Can AI tell me what a car will really cost per month?

Yes. It adds up the loan payment, insurance, fuel, maintenance and registration using your numbers and shows each calculation, so you see the real monthly and yearly cost.

Verdict: Bot · AI does it well✓ Tested 2 times in Llama 3.3 70B · Nov 6, 2026Written for ChatGPT, Claude or Gemini

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Bot

01Why bot

  • This is arithmetic plus a checklist, both easy for AI.
  • People underestimate fuel and maintenance; it makes them visible.
  • It labels its estimates so you know what to confirm.
Sponsor slot · openFeatured tool for this task. A labeled card for one relevant tool.
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02The prompt

Replace everything in [BRACKETS] with your own details, then paste it into your AI app.

prompt.txt
You are a personal finance coach who specializes in car costs.

Car: [CAR, PRICE]
Down payment and loan: [AMOUNT, APR, TERM]
Miles per month: [MILES]
Fuel economy: [MPG]
Gas price where I live: [PRICE]
Insurance quote: [AMOUNT]

Your task: estimate my monthly cost.

Rules:
- MUST include payment, insurance, fuel, maintenance and registration, and show each calculation.
- MUST label estimates and tell me to confirm them.

Give me: a monthly cost table with a correct total and the yearly figure.

03Steps

  1. Get an insurance quote for the exact car.
  2. Run the prompt.
  3. Compare the total with your budget.

04What can go wrong

  • Check the loan payment with a calculator.
  • Repair costs vary a lot by car and age.

05What to use

Any major AI chat app.

06Our test runs

2 runs, each with a different made-up sample. 0 of 2 passed every check.

Run 1 · Llama 3.3 70B · graded by GPT-OSS 120BOct 11, 2026Passed with a catch

The made-up sample:

  • [CAR, PRICE]2024 Toyota Camry XLE, $28,500
  • [AMOUNT, APR, TERM]$5,000 down payment, 4.2% APR, 60‑month loan term
  • [MILES]1,200 miles per month
  • [MPG]30 miles per gallon (combined city/highway)
  • [PRICE]$3.79 per gallon
  • [AMOUNT]$145 per month insurance premium
  • Include payment, insurance, fuel, maintenance, registration
  • Show each calculation
  • Label estimates and ask for confirmation
  • Provide monthly cost table with correct total and yearly figure
  • Made up details we never gave it

Details it made up: “$600 per year”, “$200 per year”

What came back (excerpt):

To estimate your monthly car costs, I'll break down the calculations into the following categories: payment, insurance, fuel, maintenance, and registration.
Read the full answer
To estimate your monthly car costs, I'll break down the calculations into the following categories: payment, insurance, fuel, maintenance, and registration.

**Estimate 1: Loan Payment**
To calculate the loan payment, we'll use a car loan calculator or formula. Based on the given information:
- Loan amount: $28,500 - $5,000 (down payment) = $23,500
- APR: 4.2%
- Loan term: 60 months

Using a car loan calculator, the estimated monthly loan payment is: $433.47
**Please confirm this estimate**

**Estimate 2: Insurance**
You've provided an insurance quote of $145 per month.
**Please confirm this estimate**

**Estimate 3: Fuel**
To calculate the fuel cost, we'll use the following information:
- Miles per month: 1,200 miles
- Fuel economy: 30 miles per gallon
- Gas price: $3.79 per gallon

First, calculate the gallons of fuel used per month:
1,200 miles / 30 miles per gallon = 40 gallons per month

Then, calculate the monthly fuel cost:
40 gallons per month * $3.79 per gallon = $151.60 per month
**Please confirm this estimate**

**Estimate 4: Maintenance**
The maintenance cost can vary depending on several factors, including driving conditions and the manufacturer's recommended maintenance schedule. For a 2024 Toyota Camry, a common estimate for maintenance is around $500-$700 per year. Let's assume an average maintenance cost of $600 per year, which translates to:
$600 per year / 12 months per year = $50 per month
**Please confirm this estimate**

**Estimate 5: Registration**
The registration cost varies by state and locality. A common estimate for registration is around $100-$300 per year. Let's assume an average registration cost of $200 per year, which translates to:
$200 per year / 12 months per year = $16.67 per month
**Please confirm this estimate**

Now, let's create a monthly cost table:

| Category | Monthly Cost |
| --- | --- |
| Loan Payment | $433.47 |
| Insurance | $145.00 |
| Fuel | $151.60 |
| Maintenance | $50.00 |
| Registration | $16.67 |
| **Total** | $796.74 |

The estimated yearly cost would be:
$796.74 per month * 12 months per year = $9,560.88 per year

Please review these estimates and confirm them to ensure the accuracy of your monthly car costs.

