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Dodge Truck milage

I have a 2000 Dodge 4x4 truck with 30k miles.It was getting 13-14 mpg. I changed the plugs, muffler, tires (same size). Now I only get 10-12 ! It is colder outside now also. But it is rated for 12-17 mpg. I cannot even get close. I try to drive very conservatively. Any ideas ?

Can you detect any signs the engine performance has changed, idling rougher, hesitation, less acceleration? How about tire pressure?

Now you are talking about a 20% change, quite small. Winter vs summer will do that. As will any change in driving habits, or terrain (city vs highway)

The engine seems to run smooth, not alot of “pep” compared to other brands that I have driven. Tires are at about 40lbs. I drive mostly on the highway, to lane about 55 mph 30 miles each way. I try and be mindful of accelerations and such.

The tires may have a higher rolling resistance that can yield a 1-3 MPG loss in a vehicle. Are they drastically different tires?

I have a GMC 4WD with a 4.8 V-8 and auto. It will get about 15 MPG with the type of driving you are doing and about 17 on a long highway trip in Summer. The tires are P245 75 R 16. Your gas mileage is low now but you’ll have to wait for warmer weather to compare mileage again.

I never go by the Manufactures rating on gas milage. Most of them are done under ideal conditions including temperature and humidity, altitude etc. None of us drive under those conditions.

did you replace the sparkplugs with the EXACT plug that came out of it, or did you ‘upgrade’ to a ‘better’ platinum, split, or otherh fancy plug?

Chances are the area where you are living possibly uses a winter blend of gasoline, which tends to give worse gas mileage.

Second, since the weather is colder, the engine is running considerably richer at startup, all the way until it is fully warmed up and is in full feedback mode. If you were to start using an engine block heater, you might be able to decrease this time in closed loop mode where it is running rich.

Third, unless you used the exact same tire that you replaced, that might have contributed to your loss in mileage.

Finally, you didn’t mention that you changed the air filter, or the oil and oil filter.


In addition to those other cold weather related thoughts, your tire pressure also drops in cold weather and that increases rolling resistance. Have you checked the tire pressure.

Winter causes lower mileage because of the cold weather and possibly die to winter blend gas. It’s not that much of a drop. I assume the tires have the right pressure since they are new. If the tires are different than the OEM tires, that could lead to poorer mileage, too.

I’m with cappy on this one. If your mileage changed with the “tune up”, you may have gotten the wrong plugs. This is fairly common.
It is also possible that your new muffler has adversely effected performance.

These are all great suggestions. The tires are the same size as removed 265/16 the plugs were an “upgrade”, the muffler is a higher flowing flowmaster type. The air filter was replaced, oil is synthetic.

I’d be suspicious of anything labeled “upgrade”. Perhaps you should switch back to the OEM plugs. By my calculations you are losing $60 per month, enough to justify new plugs.