Atrocious Gas Mileage Woes

[Dedaw]

Hey guys, my gas mileage has been absolute $#!& as of late. Little back story: I bought the truck in about June of this year, and I was pulling a consistent high 15s to low 16s MPG. That was a combination of LOTS of city driving and medium freeway driving. As of late, I've been getting excited if I pull out 13.5 MPG(once) and usually pull about 11.5. Since purchasing it, I have put bigger tires on it (was running 28s or so, now to 31s) and will often let it idle during my lunch at work (it's cold at 5 AM!), so I'd say maybe an hour and a half of idling a week. 90% of my commute is freeway driving, and no stop and go since there are maybe 50 other cars on the road to my commute.

 

Are the bigger tires and idling really costing me 3+ MPG? I'm hoping just doing a routine tune-up will fix things, but wondering if it's a deeper issue.

 

On a side note, my truck does not like going into reverse. Often have to try multiple times, and slip the clutch a little to get it to pop in. Heard that's pretty normal for these. Should I plan on upgrading to the newer 5 speeds sometime in the very near future?

[Guest]

Depends on how fast you're going on the freeway; these engines (4.0 / 2.5) have a "sweet spot" that will net you good MPGs on the freeway. If you go past this sweet spot though, you'll notice a slight dip in the miles.

 

Gearing is probably important as well; the increase in tire size definitely makes a difference; especially considering that the truck was probably geared for smaller tires. That's most likely the cause of your better MPGs before the switch.

 

Last time of a full tune up? Just out of curiousity.

[Dedaw]

Depends on how fast you're going on the freeway; these engines (4.0 / 2.5) have a "sweet spot" that will net you good MPGs on the freeway. If you go past this sweet spot though, you'll notice a slight dip in the miles.

 

Gearing is probably important as well; the increase in tire size definitely makes a difference; especially considering that the truck was probably geared for smaller tires. That's most likely the cause of your better MPGs before the switch.

 

Last time of a full tune up? Just out of curiousity.

Errr, fast. Usually in the 70-75 range on the way to work. 60-65 on the way home.

 

I'm sure re-gearing would help a ton, but I can't afford new gears nor new axles at this point.

 

I honestly can't say. When I bought it, I just replaced a few smalls (like air filter) that needed replacing, everything else seemed fine.

[Geonovast]

Those bigger tires are also throwing off your speedometer, which is going to affect the numbers as well.

 

Which 5 speed do you have? 89 was the transition year.

[Dedaw]

Those bigger tires are also throwing off your speedometer, which is going to affect the numbers as well.

 

Which 5 speed do you have? 89 was the transition year.

I calculate them at 3.5 MPH over what the speedo says, which is pretty close according to the radars around here.

 

I don't know, and don't know what to look for. :dunno:

[Geonovast]

Well, larger tires will affect it by a percentage, not a set amount. Those tires are about 10% bigger than your old ones, so at 30 mph with the old ones, you're doing 33 with the new ones, and 60 old will be 66 new.

 

So, if your odometer says you went 100 miles, you actually went 110.

 

This is all assuming your speedo was dead on in the first place, which they never are. It certainly won't account for all the decrease in your mileage, but it is a decent part. Your axle gearing is junk to begin with, the 3.07s really don't like bigger tires on them.

[Dedaw]

Well, larger tires will affect it by a percentage, not a set amount. Those tires are about 10% bigger than your old ones, so at 30 mph with the old ones, you're doing 33 with the new ones, and 60 old will be 66 new.

 

So, if your odometer says you went 100 miles, you actually went 110.

 

This is all assuming your speedo was dead on in the first place, which they never are. It certainly won't account for all the decrease in your mileage, but it is a decent part. Your axle gearing is junk to begin with, the 3.07s really don't like bigger tires on them.

I did forget to calculate that it, so maybe my mileage is a little better than I thought. Still not good, though.

 

I know. I wish I could afford D44s with 4.10s, but that's not possible for a while still.

[xjrev10]

Its also winter. Your fluids get a little bit thicker when it gets cold which makes things harder to move.

 

Ive read that one hour of idle time= 25 miles driven but I have nothing to back up that claim.

[ftpiercecracker1]

Tall gears and bigger tires are going to equal poor power and MPG. If the poor fuel mileage started immediately after you put the 31s on it would seem all to convinent for it to be anything else. A good tune up never hurts either, if your ECU is keeping the engine running in open (closed?) loop it will also really suck down the fuel.

 

1. Do the tune up

2. If you still have them, put the old tires back on and see if you get your mpg back. If so, theres your answer

3. If smaller tires and tune up have no effect, then your problem lies elsewhere

 

4. If your transmission is an ax-15, yes reverse gear is known to be difficult/noisy, at least mine is.

[cruiser54]

Not to mention winter blend fuels cause a fuel mileage loss.

[Geonovast]

Trans ID: http://comancheclub.com/topic/11104-how-to-tell-whether-you-have-an-ax-15-or-a-ba105-5spd-trans/

 

Both the BA 10/5 and AX-15 have issues with reverse, especially when it's cold. Double clutching can help. Also, check your fluid quality and level. Some have reported that higher-end fluids will eliminate the problem altogether (crap/low fluid in your trans can also affect mileage).

[mnkyboy]

I have some used 3.55s to get you by. I could make you a deal on install.

[buffalob]

Do the tune-up for sure.

I found that once I changed the old rotted vacume lines in my mj and every cherokee in the house,

that they idled more consistantly and improved the milage a bit as well....

