Lets talk tires for my Subaru.

[Firewind]

Originally on my 97 Paseo I had 185/60 r14, and I was getting around 38~41 MPG in early summer of 2008. Late summer I got new tires since those were getting warn down, and I step up to 195/60 size. Wider and slightly taller, and I noticed a MPG drop, I can hardly get above 36.5MPG. I have always assumed this is because of the wider tire, causing more rolling / wind resistance and I have seen a few posts around to support this idea. However my thought is how does the height play in to this equation.

The reason I bring it up is because I recently got Subaru Legacy Outback to replace the Paseo, and it desperately needs new tires. It sounds like the stock size on the Subaru is 205/70/r15. I don't think I am going to go the stretch tire route and go for a smaller tire on the rim like all the import kids do these days. However I am still wondering if I could benefit from maybe a taller tire like a 205/75, or maybe a smaller tire like a 205/65. I would assume gearing ratio's would have to come into the equation, as it could be different per each car.

Anyone have any thoughts on that?

[theholycow]

I think gear ratios are the largest FE effect of changing tire size. Next to that would be the difference in rolling resistance between one brand/model of tire and another.

There are convincing arguments for both sides of the width issue, saying that wider or narrower is better for rolling resistance. Either way, lately I don't think the difference is as large as the difference from any one model of tire to another.

Height can also affect the whole car's aerodynamic drag by lifting or lowering the car, changing how much space under the car there is for air. I also don't think this effect is as large as random rolling resistance differences from one model to another.

In my opinion, use different tire sizes to adjust gearing and handling in whichever direction you think will help you save fuel. Shop for tires that can take as much pressure as you think you'd like to use; don't buy tires rated for 35psi max if you currently run 40 or 50 psi in your tires.

[NotYourDay]

I personally agree with theholycow on the fact that, get tires rated for the PSI you put in them. Also, and this is just my opinion, I think that the lower your car is the better the MPG's you will get. In turn, The smaller the tires, the better the MPG's. Just my 2 cents

[Mayhim]

I had the same issue when I went from 13's to 14's in the CRX.

The best mileage I got was on the 13's. I can't get near it now. I, too, believe the increased height has allowed more air under the car and more drag. The rpms are lower, but I don't think that overcomes the increased drag. It doesn't drag over speed bumps now, but at a price.

[theholycow]

Mr Incredible, why didn't you get shorter tires to go with your taller rims? Were they just as hard to find as 13 inch tires (which is my guess as to why you changed wheels)?

[Ford Man]

Firewind, I know this doesn't account for all your loss, but going to the larger tires would have thrown you speedometer and odometer off by 2.1%.

[NotYourDay]

Fordman, What is a good calculation to do to get exactly how much my speedo would be off with bigger tires?

I want to put bigger tires on becuase I found a set used for dirt cheap but they are 20 bigger (if that made any sense)

[Mayhim]

HC, I wanted the taller tires to get the lower gearing and to keep above the speed bumps in the supermarket parking lots. My adjustment factor is 1.06046, but I think I lost a lot more than .06046 in the bargain.

It ain't killin' me, but it's a lot like remembering the younger babes I used to go out with...

[theholycow]

Originally Posted by NotYourDay

Fordman, What is a good calculation to do to get exactly how much my speedo would be off with bigger tires?

I want to put bigger tires on becuase I found a set used for dirt cheap but they are 20 bigger (if that made any sense)

Use the tire size calculator (which I think is linked in my sig) to calculate the percentage difference. Adjust your miles by that percentage before calculating MPG.

By "20 bigger" I assume you mean that the aspect ratio is larger by 20. That can only tell us how much taller they are if we know the width too (assuming that the width of both is the same).

[Ford Man]

Originally Posted by NotYourDay

Fordman, What is a good calculation to do to get exactly how much my speedo would be off with bigger tires?

I want to put bigger tires on becuase I found a set used for dirt cheap but they are 20 bigger (if that made any sense)

Here's a link to tire size calculator that will instantly tell you how much your speedometer and odometer will be off. If you are going to a larger tire you need to check them for clearance to make sure they won't rub anything in the fender well. I own Ford Escorts and with some tires on them there are issues with the tires rubbing the strut and spring assembly.

http://www.miata.net/garage/tirecalcold.html