alignment settings for the Ford Small Six's

Alignment settings with or without doing the Arning / Shelby drop. For 1960 to 1970 Fords and Mercury's like Falcons Comets Mustang and others with that same type of front suspension.
1. It is very important that you have no more than .25 degrees of difference from the passenger to drivers side.
2 A. For a light weight small six car 144 to 200 1.0 to 2.0 Degrees of Castor, but note that on a non power steering street use car that the more Castor that you use then the Steering effort increases when going slow like during parking so this is a good reason to also use a Roller Bearing Idler Arm for reduced steering effort.
With power steering you can use the below 2B specs for a 144, 170, or 200 small Six too.
2.B This is for a 250 six or SBF V8's use 2.0 to 3.5 Degrees of Castor see the above note also
3. -.5 to 0 Degrees Camber
4. 1/16 to 1/8 inch toe in. Less is more if it will track with 1/16
Additional notes When setting these specs all up I also will use the drivers weight in the seat. On a drag race car I use a jack to lift front of car 1 inch.
 
Have you pulled the spark plugs and looked at their color? How "on" is your rebuilt carb. The plug color will tell you a lot
As Mustang 6 said a "stock" engine is a smogger. Returning it to normal is bolt on stuff, proper timing kit, distributor recurve.
Try driving in a lower gear, with a 3.0 axle ratio and an OD transmission you may be lugging it at some speeds.
 
You appear to be serious enthusiast. Adding an AFR gauge/exh sensor as well as trad' Vacuum gauge will help determine if any fuel efficiency improvement is possible. Serious discussion on use of progressive 2Bbl type carbs is available on the forum. Holley /Weber or Weber progressive 2Bbl's (32/36) may help in MPG quest with small block six 250.

here;s related big block six discussion:



have fun

 
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I had a 98 jeep Wrangler 4 cylinder automatic and the engine had to work so hard it only got 11 mpg. I had a 98 Honda Civic ex manual and the best mileage I got was 36 mpg going from Florida to Pennsylvania doing 100 miles an hour the whole way. It would normally never get more than 30 mpg. It seems like you need to keep an engine in it's sweet spot for economy.
 
Bubba, thanks for the reply. To answer your questions (green font embedded below):

There are a number of potential causes for this major drop in MPG ranging from the engine tune up and condition of external parts used to chassis issues. so I will start first with the front end alignment settings since you have checked and fixed your brake drag issue.
Have you done any timed coast down tests? No but it seems normal - not different from any other car I've ever driven - very straight and "unencumbered" (no abnormal drag noticed)
On flat level ground how easy is it to push your car by hand? It's very easy to push it.

Here are the Alignment settings for the Ford Small Six's

Alignment settings with or without doing the Arning / Shelby drop. For 1960 to 1970 Fords and Mercury's like Falcons Comets Mustang and others with that same type of front suspension.
1. It is very important that you have no more than .25 degrees of difference from the passenger to drivers side.
2 A. For a light weight small six car 144 to 200 1.0 to 2.0 Degrees of Castor, but note that on a non power steering street use car that the more Castor that you use then the Steering effort increases when going slow like during parking so this is a good reason to also use a Roller Bearing Idler Arm for reduced steering effort.
With power steering you can use the below 2B specs for a 144, 170, or 200 small Six too.
2.B This is for a 250 six or SBF V8's use 2.0 to 3.5 Degrees of Castor see the above note also
3. -.5 to 0 Degrees Camber
4. 1/16 to 1/8 inch toe in. Less is more if it will track with 1/16
Additional notes When setting these specs all up I also will use the drivers weight in the seat. On a drag race car I use a jack to lift front of car 1 inch.
The alignment is consistent with these specs (I did the Arning mod long ago before I put the 351 in it). I have manual steering and the quick steer pitman and idler arms - the idler arm is the roller unit for the quick steering. I was only able to get a bit under 2° of caster with the shims and the camber is within 0.1° of zero. Both sides are very close to each other (within 0.1°). The toe in is 1/16"


wme013 - thank you for your reply. I'd forgotten that I did pull the plugs to widen the gap a bit. They looked normal - maybe a tiny bit on the rich side but I'd idled the car before changing them. They were not super sooty or black as one would expect for a very rich condition. I'm definitely not lugging it.

I guess I'm leaning toward the issue being either the carb is flakey (I'm sure I did everything right when I rebuilt it) or the fuel pump leaking internally. Since the plugs didn't look like the carb was putting too much fuel in, I'm going to change the fuel pump today (if I can find the one I took off of the engine when I originally got it) and see if that helps. I'll write back when I've run another tank of fuel through it.

Thank you all for your thoughts and help!
 
