I have a 95 F-150, 302, ext.cab with a 4r70w trany and 3.55 LS rear end. I am thinking about doing an engine swap to a 351 and was wondering what kind of gas mileage I would be sacrificing? I am currently averaging 14 (city) and 18 (highway).

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I’m also wondering if the 4r70w will handle the 351. I only tow around 3000 to 4000 lbs. and I really don’t drive like I’m in a race care, well unless someone pisses me off.

 

You won't sacrifice any gas mileage worth talking about...


The only thing is that the shift points for that 302 would be different for the 351 and it may not be optimal....and may not allow you to take full advantage of the 351's low end torque.


Since the 351 makes a lot more low end torque, it can use less rpm at less of a load to propel the truck, which saves mileage. Since it is a larger displacement, it will use more fuel in general, but the two usually cancel each other out and end up somewhere around the same mileage.

 

Of course with that extra torque, you're more likely to use it to get off the stoplights and there goes the mileage. I hate it when that happens

 

Best I ever got with my 302 was 17mpg on an extended trip. The best my 351 has done to date is 14mpg. The difference could be that the 302 was hooked to 3.55s and the 351 has a 4.10 behind it (and weighs a whole bunch more as a truck).

Jason

 

I get close to those numbers with my combination, 13-14 combined city, 17-18 strictly highway.

 

Depending on your area . Hills etc.

I have had 6 351's and just got rid of 96 F150 reg cab. Was really nice truck but 12 MPG didn't cut it.

Just subtract 4 MPG and you got it.I live in hilly area (Up state PA). I never got above 13 MPG on any 351 with 3:55 rears and i have all records. Even out west

My 94 F150 4x4 longbed SC 302 gets 14-17 MPG all time (with 300 LBS tools)and hauls just fine . If you haul but once a week its not worth the MPG loss. Ive had 10 302 and its all I run now. I haul a 22Ft car trailer. I'm slow on day I haul but just as fast rest time.

Hey just depends on you and needs ,I drive 500-1000 a week and even 2 MPG adds up.

 

Here's my experience, in 2000 I bought a 1992 E150 conversion (no seats though) with a 351w and an E4OD (.73:! overdrive) and a 3:55 rear end. I got 17mpg highway and 14 city consistently. My father bought a 1988 E150 clubwagon XLT 302/AOD (.67:1 overdrive), 3:55 rear end. It weighs in at about 5k pounds empty. It gets 14mpg in town, and 16 on the road. Towing the 351w got about 12 in the flats, 8 to 10 in the hills. The 302 gets 10 everywhere while towing.

 

i have a 92 f150 2wd, with a 351w,e4od trans, and 373 gear the best i average is around 13 mpg in the city.

 

Yeah in my 1993 F150 with the 351W gets im guessing about 15-16 mpg city at least maybe a bit less but i can go almost a full two weeks on one tank no joke but thats keeping it under 2000 rpm which seems hard but not that bad and i got the 3.55 rear

 

Bone stock 302 in my truck with 3.55 gears on OEM rubber: 14 City/mixed driving, 18 strictly highway

Mild build 306 with 4.10 gears turning 33" tires: 13-14 city/mixed driving, strictly highway 18+ mpg.

I religiously check my mpg each fill up. I use the same pump at the same station etc. The best I've seen on the 306 was 21 mpg. This was driving from California to Washington (zero city miles) with no lift, 30" tires and a 3.55 rear end. Also, this was on premium fuel and the timing was set at 18* BTDC.

The best tank I've gotten with my current setup at 13* and 87 pump gas is a hair shy under 19 mpg.

 

The mileage all comes down to what gearing. My '87 F150 302 T18 only gets 12 mpg if I'm nice to it, but my grandfathers '94 F250 5 spd gets 16 highway and 13 city.

 