& ditto to all the above.....

[Dedaw]

It looks like an AX-15, to me.

 

I will be doing a tune-up here, probably tomorrow as I'm sure every auto parts store is closed. Also want to do a clutch soon (it leaves almost completely at the top), but wanted to make sure I wouldn't have to buy yet another one if I got a new tranny. Would the clutch cause grinding when switching gears, or would that be internal? If it is an AX-15 (I need your guys' expert opinions), then I'll be servicing fluids on it at the same time and see if that fixes it.

 

mnky, I might hit you up on that down the road. See what next paycheck looks like, and go from there.

buffalo, do you have a vaccuum diagram handy?

 

Also, would 4.10s be excessive for 31s? Buddy said he had a set of axles he'd give me with 4.10s in them, not sure if he still has them.

[buffalob]

It looks like an AX-15, to me.

 

I will be doing a tune-up here, probably tomorrow as I'm sure every auto parts store is closed. Also want to do a clutch soon (it leaves almost completely at the top), but wanted to make sure I wouldn't have to buy yet another one if I got a new tranny. Would the clutch cause grinding when switching gears, or would that be internal? If it is an AX-15 (I need your guys' expert opinions), then I'll be servicing fluids on it at the same time and see if that fixes it.

 

mnky, I might hit you up on that down the road. See what next paycheck looks like, and go from there.

buffalo, do you have a vaccuum diagram handy?

 

Also, would 4.10s be excessive for 31s? Buddy said he had a set of axles he'd give me with 4.10s in them, not sure if he still has them.

 

I know that there are vaccum diagrams floating aroung on the forum...I literally in the past have just replaced everything I could get at with similar size new rubber lines.

Along with correcting intake manifold leaks. Also have you correctly adjusted the TPS? (took me the longest time to get that one figured out, and I would still be guessing at it without the help on this forum). The other major improvement for me came when I realized that (especially in the cold) that I had several fuel injectors that were seriously leaking.

I had not noticed it before as I have no working e-brake yet (gotta get that done dammit) So it was rare to be on my step stool under the hood with the engine running.

The colder it was the worse they leaked....Bad O-rings (my 4,0 liter has 275(ish) thousand miles on the clock)... Rather than just replace the o-rings I scored a reconditioned bosch set (4 port) off ebay for $ 70.00 shipped. major improvement over the old tired ones, they were shot, clogged and leaking, a miracle I did not have a fire really.

[mvusse]

Also, would 4.10s be excessive for 31s? Buddy said he had a set of axles he'd give me with 4.10s in them, not sure if he still has them.

 

34% lower gears for a 10% larger tires, yes, I would call that excessive.

 

3.55 is THE most common ratio out there in Cherokees and Comanches, and should work quite well with an AX15 and 31" tires. Still a little lower than stock. but some people on here would consider that a good thing.

[64 Cheyenne]

On my 88, AX-15 and on my little S10, I make it a habit to shift to first gear (synchronized?) before I shift into reverse.

Stops the gears from rotating before engagement. Another thing that that helps is to make sure you come to a full stop

before you try to shift into reverse. Sometimes just a little bit of clutch "out" will move the gears enough to engage.

Last thing...you should never bang, or force the shifter into reverse, that is how the shift rail gets bent, then you will be

pulling the tranny for sure if that happens.

 

The 88 Chief has 31s 4.10s used to have 3.07s, I like the 4.10s, think it is about perfect for me.

Still pulls medium hills without a downshift to 4th.

[maddzz1]

Have you changed the speedo gear to the correct size for the tires? If not, there is a chart on here somewhere that will tell you the correct # of teeth gear you need for the tire size. I would do that first then recheck the MPG.

[kaiserslaveray]

Can't you use your GPS on your smart phone to see what your speed is on the freeway? I've used it against my speedometer in my newer car and in friends cars it seems to be right on with the GPS. Any smartphone will have an app that will get you your speed and distance traveled I think.

[Dedaw]

Buffalo, haven't thought a thing about the TPS, will look into that. As far as the injectors, I'm sure those need an overhaul here soon, as well.

 

mvusse, I plan on upgrading down the road to 33s or 35s and don't want to regear yet again.

 

Cheyenne, it does it most often when I am backing out of a parking spot.

 

maddzz, no I have not.

 

kaiser, never thought of that. The GPS on my phone does not show current speed. Just downloaded a digital speedo. We'll see if that works.

[Dedaw]

Bump. Confirmation that it's an AX-15? Payday is tomorrow. :thumbsup:

[maddzz1]

Looks like BA-10 to me. the ax-15 has a black connector type thing in it.

Scroll down in this link there is a pic of each.

http://comancheclub.com/topic/19732-random-informative-picture-thread/

[jimoshel]

That is a AX15. The thick divider plate and no down the middle separation.

[Geonovast]

Definitely an AX-15.

[gogmorgo]

Idling is where your mpg is going. And to be honest, idling really doesn't help your engine warm up. Use the block heater if you've got one, but unless cold is below 0F, I wouldn't bother warming it up before driving, just got easy until you're up to temperature. If it's -20F or lower I might start the engine before I get in and let it run while I go and unplug, scrape windows, brush snow off &c but I don't usually let it idle unless I forgot to plug it in (or I'm parking briefly somewhere I can't plug in) and the temperature is well below 0F.

I know there are mountains, but coming from the Canadian Prairies and having lived for a few years on the Wet Coast, I have trouble believing that it actually gets cold in WA...