Peakster's Experiment #6: Mmm... DONUTS!
 

Stemming off from one of my earliest threads here at GS, I finally got off my butt and did the donut tire experiment today. Here's the breakdown:

YouTube Video of Experiment

Attachment 446Attachment 447Attachment 448

Date: May 5th, 2007
Method of testing: A-B-A
Time: 3:00pm - 5:00pm
Temperature: 19*C-18*C
Winds: coming from the North-East @ 20km/h - 26km/h
Speed: 50mph with cruise control
Route:
Attachment 445

As always, there's a fun YouTube video to watch, but here are the findings:

1st run with regular all-season tires: 56.9 mpg
run with 'donut tires' for back wheels: 60.9 mpg
2nd run with regular all-season tires: 54.9 mpg

Average with regular tires: 55.9 mpg
Average with donut tires: 60.9 mpg

8.94% FE increase with donut tires.

Needless to say I was shocked at the FE improvement. I could feel the car gliding better with the donuts in the city, but it definitely showed up on the highway too. Another thing I think could be a possiblity is that with the smaller donuts on the rear, the Geo may have had a slightly smaller 'wake' of air behind it. Thus making it more aerodynamic?

What do you other members think of the results?

I think that it's time for me to try this with the Green Machine. Has anyone had any wear problems with donut tires? I've never had to use the spare yet.

There's some visible wear on the tires already from the few tanks I've had them on. Just look at your owner's manual and it should state how much distance they're supposed to last. Mine are rated at 3,000 miles: "...so you can finish your trip and have your full-size tire repaired or replaced where you want."

I'm suprised at this. I would think that there would be more drag due to the huge gap between tire and wheel well and the lifting of the front end due to smaller tires. I don't see where the increase in FE is coming from. Couple of questions:

This is non stop right? No EOC on the route when testing?
So the distance and speed stays the same because the front wheels were not changed?
How much weight difference on the wheels? Although once at speed I won't think it would matter.

Wheel skirts would probably be easy(no bulge) and help even more with the spare. I guess now there will be a run on spares at the junk yard all across the contry.:)

I have been waiting for this experiment. Good experiment and good results. My only question is, "what psi did you have the original tires at and at what psi did you have the donuts at?"

Very interesting... I would have thought that - but it's great to see it quantized :D

My car comes with a full sized spare though :p

Yes, it was a non-stop drive. Set the cruise control and go.

Yes. The speedometer/odometer is operated by (one or both of) the front tires (Figured that out when in winter when pressing the accelerator on ice/snow). If I put the donuts on the front (I have in the past) I would have to times the mpg result by 0.91 to compensate for the smaller diameter tire.

Just from changing the wheels, the donuts are considerably lighter, but remember: I threw the stock tires in the back seat so weight shouldn't be changed due to tire/rim weight differences

Yeah, It would be a heck of a lot easier to do a wheel skirt with the donuts... maybe a future experiment ;)?

Good question, I'll check it out right now:
Donuts: ~60psi
Stock tires: ~50psi

I think the main difference is the donuts have much less surface area with the ground. I could do another test with the donuts at 50 psi too just so it would remove that factor... but that just means more work for me :rolleyes:!

i think the have less rr so thats prolly it

Sure but 10% seem kinda high for rolling resistance. Don't understand it interesting results and nice job Peak.

Less rotating weight + Better rr.


No need for another test. I just wanted to make sure the tires were at proper tire pressure.