new subscriber/v8 miles to go calculation

[Rngmeister@aol.com]

hi all, veteran of the 20v list, new subsciber here, so pardon any faulty
semantics.
anyway, on the '90 v8 my wife drives, more often than not, the "miles to go"
till empty reading on the computer gives numbers that are hilariously
incorrect.  What's the easiest way to determine the problem, or has someone
already found a simpler solution to banging on the dashboard, my wife's, which
does absolutely nothing but sometimes make the speedo quit
momentarily.....assumptions to make: car is '90v8, 115k miles.  3rd owner,
local NC piedmont car....somewhat of a mechanic (required for audi ownership
anyway)
do not assume that i: have a service manual yet,  have done much to this car
myself in the way of repairs. 

many thanks

any help would be greatly appreciated.....

rngmeister@aol.com	
Wylie Bean
Greensboro Nc
90 cq -TAP ecu/Ramair/Eibach/Bilstein Sport/Powerdisc equipped
90 v8 - stock