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| Tweet Topic Started: Apr 15 2015, 12:08 AM (855 Views) | |
| freegeo | Apr 15 2015, 12:08 AM Post #1 |
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Anyone ever try using the temporary tire or doughnut tires to get better gas mileage? They inflate to 60 psi, but i'm pretty sure they wouldn't ride very well. I'm not talking about driving on the interstate but just highway or town driving. Wouldn't they have less rolling resistance? |
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| Silver2K | Apr 15 2015, 12:25 AM Post #2 |
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They have some problems. First, they have a speed limit of like 50 mph, so you shouldn't use them on the freeway. Next, due to how narrow they are they will not provide good braking surface. If you get in an accident you will regret you took the risk. And I had an electric pump that could not get to 60 psi. For these reasons I would not even use them as spares. I carry a full size spare. |
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| freegeo | Apr 15 2015, 12:32 AM Post #3 |
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Was only thinking of trying them in town, back and forth to work. It is only a 4 mile drive one way. didn't know if it would increase MPG's or not? |
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| Old Man | Apr 15 2015, 12:54 AM Post #4 |
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Sure..probably save you .04 cents a day....right up to the time you lock your brakes up and slide right thru a red light and a dump truck catches you in the drivers door. |
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| Car Nut | Apr 15 2015, 05:32 AM Post #5 |
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EXACTLY. Couldn't have said it any better myself.
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| freegeo | Apr 15 2015, 07:05 AM Post #6 |
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Ok, I get it. It's a bad idea! |
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| myredvert | Apr 15 2015, 08:38 AM Post #7 |
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myredvert
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First, the already described issues with this size and type of tire make it "not worth it" to me. That doesn't even include the fact that these types of spares are not required to have treadwear, traction, speed, or heat ratings and typically only come with roughly 1/2 the tread depth when new. And cost considerably more to boot. Just my .02, but if you want to try to "evaluguessate" what fuel economy effects you may see from using one, then you have to look much deeper than max inflation pressure alone. If you look at the at several variables that will affect fuel economy, you will find that many of them may very well have a negative impact. My personal guess after looking at overall diameter, frontal area changes (and the middle of the wheel itself extends out further then the tread, taking some of the area reduction back and leaving you with very minimal total reduction), chassis height changes, wheel well gap increases, rotational mass changes, and rolling resistance decreases is that you shouldn't expect much of an increase, if any at all. At the very least, your tire diameter reduction of ~2.4" would make your final drive gearing so much smaller that a 115 70 14 donut spare with your proposed 4.10 would still leave you ~4% shorter overall than you have currently, which seems to be the opposite direction from what you want by changing to a taller 4.10 final drive. It would take a "3.93" final drive with a 115 70 14 donut just to equal to your 4.39 with 155 80 13s in terms of overall gearing. And, even if you changed to a white speedometer gear you would still have almost a 4% over-reporting error. With your brown gear you would have a >10% error. My advice for improving fuel economy through tire changes - use 155 80 13 tires with the highest max inflation rating you can get, and inflate them to the highest pressure that provides you with a ride you can tolerate up to the rated max inflation. If you can use a tire pressure just 10 psi higher than you currently run in your 155 80 13s, you will likely get the same if not greater fuel economy increase. If you want to go the "skinny" wheel /tire route, it would take something like the 15" aluminum spare wheels mentioned previously with real low rolling resistance tires (translated, very expensive combination) to see improvement. |
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| iveyjh | Apr 15 2015, 09:11 AM Post #8 |
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I have tried the "doughnut" spare tires. I bought five of them when I put 145/65/15 tires in my Metro. I tried the spares for a while, horrible ride and terrible rolling resistance. Not good!! |
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| perfesser | Apr 15 2015, 09:47 AM Post #9 |
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Elite Member - Former Metro owner
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Pseudo-spares are designed to keep the hub off the ground until you can get a real tire to replace it. That's all. The rubber is the cheapest they can get, so traction is a memory. Besides, they are only rated for 50 miles of use. I really doubt that you would be happy with them, although I see many of them being used a permanent replacements every time I walk through the lot at Wal-Mart. |
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| freegeo | Apr 15 2015, 12:11 PM Post #10 |
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i already run 155 80 13 on the front of the car. They are aired up to the max allowable air pressure. I'll keep working on my engine to get my mileage up. |
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| WiscoMetro | Apr 18 2015, 10:18 PM Post #11 |
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Seeking the Science of MPG
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The only way I could see this working would be to have a pair of spares on the rear axle. If they were on the front axle not only would you lose any mileage gains since the engine would have to run faster but they also would probably be over their weight limit(they are designed to only have 1 on at a time). So then if you had 2 spares on the rear it would only work with dry roads, two narrow tires would slip easily. The 50 mile limit is only to protect the tire makers from lawsuits, but they cost 2x what a normal tire does and won't last nearly as long. Even in a perfect scenario would be spending more on tires than you would save on gas. |
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| mauurgp | Apr 18 2015, 11:28 PM Post #12 |
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absolutely fresh fish
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i dont know if you are the only one to have tried, but i recall someone having tried with a guino and it got worse mpg. to me it sounded like a good idea as well, but it comes back around to the rubber - lots of rolling resistance. |
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| iveyjh | Apr 18 2015, 11:32 PM Post #13 |
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I don't remember what the mileage was, but it was significantly less and I have a MpGuino and a terrible ride. |
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Couldn't have said it any better myself.

9:50 AM Jul 11