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That excellent John, it sounds like the chassis is set up very good and is in top condition so checking that off. I think your focusing in the right direct with a 250 engine that's in very good condition then its time to look into the Carb and Distributor condition and or their tune specs, I feel with a stock built 1973 250 (after all Its also a Large Log Head) along with a T5 and 3;00 axle combo you should get real close to 20 MPG. By the way I didn't see were you mentioned witch Distributor your using in your 250 is it the 1973 Duel Advance point type?
 
That excellent John, it sounds like the chassis is set up very good and is in top condition so checking that off. I think your focusing in the right direct with a 250 engine that's in very good condition then its time to look into the Carb and Distributor condition and or their tune specs, I feel with a stock built 1973 250 (after all Its also a Large Log Head) along with a T5 and 3;00 axle combo you should get real close to 20 MPG. By the way I didn't see were you mentioned witch Distributor your using in your 250 is it the 1973 Duel Advance point type?
Thanks! Sorry, forgot to mention that I'm running a DuraSpark II setup. [Edit to add] I got the distributor from O'Reilly and I think it's for the 75 Maverick. I did the mods to the springs and tabs laid out in the tech section. It seems very happy with a base timing setting of about 17° - has about 19 inches of vacuum at idle. I don't know what the total advance is yet - I need to make a tool so I can measure that. It doesn't ping under any circumstances using 87 octane fuel so I'm thinking it's OK and not too worried about it.

I got the fuel pump changed yesterday but now the flare fitting on the pump outlet is leaking (it wasn't before) so I need to fix that before I check mileage again.
 
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That's Excellent I can see you put a lot of thought and care in your Mustang 250 build combo and even went with many of the very best in stock Ford parts (this is the same way I like doing it the same way with my own Daily Driver type cars too). Thank you with details on the DuraSpark II Ignition System parts and advance spring change Mods. That's some great Vacuum readings too witch also makes all this a little more puzzling. Yes it would be good for your fine tuning to either get some timing tape or better yet to scribe some more timing degree marks on the Damper for use with the setting of around 3 marks for 32 to 36 Degrees of Total Distributor Centrifugal Advance. Do you know what the Advance Degree Limit was set to in the Distributor there are two choices on the arm? You would also need some more Degree marks to help in tuning the Vacuum Advance Timing Limits maybe from 40 to maybe as much as about 60 Degrees that should be plenty. By chance was the Camshaft degreed during the engine assembly and was the True TDC found and marked? I am now already leaning more that the trouble is going to be focused mostly with the Carb condition and tune. Do you have a way to test your Exhaust Gas Readings or a Friend near you with a tester?
 
I put about 60 mostly highway miles on it yesterday and used about 3/8 of a tank on the gauge - that's somewhere around 5 gallons - so the fuel pump was not the problem. It just about has to be the carb then, unless I'm completely missing something else. I ordered a replacement carb from RockAuto, about the only reputable place I could find selling one. It's supposed to be here in a week or two. I must have done something wrong when I rebuilt mine or maybe it was flakey from the beginning. I don't know.

Bubba - I have created a paper ring with timing marks to paste on the crank pulley. I just haven't ever gotten around to putting it on and doing the test. I only incremented it to 30° though and I'm going to take your suggestion(thank you!) and expand that to 60°.

I don't know if the shop degreed the cam. I should have asked but it's been a few years now and I doubt if he'd remember.

I found some photos I took of the distributor innards and it looks like the arm in play is stamped 13. The other arm is stamped 18. I think that at one time I was going to change those around but don't think I ever actually did it. I remember I did fiddle with the vacuum advance adjustment and it's set to come in as soon as the adjustment allows it to.

I don't have a fuel:air ratio measuring device. I'll look around to see if maybe there's a place that rents them. We don't have smog testing stations here or I'd go to one and see if they'd check it for me.

An now for something completely different, my radio quit yesterday. I assume it's gone on strike in support of the fuel pump I replaced.

The saga continues ...
 
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Excellent Idea on making the Paper Timing Wheel to get you all the additional timing Marks for tuning! You might also make a simple Piston stop out of a gutted out Spark Plug Shell to check out your Damper's TDC Mark. With using the internal 13 Degree stamped Arm in the Distributor and the 17 Base Timing so you are at 30 Degrees total. I think you are a bit short on the Total degrees of Mechanical Advance to be in the sweet spot (of around 34 to 36 degrees). The 18 Degree stamped Arm would be just about right. Or you can leave it as is and try adding another 4 to 6 degrees in the base timing, that is if the starter doesn't start kicking back on you.

