Ok so I just recently rebuilt my engine. 95 CPI 4.3 Pretty much stock but an high flow drop in filter flowmaster exhaust dumped at the rear axle and the engine was bored .30 over at the machine shop. The new engine has about 4 thousand miles on it now but Im still getting like 10mpg on average. Or about 190 to 200 miles a tank. Now I know a brand new engine is tight and needs time to loosen up but still I feel like I should be getting at least 12-14 in the city and 16 or so on the interstates. BTW the truck does have a T-Bar crank and 31s. But everything is new as far as plugs, wires, cap, rotor, I even replaced all the CPI setup in the intake and a brand new fuel pump. So does anyone have any ideas as to why my mileage sucks so badly? Thanks guys

 

I'm not sure what the stock tires are, but I used 29 for this. With those 31s you are going almost 7% farther than you would with 29s. So your calculations are 7$ percent off from the get go. Also, it takes more power to accelerate with larger wheels, so it will decrease your city mileage. Though if you figure out how to get past 12 MPG let me know.

 

Yeah I realize that but still I was getting anywhere from 325 to 350 a tank before I built the engine and lifted it. I lifted it while it was down getting the engine so I have no reference for what I would get with stock 235 tires with the new engine. But loosing 125 miles to a tank just doesnt seem possible even still. I figure at this rate I should of just built a 350 for it because honestly it will get the same or better mileage so honestly Ill probably just end up building a 350 this summer.

 

Oh wow, hopefully someone can come up with an answer, I'd definitely be interested in finding out what's wrong. I'm also convinced I'd get better gas mileage with a V8.

 

yes if you do the math for 31's you will travel about 6.56% farther than you would with stock tires. so for instance if your speedo says you go 200 miles and you burn 15 gallons doing it, that MPG works out to be 13.33 but if you factor in that 6.56% your mpg works out to be 14.20 (.0656 x 200 = 13.12, 213.12 / 15 = 14.20)

 

You wont get better milage with a V8. Its a larger heavier engine and has two more cylinders which take more fuel. I considered it as well, but the fact was my milage wouldnt be better. Its like adding a supercharer. Your milage after will suck because it uses more fuel.

 

Not true.. Actually depending on the V8 you put in there it might be the same weight or lighter. because our V6's are cast iron and lots of V8's are aluminum. and to add to that the engine wouldn't work nearly as hard to move the truck meaning it burns less gas. it has been tried and tested.

just sayin

 

I understand it would likely be heavier, but there is clearly some unknown reason my truck gets bad mileage. The highest average I've had that wasn't all highway was 12.5. I have a friend with a 4x4 Blazer with 180k miles on it that gets considerably better gas mileage than me. There were times I'd get less than 10 mpg. I figure if my friend's V8 gets better mileage in his Tahoe, it would get even better mileage in a lighter truck.+

 

yeah like i said. but before you do that lets try some things. check your fuel pressure, clean your MAF, clean your EGR (if you got these new with the engine then dont worry about it), your O2 sensors might be bad which would most definitely drop your gas mileage to where its at. So to check this, rent a code reader that will show you the codes while you are driving and see what it says for the O2 codes then let us know what is says. Also make sure your engine temp sensor is working because if its not then your engine will stay in fuel enrichment mode because the computer thinks the engine is cold, this will also obviously drop the mileage. the code reader should also show you what the computer thinks the engine temp is (should be 180-195 depending on your thermostat).

12 MPG is way to little for even 31's, worst i have got is 13.3 and that was 100% city driving in 35MPH or less, stop and go

 

I dont have a maf and the egr is brand new. No 02 conde. I have a 95 CPI set up so hence no MAF. But I luckily do have the obd1.5 which gives me most obd2 capabilitys. Pressure is good, IE new pump and and CPI spider but yes I have checked the pressure as well.