donut tires - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Reply
 
Thread Tools Display Modes
 
Old 01-12-2009, 03:39 PM   #1
Registered Member
 
Join Date: Jan 2009
Posts: 1
Country: United States
donut tires

I put a couple donut spares on the rear of my 97 metro. Got a mile to two better milage. Only on the third tank of gas. My question to others that have done this is what causes the gain. The skinny tire?, high tire pressure? (spares are 60psi) or the lowering of the rear of the car buy 1 1/2 inches because of the smaller diameter of the donuts. The 97 sits tail end high. Has anyone ever sucked down just the rear an inch or two and checked for mileage gain?
jack
__________________

GasSavers_olddad is offline   Reply With Quote
Old 01-12-2009, 03:49 PM   #2
Registered Member
 
Join Date: Nov 2007
Posts: 1,111
Country: United States
Send a message via AIM to dkjones96
The increased mileage comes from the small spare having about a quarter's worth of contact patch with the road. It's actually very dangerous to run around like that.

The back isn't as bad as the fronts being like that but not only will the vehicle be way more likely to oversteer with those on but those spares were not designed for regular use.
__________________

__________________
- Kyle
dkjones96 is offline   Reply With Quote
Old 01-12-2009, 04:15 PM   #3
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
At 60psi, it probably does have a quarter of the contact patch.

That combined with the aerodynamic gain from leveling the car should explain it well enough. Actually, looking at a picture of a 1997 Metro, I wouldn't be surprised if it wouuld benefit from going further than leveling, down to what's called a "Cali lean" and looks (IMO) terrible.
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 01-13-2009, 06:19 AM   #4
Registered Member
 
Join Date: Nov 2007
Posts: 135
Country: United States
Also, donut spares are not designed for long-term use. If you inspected them I would not be surprised if you found significant wear on them.

You would probably be better served looking for smaller tires to fit your stock rims.
__________________

Think inside the Box!
Improbcat is offline   Reply With Quote
Old 01-13-2009, 07:33 AM   #5
Registered Member
 
Join Date: Oct 2006
Posts: 557
Country: United States
Narrow width and high pressure don't always offset the higher rolling resistance.
I did some low speed coast down tests in advance of the 2005 Tour de Sol with the Passat I had at the time. From a sandstill on a slight slope I'd release the brakes and see how far I coasted before coming to a stop. Four Continental T135-80-15 at 60 psi did not coast as far as four 185-70-14 Michelins at their max of 32 psi. The best of the temporary tire roll-down distances was not as good as even the worst of the full size tires' distance.
Lug_Nut is offline   Reply With Quote
Old 01-13-2009, 08:28 AM   #6
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Lug_Nut, I hope you don't mind if I quote that in my tire threads...that's the kind of data we should all be using, and the kind of test that's perfect for the job. I can't argue with that test. They were even almost the same outside diameter. The only thing that could be argued is whether it's the width or height that had the most effect.

Did you try the 135-80-15 tires at 32psi? I know it's totally counterintuitive but if you did I'd love to hear how that compared....I'd guess that it resulted in shorter distance than at 60psi (and shorter distance than the wider tires).
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 01-13-2009, 10:02 AM   #7
Registered Member
 
GasSavers_RoadWarrior's Avatar
 
Join Date: Feb 2008
Posts: 1,652
Usually kicking the back of a car up (or dropping the front) helps reduce drag, so dropping it would more likely increase it. Also would tend to make the back end light at speed.

It's possible that the donuts sitting an inch further into the arch reduce drag quite a bit. There's a thread around somewhere about wheel covers/spats and racing hubcaps that has pix of a racing teams experiments with rear arch aero, where they concluded that setting back the tires in the arches a little was more effective than the hubcaps.

However, getting different offset wheels and arbitrarily changing the rear track of the vehicle may have undesirable handling consequences.

In my personal experience, the "easy to get" low rolling resistance tire is the Kumho made Walmart Marshall 791 Touring. They are rated at 36 psi, which is pretty good for RR, they take 50psi, but with the size on Marvin and his weight, the max pressure I could run without losing wet weather grip was 44psi, and had to have them at 36 in winter. On Marvin it seemed like difference between those and "bad RR"
tires was 1mpg, and difference between standard inflation and 42-44psi was another 1mpg. Which is about 5% each way. Highest pressure that maintains safe grip is a matter for experimentation on individual tire and car combinations. Handling is often much improved as long as you stay below the point where the grip tails off.
__________________
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
GasSavers_RoadWarrior is offline   Reply With Quote
Old 01-13-2009, 10:14 AM   #8
Registered Member
 
Join Date: Nov 2007
Posts: 1,111
Country: United States
Send a message via AIM to dkjones96
Very interesting... Makes me wonder what all is involved in that. Kinda makes me think of the mousetrap car me and another guy made in high school.

They said to go with the lowest weight body parts and that CDs were a good wheel to use because they were light and straight. It had to go as straight as possible and as far as possible. We used wood paneling, metal hooks, aluminum shafts and LPs (large records for those of you that don't know what an LP is) and it weighed in at over 3 times what any of the other cars were. However, the furthest any of the other cars got was about 55ft while our car went 175ft before hitting the end of the hallway and when it did we were only 1.5 inches off center.

Point is, we had the same amount of power available to us that everyone else did(the mouse trap) and while the car was one of the slowest of the bunch it coasted VERY well. Only the first 30-35 ft was actually powered.
__________________
- Kyle
dkjones96 is offline   Reply With Quote
Old 01-13-2009, 11:59 AM   #9
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Quote:
Originally Posted by RoadWarrior View Post
In my personal experience, the "easy to get" low rolling resistance tire is the Kumho made Walmart Marshall 791 Touring.
Are they a badge-engineered Kumho model, or Walmart-spec tires whose manufacture was contracted out to Kumho? I can't find them on Walmart's site but wouldn't mind seeing them on Kumho's...
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 01-14-2009, 04:50 PM   #10
Registered Member
 
Join Date: Oct 2006
Posts: 557
Country: United States
Quote:
Originally Posted by dkjones96 View Post
Very interesting... Makes me wonder what all is involved in that. Kinda makes me think of the mousetrap car me and another guy made in high school.
As did I, but that was (OMG!) 35 years ago! in Jr. Hi Mine was cuckoo clock geared for revolutions, but it had no torque at all. The drive wheel spun forever when lifted off the ground but it barely moved when the entry's heavy, nearly 5 lb, weight was resting on the wheel bearings. Once the mouse trap bail approached vertical the spring force was no longer enough to move the pile across the gym floor. I think I got 8 feet or so, but the memory is foggy.
__________________

Lug_Nut is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
New Feature: Loan and lease cost labrie Fuelly Web Support and Community News 1 02-08-2012 02:12 AM
Basic Stats poorboymeyer Fuelly Web Support and Community News 2 09-22-2009 09:19 AM
More Consumers, Workers Shoplift as Economy Slows HondaTorneoSiR Automotive News, Articles and Products 2 06-19-2008 04:20 PM
Performance Underdrive Pulleys? cfg83 General Maintenance and Repair 10 09-08-2007 09:04 PM
MetroXFi changed his name! SVOboy General Discussion (Off-Topic) 26 04-09-2006 07:38 PM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 05:10 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.