I have a car that was purchased in 2010 for around 20k, it now (2016) needs new tires but im going to replace it in 12 to 36 months time. The current resale price is around 8K and the replacement of the tires is around 1k. Im guessing the replacement car would cost between 25-30K as the purpose of the car has changed.

Emotionally I think if I'm going to replace in 12 months time i think im better off purchasing now but at 36 months time im better off waiting. Im not interested in the emotion of this I would like to see a calculation that can be used to guide this decision to the one that is likely to be the best one on an objective financial basis

Remember its the calculation im after not advice on actual prices unless that forms part of the calculation.

  • 8
    $1,000 tires? Have you shopped that around? You'll probably have trouble selling a car with bald tires, and will likely pay for the cost in reduced sale price anyway. You might as well buy some tires and drive the car while you determine when it's best to sell it.
    – quid
    Aug 24, 2016 at 23:20
  • I'm really after a calculation that I can put any number into $500 to 1K could be placed into it. Also this varies lots on which countries and which type of car the tires are going on and weather its being installed does it include disposal, etc, etc. Aug 24, 2016 at 23:29
  • Are the tires the same ones originally purchased with the car? If so, they're probably already overdue for replacement. Basic tread wear isn't necessarily the major safety concern. Aug 25, 2016 at 10:30
  • There still isn't enough information in this post to calculate whether to buy or wait. What do you expect to spend per month on repairs for the current vehicle? (You can't know the answer to that, but there are estimators available.) Will you be paying cash or financing your next purchase? How much do you expect the residual value of this car to decrease each year? How much do you expect the residual value of the replacement car to decrease each year? Aug 25, 2016 at 15:54

8 Answers 8


If the car is in otherwise good shape, it's always less expensive to keep it longer.

Think of it this way: you have to buy new tires no matter what. It's just a question of whether or not those new tires are attached to a new car or your current car.

  • 1
    The buyer of the old car will have to buy new tires as well, which will reduce the amount that you get for the car.
    – gnasher729
    Aug 25, 2016 at 10:09

I don't see how anyone could give you a hard-and-fast formula, unless they know where to get some applicable statistics. Because several factors here are not a straight calculation.

  1. If you don't replace the tires but keeping driving the car, what is the increased probability that you will get into an accident because of the bald tires?

  2. How much will bald tires vs new tires affect the selling price of the car? Presumably the longer you drive the car after getting new tires, the less increase this will give to the market value of the car. What's the formula for that?

  3. If you keep the car, what's the probability that it will have other maintenance problems?


That said, it's almost always cheaper to keep your current car than to buy a new one. Even if you have maintenance problems, it would have to be a huge problem to cost more than buying a new car. Suppose you buy a $25,000 car with ... what's a typical new car loan these days? maybe 5 years at 5%? So your payments would be about $470 per month. If you compare spending $1000 for new tires versus paying $470 per month on a new car loan, the tires are cheaper within 3 months. The principle is the same if you buy with cash.

To justify buying a new car you have to factor in the value of the pleasure you get from a new car, the peace of mind from having something more reliable, etc, mostly intangibles.


I tend to agree with Rocky's answer. However it sounds like you want to look at this from the numbers side of things. So let's consider some numbers:

Cost of new car: $25K 
Value of old car: $8K
Out of pocket expense:  $17K

I'm assuming you have the money to buy the new car available as cash in hand, and that if you don't buy the car, you'll invest it reasonably. So if you buy the new car today, you're $17K out of pocket. Let's look at some scenarios and compare. Assuming:

Annual Inflation Rate: 2.5%
Old Car Depreciation Rate:  8%
New Car Depreciation Rate:  20%
Annual Investment Returns: 5%

If you buy the new car today, then after 1 year you'll have:

Starting Cash:  $17K
Value of Old Car:  N/A (sold)
Cost of New Car:  $25K
Value of New Car:  $20K
Remaining Cash: $0 (spent buying new car)
Total Assets (new car + cash):  $20000

