Added 20"s, why did my gas mileage drop??
Thread Starter
Joined: Aug 2007
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Added 20"s, why did my gas mileage drop??
Just added some chrome Avarus 20"s (20x8.5, 5x114 bolt pattern, 15MM offset with 255/35/20 tires on them) and for some reason it seems like my gas mileage is just tanking.
It seems like theres almost a 30% drop--Previously I got 290 miles out of a full tank on stock tires (avg. 18 MPG), but after changing Ive got 55 miles on a quarter tank (13.25 MPG).
The driving areas and my driving habits are pretty consistent too.
Does anyone have any experience with this or can anyone offer insight? Sorry for my ignorance.
It seems like theres almost a 30% drop--Previously I got 290 miles out of a full tank on stock tires (avg. 18 MPG), but after changing Ive got 55 miles on a quarter tank (13.25 MPG).
The driving areas and my driving habits are pretty consistent too.
Does anyone have any experience with this or can anyone offer insight? Sorry for my ignorance.
All Star
Joined: Feb 2005
Posts: 821
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From: Coal City, IL
RE: Added 20"s, why did my gas mileage drop??
that 255/35 series tire is throwing your speedometer way off. if you have a R/T, a 245/45-20 tire will keep your speedo accurate. you still may burn more gas because of the added weight and added rolling friction.
the new tire size you have is 6% too fast. that means when your speedo reads 60mph, you are really going 56mph. this will make your odometer read less miles than you really went. the rest of your loss in milage is from the computer errors in reading the data. you will need a superchips flashpaq or diablosports preditor to correct for the tire size. the dealer can't do it because you have a non available tire size in the starscan.
the new tire size you have is 6% too fast. that means when your speedo reads 60mph, you are really going 56mph. this will make your odometer read less miles than you really went. the rest of your loss in milage is from the computer errors in reading the data. you will need a superchips flashpaq or diablosports preditor to correct for the tire size. the dealer can't do it because you have a non available tire size in the starscan.
Joined: Aug 2007
Posts: 19
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RE: Added 20"s, why did my gas mileage drop??
ORIGINAL: 65standard
that 255/35 series tire is throwing your speedometer way off. if you have a R/T, a 245/45-20 tire will keep your speedo accurate. you still may burn more gas because of the added weight and added rolling friction.
the new tire size you have is 6% too fast. that means when your speedo reads 60mph, you are really going 56mph. this will make your odometer read less miles than you really went. the rest of your loss in milage is from the computer errors in reading the data. you will need a superchips flashpaq or diablosports preditor to correct for the tire size. the dealer can't do it because you have a non available tire size in the starscan.
that 255/35 series tire is throwing your speedometer way off. if you have a R/T, a 245/45-20 tire will keep your speedo accurate. you still may burn more gas because of the added weight and added rolling friction.
the new tire size you have is 6% too fast. that means when your speedo reads 60mph, you are really going 56mph. this will make your odometer read less miles than you really went. the rest of your loss in milage is from the computer errors in reading the data. you will need a superchips flashpaq or diablosports preditor to correct for the tire size. the dealer can't do it because you have a non available tire size in the starscan.
All Star
Joined: Feb 2005
Posts: 821
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From: Coal City, IL
RE: Added 20"s, why did my gas mileage drop??
use a tire size calculater to keep the diameter the same as original. for example a stock R/T comes with 225/60-18 tires (your AWD may have this size tire too) and the 245/45-20 has the same overall diameter. no problems with the speedo and odometer. here is the tire calculater to experiment with.
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html
Thread Starter
Joined: Aug 2007
Posts: 4
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RE: Added 20"s, why did my gas mileage drop??
Thanks for the info everyone. 65standard, do you know what tire size is right? I have an 07 SXT RWD.
I was wondering though, for those who throw 22"s on their magnums, do they have to correct for the tire size also? or is it only if your tires end up smaller than stock?
Also would the tire offset affect anything?
I was wondering though, for those who throw 22"s on their magnums, do they have to correct for the tire size also? or is it only if your tires end up smaller than stock?
Also would the tire offset affect anything?
All Star
Joined: Feb 2005
Posts: 821
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From: Coal City, IL
RE: Added 20"s, why did my gas mileage drop??
the stock sxt rwd tire is a 215/65-17 and the closest tire to keep the speedo happy is a 245/40-20. you will be 1% fast.when your speedo reads60mph, you will really be going 59.4
most people don't even know or care if their speedo is off when they add 20's or 22's
most people don't even know or care if their speedo is off when they add 20's or 22's
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Champion
Joined: Aug 2005
Posts: 3,313
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RE: Added 20"s, why did my gas mileage drop??
It is likely that your tire rolling resistance is now worse:
http://auto.howstuffworks.com/tire4.htm
also
http://auto.howstuffworks.com/hybrid-car4.htm
besides weight, there is a "close cousin" called 'rotational inertia'
that can be higher with large diameter metal wheels unless they are designed correctly. This may also be slightly affecting your acceleration, braking....and fuel economy
http://auto.howstuffworks.com/tire4.htm
also
http://auto.howstuffworks.com/hybrid-car4.htm
besides weight, there is a "close cousin" called 'rotational inertia'
that can be higher with large diameter metal wheels unless they are designed correctly. This may also be slightly affecting your acceleration, braking....and fuel economy
Thread Starter
Joined: Aug 2007
Posts: 4
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RE: Added 20"s, why did my gas mileage drop??
Thanks for posting the link. Still have a few questions though.
Would the rolling resistance still change even if the overall tire diameter was the same as the stock tires? (Using the miata.net calculator, I found out my current tires are 4% smaller than stock.)
Since 20" rims would be heavier than stock, I can see how this is a factor in decreased gas mileage. But is there anything else that could be affect rolling resistance?
Would the rolling resistance still change even if the overall tire diameter was the same as the stock tires? (Using the miata.net calculator, I found out my current tires are 4% smaller than stock.)
Since 20" rims would be heavier than stock, I can see how this is a factor in decreased gas mileage. But is there anything else that could be affect rolling resistance?