I have had two '99 4 cyl Frontiers that were automatic. Don't think there is any real difference in the '98 and '99. I got about 24-25 mpg driving mostly on the highway and some city driving. With a manual transmission, you will probably get a mile or two better unless you wind it out before changing gears.
The EPA range estimate for the 2009 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2014 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2010 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2007 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2008 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2006 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2004 Nissan Frontier is 318.0 mi. in the city, 381.6 mi. on the highway.
The EPA range estimate for the 2012 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2013 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.
The EPA range estimate for the 2003 Nissan Frontier is 318.0 mi. in the city, 365.7 mi. on the highway.
The EPA range estimate for the 2001 Nissan Frontier is 318.0 mi. in the city, 381.6 mi. on the highway.
The EPA range estimate for the 2005 Nissan Frontier is 400.9 mi. in the city, 485.3 mi. on the highway.