Does a cooler thermostat reduce gas mileage on our trucks for some reason?

I ask this because I filled up the other day after switching back to stock (previously 9 MPG on SCT performance tune) and I still only got 10 MPG. Should I change it from the 180* back to the stock 195* and see how that works? I don't know what else to do. The window sticker on this trucksays 13-city/17-highway and I am nowhere near that even with the cold air intake, headers, and cat-back.

I drive conservatively (never above 2k RPM in town)and I run premium in it. I recently changed the cap, rotor, plug wires, and plugs at about 108,000 miles. It has just under 110,000 on it now. I'm not sure what pressure I should have in my tires but they seem to be properly inflated and they are wearing evenly across the tread.

 

it may but it is not significant enough to swap IMO... I tried it to when I had my 2000 5.9 gasser. one cheep trick that will help is to wrap your fuel rails with insulating tape to prevent heat soak, tape runs under $20 bux. make sure you use a mopar cap & rotor, and autolite 3923's.

what size is your air filter & is it well insulated? (from engine comp. heat)

 

I think tire pressure might have something to do with it. They all have 42 pounds in them now but they had about 32 when I checked them at first a few minutes ago. They have a C load rating and the max is 50 psi per tire, cold. The cap and rotor are from NAPA, the wires are MSD, and the plugs are the stock replacement 5124s (I think). Will going to the 3923s help mileage for some reason?

I'll have to try that insulating tape trick to see how it works. Oh, and the air filter is situated like the one on this truck but it is a K&N kit from Summit Racing.


Here is what mine looks like with the little barrier between the filter and the engine compartment.

 

change your plugs. 5124 i never heard of for our dodges

 

Steve,

My truck is all stock and the Autolites made it run like crap. Mileage went through the floor, acceleration was lousy, and it was misfiring on two cylinders. When I put the Champions back it ran like a top again.

 

I'm not sure on the exact number but I do know that the guy at NAPA looked them up so they ARE the stock spec plugs. I'm just not sure what the number is.

 

A switch to a cooler thermostat will decrease fuel economy (but I doubt by enough to be significant) because the cooler running temperature will cause a richer mix (A/F ratio).
There are many benefits to runninga cooler t-stat, but fuel economy is not one of them.
In my case, I was running a little on the lean side with a CAI, LT Headers & a tuner set for performance (which advances the timing). Not lean enough to generate a CEL, but lean none the less. The addition of the 180* thermostat richened me up just enough to bring me almost perfectly back to stock spec.

With your setup (CAI, LT Headers, etc.) I'd venture to say you were also running a good bit on the lean side. Probably explains the worse gas mileage when you added the t-stat. In my opinion, I'd give up the mile or two per gallon and run with a more proper A/F ratio you are probably at with the lower temp t-stat. A small price to pay for long engine life. Even if you are not lean enough to popa CEL, long term running lean is not a good thing.
Of course you can take it someplace and have your A/F ratio checked with/without the lower temp t-stat to confirm if you need to be running with it or not...

 

The 180* t-stat is still in the truck though.I set the programming back to stock and only picked up 1 MPG. I keep hearing about guys here with the same kind of truck I have that are getting 13+ in town and I am wondering what could be causing my MPG to suck this bad. Maybe these guys that are getting great mileage are simply idling down the street at 10-15 under the limit.

Anyway, now that the programming is back to stock, the exhaust smells like it is running rich to me. Not quite as bad as with the performance tuning, but still pretty strong. So I basically have everything in my sig except for the SCT tuning right now and I'm getting 10 MPG in town. That is not good.

WhatI should have done is check the mileage before I started modifying the truck. I have always been told that minor bolt-ons like a CAI, headers, tune, etc. will actually increase your mileage as long as you drive the same way you did before modifying it.

 

Dumb question, and maybe someone's mentioned this before, but how's your plenum gasket?

I have all the mods in my signature, I drive it with a lead foot, and I get 11 mpg on average in town on 89 octane fuel. Some days I get 10.5, some days I get 11.5 or 12, but on average I usually tend to run very close to 11. So something is off to make you get 10 mpg.

 

I don't know what these guys are doing to get 13 mpg in town. When I had my '98 RCSB 4x4 with the 5.9L I averaged about 10 in town and if I stayed at or below the speed limit I might squeeze 13.5 on trips...