If you keep the old car, after 1 year you get:

Starting Cash:  $17K
Cost of Tires:  $1K
Value of Old Car:  $7360
Cost of New Car:  $25625
Remaining Cash:  $16800
Total Assets (old car + cash):  $24160

After 2 years, you have:

Starting Cash:  $17K
Cost of Tires:  $1K
Value of Old Car:  $6770
Cost of New Car:  $26265
Remaining Cash:  $17640
Total Assets (old car + cash):  $24410

And after 3 years, you're at:

Starting Cash:  $17K
Cost of Tires:  $1K
Value of Old Car:  $6230
Cost of New Car:  $26920
Remaining Cash:  $18520
Total Assets (old car + cash):  $24750

Or in other words, nothing depletes the value of your assets faster than buying the new car. After 1 year, you've essentially lost $5K to depreciation.

However, over the short term the immediate cost of the tires combined with the continued depreciation of the old car do reduce your purchasing power somewhat (you won't be able to muster $25K towards a new car without chipping in a bit of extra cash), and inflation will tend to drive the cost of the new car up as time goes on. So the relative gap between the value of your assets and the cost of the new car tends to increase, though it stays well below the $5k that you lose to depreciation if you buy the new car immediately. Which is something that you could potentially spin to support whichever side you prefer, I suppose.

Though note that I've made some fairly pessimistic assumptions. In particular, the current U.S. inflation rate is under 1%, and a new car may depreciate by as much as 25% in the first year while older cars may depreciate by less than the 8% assumed. And I selected the cheapest new car price cited, and didn't credit the tires with adding any value to your old car. Each of those aspects tends to make continuing to drive the older car a better option than buying the new one.


It depends how detailed you want to get in your calculation, but fundamentally, 1K < 25K.

On a very basic level, divide the cost (less what you sell it for) by the time you'll have the car for. If you junk it, $1K/12 month = $83/month to buy tires to have a car for a year. If you sell it for $1K, then it become $0/month. (Plus other maintenance, etc..., obviously).

If you pay 25K and keep the new car for ten years and sell it for nothing, it becomes roughly $208/month (plus maintenance).

If you want to get more accurate, there are a lot of variables you can take into account--time cost of money, financing, maintenance costs of different vehicle types, etc...


Would you buy this used car, in its current condition but with new tires, for the price of the tires?

If so, buy the tires.


New tires will increase the resale value of the car; while not by the full cost of the tires, it will not be entirely a sunk cost.

You'd need to factor that in and find out how much the new tires increase the resale value of the car to determine how much they would truly cost you. However, I suspect they would cost you less than a $25,000 car a year early would.

That new car would cost some amount over time - it sounds like you buy a new car every 8 years or so? So it would cost you $25/8 = $3.3k/year. That would, then, be the overall cost of the new car a year early - $3.3k (as it would mean one less year out of your old car, so assuming it was also $25k/8 year or similar, that year becomes lost and thus a cost).


There are a few factors I like to consider when I'm reasoning financially over my households cars.

How many KMs will the car travel each year because I like to factor in how often tires will need to be changed, how much tires for my models cost as well as how gas efficient they are.

Knowing how much the car is driven and in what environmental/road conditions is also important factors to know because that will help guestimate possible repairs cost.

Also possible taxes should be taken in to consideration. For example a few years ago I had a diesel Citroen C5 that had yearly taxes of roughly 500$. The replacement costs only 150$ a year in taxes. So switching cars 3 years early would have saved me 1050$ in taxes.

So some information on possible taxes, how far you drive each year, what environmental conditions, type of driving (daily long rides or just short etc..) as well as the fuel efficiency of both cars would help to better calculate your costs for say three scenarios. Car change in 12, 24 and 32 months respectively.


If You use the car regulary, I don't think that driving on the bald tires for 3 years is a reasonable option.

Have You considered buying used tires? Those will be cheaper and will last till You get to replace the car.

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