2.3L ('83-'97) Reconsidering awful MPG

[lowspeedpursuit]

Obviously "so I slapped a lift and big ol' tires on my truck and now mileage sucks, what do?" threads are a dime a dozen. As I slowly made my truck bigger and heavier, I figured I couldn't complain too much about whatever I ended up with... until I was looking up something unrelated and saw most people seem to post their 2.3Ls are still reliably getting 20+ mpg average, even with larger tires.

I get 16 mpg, highway, on a good day. 31"s on a ~3" combo lift, 4.56 gears, and it was maybe 1mpg better on 29"s. New O2, MAF, etc. No codes except the soft ones from running a manual swap with the auto computer.

Now that I've realized this isn't exactly expected, what am I missing here?

 

[franklin2]

Pump the tires up to max pressure, do not drive as fast on the freeway, and get less aggressive tires. You are up in the wind now, so high speed driving is going to have a lot of wind resistance. If you have knobby tires, you know that noise they make going down the highway? That is energy turned to sound, aggressive tires have more rolling resistance.

 

[scotts90ranger]

Yeah, that's part of it for sure... Back when my '90 was on 31's with 4.10's it got around 20mpg... rpm is a thing too, my '97 likes running under 3000rpm for cruise for mileage, with more aggressive tires it got 21mpg now with some street tires it averages 23mpg...

I'm going to assume you have your speedometer corrected for the gears and tires?

 

[lowspeedpursuit]

I am the dumbest man alive, although at least I figured it had to be something stupid.

Before the last thread I posted about finishing my cruise control, this truck rarely saw the highway, and I often drive on sand away from air stations, so... my tires live at 27psi. A quick google says a >5psi difference can account for 5 mpg as well.

Speedo is accurate to within 2mph, which is less than jitter. Tires are Yokohama Geolandar ATs, so fairly conservative, although wider grooves than the old DuraTracs.

Gotta drive up to the city sometime next week, so I'll reassess at 30-35psi.

 

[Blmpkn]

How fast do you drive on the highway? And how heavy are the 33"s? Fuel leak?

Big tires and lift or not.. 16mpg still seems awfully low.. Unless you drive like a teenager

 

[lowspeedpursuit]

I'm on 31"s, I'll edit the top post to be more clear. Looks like 42lbs. ea. Can't remember if the 15" aluminum B-series wheels are 15lbs or 20lbs ea, I just know they're 5lbs less than steelies.

I'm around ~70mph highway cruise. Might accelerate "aggressively" depending on where you draw the line, but the truck's so slow that anything less would get me run over.

 

[lowspeedpursuit]

Well, no improvement at 35psi, so I guess back to no great ideas at this point.

Are people on even bigger/heavier setups miscalculating their MPG somehow, or is my truck broken in some obscure way?

 

[Frank S]

What transmission are you running? You need to figure out what your gearing SHOULD be with those tires. I use the calculators at www.4lo.com (just because it's easy to remember). The tire diameter calc is for LT tires (more aggressive tread car tires, really) so won't help you much there. The diameter should be as you stated though, 31" or 29". If this is 2WD just put in "1" for the transfer case ratio (or even if 4x4 in high range). You need to cruise at the speed the engine is making good torque. For older, larger engines that's around 2000 rpm. The Ranger version Lima 2.3L torque peak is 2600 rpm, so you need to be running close to that at 70 mph in OD... actually closer to 3000 rpm. Yeah, that's turning a lot, but it doesn't make good low speed torque like even an old 200 six -- and that one peaked at 2400 rpm. I'm more used to the AMC sixes, which peaked in the 1600-1800 range (stock). Even with those we want to cruise around 2000 rpm, a little over the torque peak. If you're not the engine is working a lot harder than it needs to, and that is killing your gas mileage and low speed power. You have to gear it higher to make up for it.

You mentioned gaining 1 mpg by dropping the tire size from 31" to 29". With the 31" diameter tires you should be running around 2700 rpm, with 29" 2900 rpm. What you really need to do is get a 26" tire (~3250 @ 70 mph) and see if that helps. Reducing the diameter has the same affect as increasing the gear ratio (bigger number). At some point the high rpm will start to negate any gains, but running at 3000 rpm with a small engine and big tires is not going to get you there. Mpg will probably bump up again, another 1-2 mpg. The rev limiter is 6250 rpm, I think. You just have to rev up a small engine to make any power. You need a 5.01 rear gear (I know they don't make that ratio, but closer to that) to run 70 mph in OD at 3000 rpm.

 

[lowspeedpursuit]

There may be some merit to what you're saying. I regeared based on what was on the truck when I got it, which was 215/75R15, and also based on the fact that my jeep is definitely overgeared, and it's awful to drive on the highway at all. Per the door sticker, stock tires were smaller, something like 195/70R14, so my current setup turns very slightly lower RPMs than that, which runs counter to conventional wisdom.

Practically speaking, it's probably a non-starter. Setting aside that gearing deeper would make it impossible to calibrate the speedometer, the only D35 reverse gears still available right now are 4.10 and 4.56, plus my stock 3.73s. When 4.88s were last available, IIRC they were hitting $400+ anyway.

We're not hypermiling here. The point of the truck is to do truck stuff. There's no merit to dropping to a 26" that's smaller than what I got it with. I guess I could run the same experiment by bumping up to 33"s and cruising in 4th, but then I'm definitely undergeared for normal driving.

 

[Frank S]

Unless you're going in the mud a lot or for some other reason really need the ground clearance a shorter tire won't hurt. Doesn't look as cool though, I know... It will pull better with the shorter tire also, not just cruise in fifth better.

 

[lowspeedpursuit]

I put the 29"s on originally because that's the minimum for me to be able to crawl under the truck without jacking it up. The 31"s were the eventual result of only 4.56 gears being available at the time, when I had originally targeted 4.10. I still almost got high-centered in the dirt literally this morning.

In any event, it really depends on your goals. If my primary goal was "better MPG", then yeah, obviously dropping lift or tire size back down will help with that. But, "how do I improve MPG?" isn't the question.

The question is, what am I missing? Why does my truck seem to get notably worse MPG than other posters with similar setups.

Earlier in this thread, scotts90ranger quoted 20mpg, also on 31"s, with taller gears, for example. What's the functional difference, then, between his truck and mine?

 

[bobbywalter]

manual swap with a auto computer.


that is a definite possible mpg killer.

 

[lowspeedpursuit]

I know relatively little about engines themselves. Is that because the auto computer runs less timing advance, and that reduces both MPG and power?

I have a manual computer, but '94 2.3L manuals always had EGR, and autos did not. If I swap the computer, I get a permanent CEL, and my understanding is that will just pull the timing anyway.

Is there a way forward there without building an EGR system?

 

[bobbywalter]

turn off the egr on the manual pcm

 

[lowspeedpursuit]

Is that a thing? Even assuming I chipped the manual computer, I wasn't aware anyone ever bothered to decode the relevant tune.