9 MPG in I6????

Hello,

I was wondering if someone can tell me if 9 MPG is to be expected on a 74 CJ5 with a 4:27 rear-end on 33" inch tires? These gas prices are killing me...it's my daily driver.

Also, there is some light blue smoke out of the exhaust and it smells strong...most noticeable at idle. Speaking of which, I've got some carb issues and what is a good carb to get should I need to replace mine? (I haven't the slightest idea what kind is there now). Any ideas on carb cost?

Thanks much!

Derek

 

Sounds like you are running rich. but your not going to get steller milage even on a properly running engine.

 

Is your speedo calibrated to run 33" tires. Since the factory tires were more like 29" tall thats a difference of about 17% that your speedo would be off. Speedometers are not off a set amount, they are off a percentage. You'll need to calculate that along with your gas mileage if its not been corrected.

 

Motorcraft 2100 is a popular swap. If you have the cash, I would fuel inject with a Howell kit. Wish I had the cash to do that upgrade. The others are right, if your speedo isn't calibrated for 33's your mileage estimate will be off.

High5

 

True about the wheel height, but at most that accounts for 2 mpg. You should be running in the mid to high teens, unless you have a very heavy foot. <edit> Also, the aerodynamics of a Jeep are, well, not very good. Air resistance goes like the square of velocity, so you're gonna burn a lot more fuel if you drive fast. Keep the speed down if you want to save gas.

The stock Carter BBD is a fine carburetor; only go to a 2V if you want more performance. Probably not worthwhile unless you change to a 2V manifold too.

The EFI would be good even with the stock manifolds, giving you better drivability, economy, a little more power, and better offroad-ability. If you're handy with electronics, the mega-squirt is another option besides a bolt-on kit like the Howell. http://www.bgsoflex.com/megasquirt.html

 

blue smoke = oil (rings, valve guide seals,etc)
black smoke = fuel (aka rich, carb rebuild, adjust, clean or all of the above)
white smoke = water (head gasket, head cracked)

I don't think your mileage could improve a whole lot with out going to fuel injection and 29" highway treads and 2.73 gears. Fine tuning might get you a couple mpg's though and at 2.59 a gallon that certainly makes a difference.

My Grand Wagoneer was getting 6 mpg. Sold it last night.

 

Hmm. I think you should be able to get 15 or 16 mpg even with the 4.27 gears. It's been a long time, but I recall that's what I got with my '75 CJ-6 (258, T-14, 3.73s) and it had 31s and a metal cab. Mostly freeway driving - I didn't drive overly fast, but kept up with traffic.

 

Thanks for all the ideas. There is no way I can afford an EFI upgrade at this point.
When I'm driving on the highway, I have the pedal floored..it's the only way I can keep up with traffic. As I'm going with traffic, my tach reads 3 thousand RPM and traffic moves at about 65-70MPH, with 60-65 being my top speed. Is that the norm?

I had 31" tires on previously, and since I added the 33's, theres no real discernable difference in performance/mileage.
Thanks!

 

Realize that the tire height will affect the accuracy of your speedometer, so at 65 mph you are actually going about 74 mph.

The RPMs are no problem - at 65 you should be doing 2800 to 3000 RPM, depending on the actual size of your tires. A 258 should have plenty of reserve at that speed on level terrain, 1/4 to 1/3 of the throttle I would guess.

 

How would one re-calibrate the speedometer for accurate use with larger tires, such as the 33's?

 

You would have to change the gear on the transfercase end of the Speedo Cable. I do not know what you would have to change it to though.

 

AFAIK you could a) use a different gear in the transfer case, or b) buy a device that goes inline with the speedo cable to change the relative speeds (basically a little gearbox - Stewart-Warner sells/sold them) or c) use an aftermarket electronic speedometer that can be calibrated to various tire sizes and gearings. Those are the only options that I know of.

 

Just make sure every thing is working correctly. My vacuum advance was shot for a while and my gas mileage was horible.. Now that it is fixed my mileage is a lot better. As for a carb to swap to i used a motorcraft 2100 It required a bit of fittling to get the manual choke to work right. To fix the speedo you will need to get a diffrent spedometer gear.

 

Hi all

Another idea to correct the speed, is to read it on the tach. I did a little Microsoft Excel chart with the gearing of the tranny, the gearing of the t-case (hi or low),the gearing of the axles and the tire size. This way, I know that with my drivetrain (stock 1977), at 3000 rpm with 31's, I'll run around 78 mph. Here in Québec the two most used speed limits are 50 km/h and 100km/h (basically 30 and 60 miles / hour)

So in second high, at 2100 rpm, I'm at 50 km/h, and at 2400 rpm in third high, I'm at 100 km/h. This is all I need to know.

If someone wants the chart, just let me know, I'll send it


Bob in Sherbrooke, Province of Québec

 

My 74 with 258 I-6 is original as far as I know. After a carb rebuild, I get ~ 16 MPG. I have never gotten to 17. My driving habit is local run around town, almost never on the highway. If I do get on the highway, I can get to 60 down hill with a strong tail wind. 55 is about max. Everybody, & I mean everybody, passes me like I am standing still. Kind-o-scary.

 

We have a '79 CJ7 with the 258 six and are getting 24 mpg highway with 5:38 gears, stock wheels and tires, and dual exhaust. We get about 16-18 in town.

 

ok, so it looks as if I'll go the carb route. Out of curiosity, what does just a normal replacement carb run if it comes down to that? I'm a student so I can't go balls out with it.

You guys have been great by the way. Thank you.

Derek

 

If you are driving around with the the accelerator to the floor you are lucky to be getting 9. You definitly have something wrong.

 

Use your GPS to get a mental fix. I found that while running 35" tires, for every 25 miles I have to add 5 miles to the odometer. I take this into consideration when calculating my mileage. I also reference my tach as my speedo.

 

can't imagine your rpm's on the highway!