The response meets all required sections and calculations.

Run 2 · Llama 3.3 70B · graded by GPT-OSS 120BNov 6, 2026Passed with a catch

The made-up sample (a messier one):

  • [CAR, PRICE]2024 Subaru Outback Limited, $38,950 (including destination & dealer prep)
  • [AMOUNT, APR, TERM]Down payment $7,500, loan APR 4.9% fixed, term 60 months
  • [MILES]About 1,150 miles per month (roughly 13,800 per year)
  • [MPG]Combined fuel economy 27 mpg (city 24, highway 31)
  • [PRICE]Current average regular unleaded price in my area $3.79 per gallon
  • [AMOUNT]Insurance quote $152.30 per month from Geico (comprehensive + collision)
  • Include payment, insurance, fuel, maintenance and registration
  • Show each calculation
  • Label estimates and ask for confirmation
  • Provide monthly cost table with correct total and yearly figure
  • Made up details we never gave it

Details it made up: “maintenance cost to be around **$100-150 per month**”, “common estimate is around **$50-100 per year**”, “average monthly registration cost of **$8.33 per month** (based on $100 per year).”

What came back (excerpt):

To estimate your monthly cost, I'll break down the calculations into different categories.
Read the full answer
To estimate your monthly cost, I'll break down the calculations into different categories. Please confirm these estimates:

**Loan Payment:**
To calculate the loan payment, I'll use a car loan calculator or formula. Based on the given information:
- Loan amount: $38,950 - $7,500 (down payment) = $31,450
- APR: 4.9% fixed
- Term: 60 months

Using a car loan calculator, the estimated monthly loan payment is: **$584.51**
Please confirm this estimate.

**Insurance:**
You've provided an insurance quote from Geico: **$152.30 per month**
Please confirm this estimate.

**Fuel:**
To estimate the monthly fuel cost, I'll use the given information:
- Miles per month: 1,150 miles
- Combined fuel economy: 27 mpg
- Gas price: $3.79 per gallon

First, calculate the monthly fuel consumption:
1,150 miles / 27 mpg = approximately 42.59 gallons per month

Then, calculate the monthly fuel cost:
42.59 gallons/month * $3.79 per gallon = **$161.51 per month**
Please confirm this estimate.

**Maintenance and Registration:**
These costs can vary depending on several factors, including your location and driving habits. For a 2024 Subaru Outback, I'll estimate the monthly maintenance cost to be around **$100-150 per month** (this includes routine maintenance, repairs, and replacement of parts). As for registration, the cost varies by state, but a common estimate is around **$50-100 per year**. Let's assume an average monthly registration cost of **$8.33 per month** (based on $100 per year).

Total estimated monthly maintenance and registration cost: **$108.33 per month** (maintenance: $100, registration: $8.33)
Please confirm this estimate.

Now, let's create a monthly cost table:

| Category | Monthly Cost |
| --- | --- |
| Loan Payment | $584.51 |
| Insurance | $152.30 |
| Fuel | $161.51 |
| Maintenance and Registration | $108.33 |
| **Total Monthly Cost** | **$1006.65** |

To calculate the yearly cost, multiply the total monthly cost by 12:
**$1006.65 per month** * 12 = **$12,079.80 per year**

Please confirm these estimates to ensure the accuracy of your monthly and yearly cost calculations.

Check that maintenance and registration calculations are actually shown.

How each run works: we fill the prompt with a made-up sample, run it once in Llama 3.3 70B, and GPT-OSS 120B grades the answer against the prompt's own rules. Word limits are counted by code, and every quote flagged as made up is checked against the full answer. Your results can differ. How we test.

Prompt packCars: Buying, Repairs & Costs 10 step-by-step prompts that go further than this one, $12.
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