What's guys, I just bought my first 1985 GMC Jimmy. It has a crate 350 engine with a NP208/700R4 combo and 373 gear. It is completely stock to my knowledge and driving it around would for sure support that theory other than the shift kit put in the tranny. It smells that it is running a little rich so I'm planning on replacing the carb, but long story short it gets around 10 MPG. Should I be expecting that or a little better? Thanks in advance for all input.
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
gas mileage
- Thread starter new guy
- Start date
What driving conditions? City, highway, combination? How do YOU drive?
16-17 freeway is normal, 10-13 city/freeway is pretty normal too.
I'd reconsider replacing the carb, it MIGHT need some work, or you might have other things going on like bad plug or wires, worn out distributor cap/rotor, vacuum leaks, etc. A thorough inspection of the engine bay might find problems that are easy to correct.
16-17 freeway is normal, 10-13 city/freeway is pretty normal too.
I'd reconsider replacing the carb, it MIGHT need some work, or you might have other things going on like bad plug or wires, worn out distributor cap/rotor, vacuum leaks, etc. A thorough inspection of the engine bay might find problems that are easy to correct.
Combination driving, I live in the country but work in a small city so I pass through 3 redlights on the way to work and I usually get caught by one. As far as my driving habits on it I'm really easy on it cause I knew going into it that it wasn't going to get good mileage anyways. Plugs and wires are both new but the distributor I don't know when it was replaced last. As far as vacuum leaks I haven't ran across any but I'm no mechanic either.
smier
1/2 ton status
What size tires? By just going from 31" street tires to 32" mud terrains cost me almost 2 mpg average...
It has 31x10.50 BFG all terrain ta's on it. If I put one of the new air raid air filters, put an electric fan help?
Nope, not appreciably.
Best I ever saw with a 305 or 350 was 17MPG. Even running 235/75R15's or 31's. Dual "cold air" (I really hate that now that every econobox "tuner" uses it) snorkel air cleaner or without.
Electric fans were worth 0MPG over the stock clutch fan.
Best I ever saw with a 305 or 350 was 17MPG. Even running 235/75R15's or 31's. Dual "cold air" (I really hate that now that every econobox "tuner" uses it) snorkel air cleaner or without.
Electric fans were worth 0MPG over the stock clutch fan.
Ain't going to make a lick of difference. Last test I saw was that engine fan swap was worth 5HP. If you were increasing engine efficiency (and thus power) you would see it in the MPG, and it's not there.
You get a hell of a lot better bang for the buck out of "serious" modifications...long tube headers, camshaft, heads, etc.
You get a hell of a lot better bang for the buck out of "serious" modifications...long tube headers, camshaft, heads, etc.
The fan is a left over from a drag car that I had. On it I picked up a couple of MPH on the track and a better et. This being my first blazer/jimmy I don't know much about them so I greatly appreciate all the advise and look forward to being a member of this forum. When you mentioned headers cam and intake what do you suggest? I'm not looking for anything extreme the Jimmy is used to haul my camper as well (its a pop up) so no danger in hauling something to big or heavy there lol. I could easily be persuaded into a new set of heads as well if you don't mind sharing advise there as well.
ive got a 90 with a tbi 350,700r,31-10.50 tires,had it since 92,bone stock,hiway only mileage at about 60-65 14mpg,usally i run a bit faster than that and it drops to about 12.8,around town about 11-12
I'll take that quote out of context, since it's so appropriate:I'm not really an authority on these trucks, don't care to be either, but mine seems to like it in the 10-12mpg range no matter what. I've gone through the following iterations:
305, Q-jet, 3.08 gears
350, Q-jet, 3.08 gears, 32" tires
350, Q-jet, 4.11 gears, 33" tires
350, Q-jet, 4.56 gears, 35" tires
350, TBI, 4.56 gears, 35" tires
That's mixed driving, highway and trail, several carb rebuilds, over the years. Sure, the weight has increased over the years due to more batteries/bumpers/winch/spare parts/unnecessary need-to-haves.
Fuel efficiency with the aerodynamics of two stacked bricks - well?
Anyways, I've stopped worrying about fuel efficiency a while ago.
You results may vary.
P.S.... and even my new truck, a supercharged FZJ80 Toyota LandCruiser - guess what, 10-12mpg...
Vortec heads alone are typically worth 40HP if coupled to the right camshaft. They aren't high lift/high flow heads, but they mix and burn air and fuel extremely efficiently. I'd have to look around to compare price, either go with new Vortec heads (they are prone to cracking, and IMO not worth buying used at this point) or any of the aftermarket vortec replacement castings from companies like RHS, etc. You won't need anything really fancy (springs, etc) if you are sticking with a fairly mild cam.
I'd defer to others that have run your combo as to camshaft. Mine is an aftermarket roller cam, way too much money for what it is. A hydraulic flat tappet would have been fine in my case.
Electric fans are a crapshoot in these trucks. The trucks can be hard to cool when you are crawling and the ambient temperature starts to climb. The stock fan moves a ton of air (never seen anything stating actual numbers) and it's hard to find electrics that move as much...Ford Windstar fans apparently do a good job, but look at some of the threads on their usage, they are pretty large, and the OEM stuff seems to do a better job of cooling than most aftermarket. IMO electric fans are nice if you need to shut them off for water crossings. Also good to get AC temps down since their flow isn't tied to engine RPM...at idle, if they are strong fans, should bring AC temps down. Outside of that, large electrical draw when on (enough alternator draw to stall my 350 if they come on when the motor does!), questionable cooling capacity, and added complexity.
All that said, I'd make sure your carb is in tip-top shape first. Even if it's not a problem, whatever you decide to do, it will still be there, so good idea to make sure it's working the best it can, first. Never know, between that, timing, and making sure there are no vacuum leaks, there might be a bunch of power to be had. Enough that the more expensive stuff might not feel necessary, or at least something you can wait for.
I'd defer to others that have run your combo as to camshaft. Mine is an aftermarket roller cam, way too much money for what it is. A hydraulic flat tappet would have been fine in my case.
Electric fans are a crapshoot in these trucks. The trucks can be hard to cool when you are crawling and the ambient temperature starts to climb. The stock fan moves a ton of air (never seen anything stating actual numbers) and it's hard to find electrics that move as much...Ford Windstar fans apparently do a good job, but look at some of the threads on their usage, they are pretty large, and the OEM stuff seems to do a better job of cooling than most aftermarket. IMO electric fans are nice if you need to shut them off for water crossings. Also good to get AC temps down since their flow isn't tied to engine RPM...at idle, if they are strong fans, should bring AC temps down. Outside of that, large electrical draw when on (enough alternator draw to stall my 350 if they come on when the motor does!), questionable cooling capacity, and added complexity.
All that said, I'd make sure your carb is in tip-top shape first. Even if it's not a problem, whatever you decide to do, it will still be there, so good idea to make sure it's working the best it can, first. Never know, between that, timing, and making sure there are no vacuum leaks, there might be a bunch of power to be had. Enough that the more expensive stuff might not feel necessary, or at least something you can wait for.
K5dreamer
1/2 ton status
33in tires, th400, 3.08 gears at 55-60mph was around 22mpg for me 
must have been that magical invisible carb on my 6.2 diesel
but in all seriousness, 10-13 mpg mixed driving dosnt sound too hateful to me for your setup.
must have been that magical invisible carb on my 6.2 diesel
but in all seriousness, 10-13 mpg mixed driving dosnt sound too hateful to me for your setup.
