With insane gas prices, people are looking at motorcycle gas mileage. More people are considering motorcycles to reduce their gas costs. I've gotten over 50mpg on my 2004 Road King Custom, which has the five speed transmission. The new six speed will no doubt give you better mileage.
As you can see by the list below, Harley-Davidson motorcycles get great gas mileage ranging from around 53mpg for the big Screamin Eagle Ultra Classic and the Screamin Eagle DYNA, both with the 110 cubic inch engine to around 60mpg for the 883 Sportster.
If you're considering a motorcycle to lower your gas costs, keep in mind the type of riding your going to be doing. If you're thinking about an 883 Sportster to commute to and from work, remember the types of roads you'll be traveling on. The Sporster gets great gas mileage but I would not want to spend a lot of time on the highway with it. If you are going to be doing a lot of highway riding, I would consider something larger. Either way, you'll be saving money on gas and having fun doing it. Here is a mileage estimate list for Harley motorcycles.
Sportster 883 models: 60 HWY / 45 CITY
Sportster 1220 models: 57 HWY / 42 CITY
DYNA Super Glide: 54 HWY / 35 CITY
DYNA Super Glide Custom: 53 HWY / 34 CITY
DYNA Street Bob: 54 HWY / 35 CITY
DYNA Fat Bob: 53 HWY / 34 CITY
DYNA Low Rider: 53 HWY / 34CITY
DYNA Wide Glide 105Th Anniversary: 53 HWY / 34 CITY
Screamin Eagle DYNA: 53 HWY / 36 CITY
VRSV V-Rod: 42 HWY / 34 CITY
Night Train Softail: 54 HWY / 35 CITY
Softail Custom: 54 HWY / 35 CITY
Fat Boy: 54 HWY / 35 CITY
Rocker, Rocker C: 54 HWY / 35 CITY
Softail Deluxe: 54 HWY / 35 CITY
Heritage Softail Classic: 54 HWY / 35 CITY
Electra Glide Standard: 54 HWY / 35 CITY
Road King, Road King Classic, Road Glide, Street Glide: 54 HWY / 35 CITY
Electra Glide Classic: 54 HWY / 35 CITY
Screamin Eagle Road King: 53.5 HWY / 37.7 CITY
Screamin Eagle Ultra Classic: 53 HWY / 36 CITY