Tire size and FE

[Apabob]

I dont know how often this has been referenced here but i just thought id mention an example of tire size and how it affects my FE.

I have a small, slow car with a 4 speed manual tranny. Needless to say, highway speeds really tend to wind the piss outa my engine.

Ive recently switched my small diameter snow tires (155/80 R13)out for some substantially larger diameter all-seasons (185/80 R13).

The advantages are tremendous for my particular situation. The greater diameter of the tires effectively stretches my gear ratio, making traveling at 70 mph feel a little less like im whipping a horse to death.

The other side of that particular coin, is less mechanical advantage on takeoff (dead stop). Thats not at all an issue for my little diesel but could be something to consider in another vehicle.

And finally, the larger wheels just plain roll better. They are not slowed nearly so much by bumps in the road and my car will roll so much further every glide its amazing.

Ive seen psi and low resistance rollers mentioned here alot, and with very good reason, but i thought tire size deserved a bit of attention too.

Something to consider...?

[PaleMelanesian]

Someone did controlled testing on this and found it to be a benefit. Sentra se-r I think it was? The larger tires will hurt you if you do a lot of stop-n-go and braking. There's a lot more momentum in the larger diameter spinning tire that's wasted if you brake. If you coast instead, you get that momentum back in extra distance traveled so it's a wash. Then the lower rpm can help you.

[Apabob]

I didnt consider the greater energy present due to the Centrifugal force of the larger dia tire. Thats an interesting consideration.

I reealy notice how much less small bumps affect speed. The mechanical advantage (leverage) of the larger diameter tire allows for smoother rolling. They just climb the bumps better!

[ALS]

You can play with a Gear/Tire calculator to see the highway benefits of going to a larger diameter tire. On my old Volvo by switching to a 205/55/16 tire (optional) over the stock
195/60/15 it was about a 100 rpm drop over all at 65 mph. Hey every bit helps. I wish when I got new tires for my 960 I had gone to a 205/60/16 over the stock 205/55/16's.

Be carefull when doing this so that you don't get rubbing issues.

[Dream'R]

By my calculations you have increased your tire diameter by 6.8% which is a significant difference. As I'm sure you realize, your speedometer and odometer will be altered by the same difference. I'd be curious to hear if your fuel economy has improved by even more than compensating for this factor. It would seem very likely if for no other reason than the change from snow tires to all seasons usually reduces rolling resistance. My Civic hybrid's mpg improves at least 5% when I switch my tires over in the Spring.

I have no evidence to back this up but my instincts tell me that taller, narrower, high pressured tires roll easier -- think road bicycles versus mountain bikes!!

Cheers,

Roger

[Right Lane Cruiser]

A small nitpick here -- it isn't "Centrifugal force" (which doesn't actually exist -- centripetal force does) but rather angular momentum.

[PaleMelanesian]

http://xkcd.com/123/

[herm]

are there any LRR tires in 13" or 14" sizes?.. its discrimination!

[PaleMelanesian]

I know!

but really, there are some. Especially the one Sean's about to mention.

[Right Lane Cruiser]

Sure there are, Herm -- the first gen Insight uses P165/65R14. Only that size of the OEM spec'd tire is ultra LRR construction but it does exist.

Great cartoon, Andrew! And nice premonition, too.