Question about 2 stroke fuel economy
 

Right now I'm interested in purchasing an "Enduro" style dirt bike that is suitable for on road use and is legal for riding on the road. I found a dirt bike for $500 - it's a 1980 Suzuki PE 175cc, 2 stroke engine. I've always heard that dirt bikes get pretty poor fuel economy, and from reading on various forums I've seen numbers as low as 15-20 miles per gallon while riding on trails. However, I've never seen any numbers posted for a 2 stroke dirt bike riding on a paved road at a constant cruising speed. The Suzuki PE 175 has a top speed of 75 miles per hour, so what kind of mileage could I expect to get out of it cruising at say 45 miles per hour?

You cam get decent economy, but it'll likely need to be regeared.

Two-strokes are inherently less efficient than four-strokes because of the way the ports are arranged - there is a significant amount of time when both the intake and exhaust ports are open and the exhaust closes after the intake. As a result a good portion of the incoming charge is lost out the exhaust. There are all manner of devices to try to minimize the effect, such as expansion chambers, slide valves and so on but they (other than expansion chambers) start adding a lot of complexity and the initial advantage of the design - low parts count, light weight, etc - start to get lost. The exception to that being direct injection...

If Envirofit has a kit for that motor, it'll convert it over to electronic direct injection; cleans the emissions up, boosts the mpg up 30-40 percent. Combine that with a regear and an optimized expansion chamber, it could do pretty well.

What I don't like about the synerject kits is that they still have the incoming air going through the crankcase, with the oil in it. A system that gets rid of that, e.g. using a scavenging air blower rather than the crankcase for scavenging, would be better.

The other disadvantage of 2 stroke engines is that they tend to need to be rebuilt more often.
I owned a Yamaha RD350 for a while, a street two stroke motorcycle and it sucked down the gas, getting 40mpg or so and it was a light motorcycle so I can only imagine what an enduro would get.
There are nice 4 stroke off road bikes out there, why not choose one of them? you'll spend more time ridding it then fixing it then if you get a two stroke.

I think one of the reasons two strokes get rebuilt more often is the lubrication system; it's pretty easy to starve the cylinder of oil. On the plus side, they are a lot easier to work on than four strokes.

I put 25,000 miles on a 1971 Suzuki TS-90 two stroke enduro. I used to commute 32 miles each way into Denver, and averaged 75 mpg on that commute. I also did the same commute on a 1966 Suzuki T-20 two-stroke 250 cc. twin, the 6 speed X-6 Hustler, and averaged 55 mpg with it. Two stroke efficiency is all over the map. I had a Suzuki T-500 two stroke 500 cc. twin that only got 33 mpg.

With the technology out there now, I can't see why with a custom expansion chamber, efi, good oil and tuning that these engines can't get equal to a 4 stroke.

because they are still not efficient under most of their rpm range and get worse away from torque peak, and they still burn oil.
2 strokes, while they have good power to weight, have poor power to fuel (bsfc), compare carb to carb, compression to compression. And the extra fuel per horsepower equates to extra pollution.
from Using BSFC #'s