Mileage

This program calculates vehicle fuel efficiency and related statistics based on odometer readings, fill-up cost, and price per gallon. It prompts the user for the current and previous odometer readings, tank capacity, fill-up cost, and cost per gallon, then computes miles driven, gallons used, miles per gallon, cost per mile, and projected range on a full tank. The expression CF/CG (cost of fill-up divided by cost per gallon) is used repeatedly to derive actual gallons consumed rather than accepting gallons directly as input. After displaying results, the program waits for any input via INPUT A$ before clearing the screen and restarting with RUN.

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Program Analysis

Program Structure

The program is divided into two broad phases: data collection (lines 5–110) and results display (lines 125–220), followed by a simple loop mechanism (lines 230–250). Line 5 displays a title banner using inverse-video characters. Lines 10–110 prompt for five input values in sequence. After a CLS at line 125, the results are printed across multiple PRINT statements. Lines 230–250 wait for a keypress via INPUT A$, clear the screen, and restart the entire program with RUN.

Input Variables

Variable	Meaning
OD	Current odometer reading
OR	Previous odometer reading (last fill-up)
A	Miles driven (OD - OR)
GAL	Tank capacity in gallons
CF	Cost of this fill-up in dollars
CG	Price per gallon in dollars
TG	Gallons used (CF/CG), computed but not used in output

Notable Techniques

Rather than asking the user directly how many gallons were purchased, the program derives this value by dividing fill-up cost (CF) by price per gallon (CG), giving CF/CG. This expression is computed inline four separate times in the output section (lines 130, 160, 200, 220) rather than using the stored TG variable, which is assigned at line 120 but never referenced again — a minor inefficiency and potential source of confusion.

The PRINT ,,"..." idiom (double comma) advances to the third print zone before outputting text, providing a consistent left-indent effect across the results display without using PRINT AT.

Line 220 applies INT to the estimated full-tank range before adding it to the current odometer, ensuring the predicted odometer limit is displayed as a whole number rather than a decimal, which is appropriate for an odometer context.

Bugs and Anomalies

• TG is assigned at line 120 (LET TG=CF/CG) but never used; all subsequent references recompute CF/CG inline, making the assignment redundant.
• The variable name OR shadows the BASIC keyword OR on some Sinclair variants; while valid as a numeric variable on the ZX81/TS1000, it is an unusual choice that could cause confusion.
• If CG is entered as zero, a division-by-zero error will occur at line 120 and throughout the output section; there is no input validation.
• Line 230 uses INPUT A$ as a pause mechanism (waiting for the user to press Enter), rather than the more idiomatic PAUSE 0 or INKEY$ loop; this requires the user to press Enter explicitly to continue.