That year/model has a 10.6 US Gal. tank
The original mpg rating for the 88 Excel w/4-speed manual transmission was 27/33. (lower for automatic and higher for overdrive-equipped manual)
Assuming as always that is an optimistic figure, you would be hoping for at leat 25/30 - so 250 to 300 miles on a tank would be closer to what you should be getting.
Along with the tune-up, put in a new air filter and get an oil change - (10w30 recommended). Also check your tire pressure all around - low pressure and worn tires can also degrade mileage. Summer tires will do a lot better than all-season radials
In all, these are not very powerful vehicles (68 hp) so how you drive can also make a big difference in fuel consumption.
Best wishes