I know that this is an old post, but in case anyone in the future wants to know, here is my answer to the original question up top. He asked if swapping a 302 for a 351 would change his gas mileage and how much. The tranny and rear were staying the same. Now, one person mentioned that since it's an auto tranny that the computer shift points are different, and that may be, so it might be best for others doing the same kind of swap to get a brain that matches the motor, tranny, and vehicle year and model also. Mine is a 1992 5 speed, so there was no reason to change the brain. Anyway, what I did was take a 1992 Ford F-250 4x4 that had a wore out 351 Windsor, a ZF 5-speed, and a 3.55 rear end in it and the body was beat all to hell, and then took a 1992 Ford F-150 2x4 that had a 302, an auto tranny, and a 3.55 rear end in it and an excellent body and put them together. I took the body off of the F-250 and put the F-150 body on the F-250 frame, and then put the 302 in it and the 351 brain because of the ZF 5-speed tranny. Now that my truck was a 302 attached to the ZF 5-speed and 3.55 rear end I was able to figure out that I got about 18 hwy and about 15 city. About a year later I rebuilt the 351, put it in the truck, and figured out that I now got about 16 hwy and about 13 city. So, just daily driving I lost about 2 mi to a gallon, but facts are facts and the more ci's you have the less mi's to gallon that you're going to get. But that's not all to figure in here. He said he uses the truck to pull cars. 302's are not much for pulling because they are a fairly short stroked engine and therefore low on torque. That's why the only truck's and vans that you find them in stock are light duty, because they don't put out enough torque for heavy pulling. Now several years before this I built a 1989 Mustang. I took the 302 and bored and stroked it to a 347. It shocked me because my hwy mileage went from 18 to 25. I figured out that the only way possible was because I increased the torque so much that in 5th gear it originally was pulling 60 mph at about 1,600 rpm's and after the engine build it was pulling 60 mph at about 1,300 rpm's, so basically the torque increased, the weight didn't, so the torque to weight ratio changed so much that it wasn't running at much more than an idle after the build and therefore drastically increased the fuel mileage. Now, a 351 with stock equipment has alot more torque than a 302 does, even a bored and stroked 302 to a 347 if it's running stock 351 heads and fuel system is going to have less torque than a 351 does from the factory. You put a load on a 302 Ford and it will bog down quite a bit, you do the same to a 351 Ford and it won't bog down very much at all. So, even though the guy was putting a larger motor in the truck and because of the ci difference should lower the gas mileage, the fact that putting the 351 is barely going to change the weight of the truck, around 50 lbs difference is about it, and putting 3,000-4,000 lbs on a 302 is going to seriously bog it down but won't really make a whole lot of difference on a 351 I would figure that while pulling the 3,000-4,000 lbs on it that the gas mileage would be about the same, but unloaded it's going to drop about 2 mi to a gallon. So, it's really up to how you use the truck as to wether you want a 302 or a 351. If you're driving alot of miles and don't haul/pull alot then you definately want the 302. If you haul/pull alot then you want the 351. Also, the 351 is alot more fun from 0-60 mph, but the 302 is alot more fun from 60-100. The 351 has alot more low-end torque, but the 302 is shorter sroked with the same bore so it has more torque at high rpm's, so the 351 will get to 60 faster but from 60 on up the 302 will start to build speed faster. That's why if you're building a street rod you want a long stroked engine with a medium bore that you can get out of a stock block, but if you're building a circle track car then you want an aftermarket block that's capable of big bores but keep is short stroked, that way you can get about the same ci's and same compression, but you can run alot higher rpm's and it raises the rpm range that your big torque hit's, making it about the same torque as a long stroked engine, just at higher rpm's, and because of the fact that it makes your torque peak at a higher rpm it makes your hp peak at a higher rpm. But, on a street car you want your torque to peak at about low to mid range rpm's and your hp to peak at mid range rpm's, and a truck you want your torque to peak at low rpm's and your hp to peak at low-mid rpm range.

 

Interesting take on the subject. With my 351 I have seen up to 15mpg, but I hope for better after a couple tweaks.

 

If you increase your torque to a point that your truck is running as close to idle in 5th at about 60 mph as possible then it'll help with the gas mileage. All I did with mine for now is put a Comp Cams 35-255-5 cam in it, put an Edelbrock Performer truck intake on it, bored it .030 over, and shaved .010 off the head deck on the stock E-7 heads. One thing you can do to help it is get a set of heads off of a 1999-2000 Ford Explorer 302, I was going to do that until I decided that I was going to put a 408 stroker kit in mine and knew that I'd need bigger heads, but for a basically stock 351 they're excellent and the only thing that you'll have to do to make them fit is get the head bolts holes drilled out because a 351 uses bigger diameter head bolts, but I'd suggest shaving .010 off the head deck because 351's have always had low compression and the only other ways to increase compression is to bore the block, put in flat top or domed pistons, or both. The intake and cam that I put in mine increased the torque by about 70 ft lbs, I also put a set of long tube headers on it that increased the torque by about another 25 ft lbs, and I don't know what the shaving the heads or boring the block did for it, but it has so much power going to the tires now that it's hard to take off without wheel hopping the rear end, so it all done some good ****, more than enough addition to power to make up for the 2 miles to a gallon loss by going back to the 351. I can't wait until next year when I put my 408 kit in it and my new big valve heads on it, then it's really going to be a monster. Oh, since yours is a '97 351 then you can put a hydraulic roller cam in yours and get more juice out of a basically stock motor than I did with the cam that I put in, probably not a major difference, but it will probably help yours on gas mileage over mine and add probably another 5-10 hp because mines just a hydraulic mechanical cam which means mine has more friction on the bearings than what yours would, but if you get the hydraulic roller version of mine I guarantee that you'll love the torque gain.

 

Well Dan, it may be an old thread, but it certainly still remains a valid and interesting topic.

All the older responses and your own experience with the 302 to 351W swap agree with the logic that a larger engine will use more fuel. You also noted that the increased torque of the longer stroked engine may allow you to run lower RPM's for a given speed, although this requires a change in gearing. When all the dust settles from rebalancing the math, you may very well realize a mileage advantage with a larger engine, all other factors remaining equal. These other factors make this a very interesting discussion.

There is a given amount of energy in a unit of gas and a given amount of work must be done to overcome inertia and frictional losses including rolling and wind resistance. Weight will compound all these factors, but we are assuming all remains constant with the exception of the change in engine displacement. The 1st law of thermodynamics comes into play here. If we are using more fuel to do the same work, where is the extra energy going? The answer must be in heat, but real world results are pretty hard to observe as we have no accurate means of monitoring how much heat is dissipated by the cooling system.

Results may vary in the real world and I had my own experience with a 302 to 351W swap many years ago with a 73 Bronco, (which I wish I still had). I replaced the worn out stock 302 with a fresh 351W. The 351W was running flat top pistons at around 10:1 compression, stock heads with a 69 Mustang factory 4bbl manifold, a Carter Competition series 650 cfm carb, long tube fenderwell headers and a Comp Cams camshaft. Everything else in the truck remained the same. The seat of the pants experience was incredible and I actually gained 1 mpg in fuel economy.

I think the answer in the fuel economy increase lies in the increased efficiency of my 351W build. All engines are inefficient, wasting fuel as heat energy instead of converting it to rotary motion. Loading an engine increases the inefficiency and this will affect a smaller displacement engine more rapidly than a larger displacement engine. I think the engine build also affects efficiency. Possibly the increased compression ratio was the primary cause in my example?