I do think it would be interesting to know if they Degreed the camshaft during the install or not and also witch of the Timing Chain & Gear sets that they used in your 250 build. There were 4 different timing gear sets and 2 different Chains that Ford made for the 250's over the years the one I like to use for these street 250 Builds is the early narrow chain set used from 1969 to 1972, this was the Cloyes # S414 Cam Gear used with the # S415 crank gear this version puts the Cam straight up. The second set used in 1973 to 1976 250's also wasn't too bad they only changed the timing a little by using a different Camshaft Gear. It used a later 250 cam gear the Cloyes # S442 and still used with the same # S415 crank gear. So by changing the Cam Gear to the the Cloyes # S414 its back the same specs as the early set. My 1977 Maverick Timing Gear & Chain set wasn't quit as good and used 2 different gears to change the Camshafts timing events yet it ran pretty good for the stock set up, I ported the head on it and wanted to go with a better aftermarket camshaft and a few of the other better stock parts there wasn't the money for such frills back then.

I right now do feel your Carb is were to really look wish, I could inspect it up close real good to see. LOL yes that old Radio didn't like what your trying to do but maybe its a only a simple Fuse that gave up in protest. Best of luck
 
I can confirm that when the #1 piston is fully up the timing mark is at TDC on the pointer. I learned this during a previous adventure with putting the distributor in after not marking the rotor position before I removed it. Beyond that, I don't know what's inside.

I had to look back in my stuff to decode the stampings on the distributor. The numbers are supposed to indicate half of the total advance that the slot on a given weight provides. So 2x13+17=43, plus whatever the vacuum advance adds. I'll have to plot the curve one of these days. I should probably do two curves - one with mechanical advance only and one with both vacuum and mechanical advance.

I have another RBS but it didn't have the right throttle linkage on it for my car (it has the pull cable linkage). In the past I'd tried swapping the throttle shaft from the push linkage into the cable linkage carb but it didn't work well - something wasn't compatible. I spent some time the last couple of days experimenting with a bent up piece of scrap steel strap from my junk bin welded onto the cable pull linkage carb to get a link in the "right place" for my push throttle rod. I had to do a few iterations to get the geometry of the new piece right but finally got it done yesterday. I drove it on a short trip around here to see if there was a difference (and if my pathetic welding skills made an adequate joint). The car seemed to have quite a bit more pep. I filled the tank and plan to take it out for another longer jaunt today to see what the fuel economy is with this carb.

I'll post again after I've put some more fuel through this carb setup and let you know what happened.

EDIT: Did a 50 mile mostly highway jaunt and used about 1/4 tank of fuel on the fuel gauge. I think that's about 3 gallons - so maybe 16 mpg. I'll run the rest of this tank through it (happily, that will take longer now :)) and see what the average comes back as. It's definitely much better. Oh, and the radio has decided to work again. I'm glad I didn't have muck around behind the dash trying to find that problem.

You've helped me a lot and I really appreciate it! Thank you.
 
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RBS is hi cfm. May B try a lower flow 1?
The A/F gauge'n O2 sensor R the way to go for fine adjustment.

(Mr.) MPGs (stang then bronk) member has a l o n g thread (w/our help) froma while ago (close toa decade) on
gettin max mileage. Check it out. Don't leave exhaust ideas out of the equation. We seek MPGs as part of the
"performance' (along w/pep) designation. Personally, U, John, deserve in the 20 mpg range !!! Tell me Y not~
Tires cant reach 40 psi? Brakes draggin/ol wheel bearings? 2 air cleaners? C'mon now...pass 20 mpgs~
 
RBS is hi cfm. May B try a lower flow 1?
The A/F gauge'n O2 sensor R the way to go for fine adjustment.

(Mr.) MPGs (stang then bronk) member has a l o n g thread (w/our help) froma while ago (close toa decade) on
gettin max mileage. Check it out. Don't leave exhaust ideas out of the equation. We seek MPGs as part of the
"performance' (along w/pep) designation. Personally, U, John, deserve in the 20 mpg range !!! Tell me Y not~
Tires cant reach 40 psi? Brakes draggin/ol wheel bearings? 2 air cleaners? C'mon now...pass 20 mpgs~
Thanks Chad. I'm pretty good at getting good fuel economy (I'm cheap). Actually I think my issue is resolved. This engine has gotten really poor fuel economy since I put it in and I knew it should do much, much better. I knew that "something" was not right with it but never could figure out what the core issue was, until Bubba helped and let me bounce ideas around. I believe that I did something wrong when I rebuilt the carb and I have a different one on it now that seems to be working better. I still have 3/4 of a tank of fuel to go through to get an accurate idea of the mpg now that I've changed the carb. All I really need is the time to do it so I can confirm that the issue is, indeed, resolved.
 
same carb or different model?
 
"...same model..."
seeking MPGs would mean lower flow than that 215 CFM, no?
What's the break pont btw "enuff stuff" to push a brick down the road and 'over kill' (poor MPGs)?

I've no idea but a lill YF of 170 CFM pushed my bronk pretty well (before I swapped in an RBS)
up off rd hills (300 ft elevation change), w/a sno plow full of wet heavy sno, skiddin logs out da woods'n upa hill, etc.
I'm sure it got better MPGs (esp w/o those loads) then a hi flow, no?
 
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