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The difference in mpg between that and another 2000 wrangler (with the original engine) would be negligible. As long as it was maintained properly.

If anything, the one you're looking at wasn't maintained properly and that's why he needed to change the engine.

Request the bill of sale for the "new" engine. Unless it came from Chrysler as a crate engine, it isn't factory-brand new.

If the engine was completely re-built by a trusted engine re-building shop, it will get better fuel economy than a brand new one from Chrysler. Every part inside a crate engine from Chrysler is manufactured by the lowest bidder. Chrysler makes the block but nearly all the moving parts inside are furnished by outside suppliers.

If the motor was re-built and sold through an auto-parts store it is far from brand new! If this was the case, the engine is loaded with low dollar parts and nothing was done to measure, machine or hand fit anything.

Fuel economy comes from carefully selected pistons and rings and a cylinder hone hone job as per the piston ring manufacturer. The rods, (even if they're brand new) should be re-conditioned. The block decks should be trued-up parallel with the center-line of the crankshaft. The rotating assembly should be balanced. Every bearing should be hand fitted.

Small engines in big old fat heavy cars don't get as good fuel economy as a light car with a bigger engine. Your 2000 Jeep Wrangler will never get 28 mpg on trips as will a brand new 430 hp. Grand Sport Corvette.

compared to what? the old engine it had? probably not, especially if they used the old external components like the spark plugs, filters and wires. The only time you might notice a tiny significance is when you do a tune-up which is when you replace old spark plugs, engine oil and filters with new ones and transmission oil (change time varies from car to car). Also proper air in the tires helps and not having a ton of extra weight inside the vehicle as well. What also helps is cleaning the fuel injectors with a cleaner every six months.

The engine is probably rebuilt, rather than new. The quality can very, but if it is done by a capable outfit, it may be a good engine. If they bored out the cylinders, it will have a greater displacement (cc's) so it will have slightly more horsepower and worse fuel economy.

Find out if it is truly a new (factory) engine or, more likely, a rebuilt. Make friends with a gear head (if you don't already have any gear head friends) and have them figure out how good the rebuilt is.

The answer is no. Better gas mileage depends on which KIND of engine and horse power your have has coupled with your driving habits. Small engines with low horse power gives you better mileage than the opposite.

Dont worry, but dont, a newer engine wont give you better MPG, not if compared with a good running same (but older engine)