Bigger tires = Better mpg?

[Wojtek]

When I was comming back from the store today I thought of something.

If you equip your car with bigger tires, wont this boost your mpg by a little?

Say 15's and 16's (38.1cm and 40.64cm)

circomference of the 15's is (pi*38.1) 119.69cm
circomference of the 16's is (pi*40.64) 127.67cm

So if for one rotation of the wheels axis, your car makes 8cm more, then dosnt this translate into an reduction of gas usage?
93.7% of the gas with 16's compared to 100% with 15's?

And by putting bigger tires (17's) you reduce even more the gaz consumption?

Am I right or I missed something?

[pro5ject]

Yeh but the larger tyres / rims would most probably weigh more.

Unless say you had steel rims and high profile tyres first, and then went to larger alloy wheels, and low profile tyres.

The power to weight ratio would be the only way to work it out.

But in theory if the tyres / wheels weighed the same, then yeh it would make a difference (maybe 1km extra with the same amount of fuel for say every 6000km's or something) I guess - if the car is no heavier.

[keith]

i recently switched my jeep from 33" to 30" tires and the smaller tires have noticeably increased the mpg. the larger tires were even on aluminum wheels, as opposed to steel wheels on the smaller tires, and were surprisingly lighter... but the power needed to turn the larger tires outweighs the speed gained by the larger circumference.

it may work out for you if you switch the differential ratio, but then you're getting into hundreds or thousands of dollars.

in either case, you have to switch the transmission's speedo gear too, which was a slight pain but worth it.

[Gayowulf]

bigger wheels reduce gas mileage: they require more power to turn, and are usually heavier.

keith summed it up pretty well.

[Webdude]

Take into account a 12 speed bycycle. Well, lets say 2 gear for ease of argument. One is easy to start off on, but you have to peddle fast to keep up speeds. Thats the smaller wheel.

Uses more gas to keep up speed, but uses less gas to get up to speed because easier to turn.

The larger wheels are like starting on the hard gear. Uses more fuel to speed up, but less fuel to keep up the speed.

So in stop in go travel, you want small wheels, while if you do a lot of highway travel, bigger wheels are better.

Weight doesnt figure in much except at speed up. You havent been calculating in the power of momentum... or the old "things in motion tend to stay in motion".

[notnamed]

Ha, I read that as "old things in motion tend to stay in motion," goes with your avatar :biggrin2: