Tire Concerns: How an Engineer buys tires

by Margo
(Delaware)

Often I make a spreadsheet, apply weighting factors, and have the spreadsheet tell me what I should do. For example when ordering at a restaurant I make a mental list of all of the items on the menu that sound good, and then I choose the cheapest one. This doesn't mean I order purely by cost - if all that sounds good is filet mignon, that's what I order, but if the filet and the side salad sound equally good, then I'll order the salad.


When I left for work one morning, I noticed that my right front tire looked like it was lacking air. I stopped at a gas station to fill it after work and the others were just about perfect.

When I got back home yesterday, the tire looked even lower than it was before, so we went to a tire dealer.

The shop said we needed four new tires since they were are badly worn.

We were given three options with different warranties and different prices: 45k miles for $300, 50k miles for $370, and 70k miles for $428. These are different brands and models, but it was almost impossible to find information on the internet for any of them. The dealer said they're pretty much the same.

Since I understand that having a tire that lasts longer (more miles) is valuable, so I assume that each tire lasts just as many miles as it is warrantied.

Between Option 1 and Option 2, I pay $70 and get 5,000 miles, but between Option 2 and Option 3, I pay $60 (engineers round, too) and I get 20,000 more miles. Almost automatically, I vetoed Option 2 - since, if I'm paying more for extra miles, I might as well go all out.

We found some data on "low rolling resistance" (LRR) tires. In essence they're higher efficiency, and you save fuel.

None of the three options we'd been offered are low rolling resistance, but Tire Rack.
has a pretty nice tire chooser that shows two LRR tires for our car. These tires could improve our fuel economy from 1.5% to as much as 4.5%.

Now that I had some numbers I could create a spreadsheet to aid in decision-making.

I had five rows: three cheap sets of tires, plus the two LRR sets, which have a higher price but we assume they'll help us save fuel in the long term.

Based on the number of miles they should last, I calculate fuel savings and subtract from the total price of the tires. For the cheap tires I don't assume any fuel savings.

Then I divide my gas-offset price by the total number of miles we expect each tire to last, and calculate the cost per mile. Naturally it is a small cost ranging from $0.0044 per mile to $0.0074 per mile.

As I initially expected, Cheap Option 2 was the most expensive per mile. And, providing that the LRR tires are at least 2.25% better than non-LRR tires (the improvement which I used for calculation), the tires that have the highest price will actually be the cheapest over the long run.

So those are the tires I bought. In addition, the warranty was the longest, and it's worth something to not have to worry about tires again for a longer time.