What Does the Keep vs Change Car Calculator Do?
Keeping a car and replacing it are future-cost alternatives. This calculator compares the depreciation and running costs expected from today over one chosen period, adding any one-off switching fees. It helps separate a desire for a replacement from the claim that replacement will save money. Reliability, accessibility and other practical reasons can still matter even when keeping the current car has the lower modelled cost.
How to Use This Calculator
- Enter what the current car is worth today and an estimate for its value at the end of the comparison. Use today's value rather than the amount paid years ago.
- Enter the replacement's purchase price and expected value at the same future date. Add one-off switching costs that are not already included in those prices, taking care not to duplicate fees.
- Enter annual running costs for both options and a common number of years. Include the same categories on both sides and use finance interest rather than counting loan capital again alongside depreciation.
- Calculate and inspect the period difference and required running-cost saving. Test another resale value or repair allowance if a modest change could reverse the decision.
The Maths & Assumptions Behind the Tool
Keep cost = current value − future value + annual current running costs × years. Change cost = replacement price − replacement future value + replacement running costs × years + switching costs. The comparison is change minus keep over that selected period. The annual saving required to cover extra depreciation and switching costs divides those extra amounts by years. It is not a forecast of a separate break-even date beyond the entered resale horizon.
Worked example
Over three years, keeping a car worth £10,000 now and £6,000 later costs £4,000 depreciation plus £3,200 × 3 in running costs: £13,600. A £22,000 replacement worth £14,000 later costs £8,000 depreciation, £2,200 × 3 running costs and £300 switching fees: £14,900.
Keeping is £1,300 cheaper. The replacement would need to save £1,433.33 per year in running costs to offset its £4,300 extra depreciation and switching cost over this three-year period. The entered annual saving is only £1,000. This threshold is not a prediction of the payback year.
Why This Matters for UK Drivers
The current car's value today represents money tied up in the option to keep it. The relevant depreciation is the difference between that value and the estimated value at the end of the period. Its historic purchase price is not entered. For the replacement, use the actual purchase price and a resale estimate for the same future date. Annual running costs should cover the same categories on both sides, including finance interest where applicable.
Repair risk deserves scenarios rather than certainty. An older car may need more work, but a possible expensive repair should not automatically be treated as a guaranteed annual bill. The extra-repair comparison shows how a one-off allowance changes the result. Avoid adding repairs twice if they are already in the running-cost estimate. The tool reports the annual saving needed over the selected period; it does not extrapolate those end-of-period values into an apparently precise future payback date.
Interpreting the result in practice
When considering an older car, list known work separately from uncertain repair risk. A quoted repair needed now is different from an allowance for something that might fail later. Model a base case and a repair case, then consider how much uncertainty you can tolerate. A purely financial result does not account for accessibility, reliability needs or the inconvenience of downtime.
Selling the current car helps fund the replacement, but sale proceeds should not also be subtracted from the replacement's depreciation calculation here. The value of keeping that car is already represented by its current value and future value. Mixing an incremental cash outlay with both complete depreciation paths can double-count the same asset. Use a separate payment schedule when you want to assess the cash needed on the changeover date.
Frequently asked questions
What should I include in annual running costs?
Fuel or charging, insurance, tax, servicing, MOT, tyres, repairs and any other recurring costs that differ between the cars.
Why use today's value for my current car?
Keeping a car has an economic cost because the asset you own today can still depreciate. Ignoring that value can make “keep” look artificially cheap.
Should finance interest be included?
Yes, if it is material. Add expected finance interest to the replacement's annual running cost or one-off switching costs.
The hidden cost of changing cars
What the number tells you
Changing to a newer or more efficient car can cut fuel and repair costs, but the replacement vehicle may lose value faster. Comparing only monthly fuel bills can therefore make a change look better than it really is. This tool puts depreciation and running-cost differences into the same ownership period.
Use realistic inputs
Use current private-sale or trade values rather than the price you originally paid for your existing car. For the replacement, make a reasonable future-value estimate at the end of the comparison period. If those values are uncertain, run a low and high depreciation scenario.
What is outside the calculation
Non-financial reasons can still justify changing: reliability, safety technology, space, emissions or personal preference. The calculator is intended to clarify the financial trade-off, not decide whether you should keep a particular vehicle.