I have an '85 Monte with the stock 14" steelies and the 305. When I bought the car I averaged about 8-9 MPG with the set of 1987 snow tires (that still held air!) that it came with. I had to replace them with newer all season tires to pass inspection. These tires are significantly smaller in diameter than the snow tires, and at least one of them has a noticeable weep. Around that time my MPG dropped to an apalling 6, even after a carb tune-up to lean things out a bit. My question is this, is the smaller diameter the main reason behind the loss in economy, or is it the loss of tire pressure? (I pump the leaky one up every few days, it seems to have slowed over time). I can't get the leak fixed by any big box store, the shop that fitted them sold me old tires and no RMA certified place will touch them. Could it be that I need an alignment?
Question about tires.
- Thread starter Bludacious
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Let's try to answer: time for tires 101.
(As an aside, what size tires ARE you running??)
There's 3 numbers to modern radial tires. You may see something like 195/75/14 for a stock/original tire size.
First number is the width - in this case 195 mm.
Second number is called aspect ratio, meaning, what percentage of the width of the tire is the sidewall in height - in this case 75%
Third number is the size of the rim - basically the hole in the middle although that hole is slightly smaller to account for bead surface on the recessed lip.
All together they give you the diameter of the tire, which, multiplied by pi (3.14 is close enough) tells you how much road surface is covered by one rotation of the tire.
Let's give you a unrealistic/exaggerated example to demonstrate why this matters for your mpg question.
Say a tire is 10 inches tall. One revolution of that tire covers (10×3.14) approximately 31.4 inches on the ground.
Say you have a 20 in tall tire. Well, one revolution of THAT tire (20x3.14) covers approximately 62.8 inches on the ground.
When your odometer gives a speed, or counts a mile (where you're getting your mpg from) it uses a little color coded gear based on your rear axle ratio AND your factory tire size to account for how many times a minute the tires are spinning around. It converts that for you into mph and miles on the odometer.
Using a larger tire than stock causes your odometer and speedometer to read LESS than you're really going. Likewise, using a smaller tire than stock causes the odometer and speedometer to read MORE than you're really going. (Unless you swap your speedometer reduction gear on the side of the trans by the speedo cable)
Make sense so far?
So, size of tire makes a difference.
ALSO, type or rubber makes a differecent both in traction and mpg. As tires age, rubber hardens (leads to cracks, worse grip, etc). Snow tires use softer rubber compounds to resist cold and have the sipping work better, but, wear out faster. All season tires have firmer rubber to last longer. Sport tires have softer rubber but not as soft as snow tires to get better grip at expense of longevity. That's why the mileage ratings are worse on snow/summer vs all season, and why you see ads for fuel maximizing tires with better rubber compounds.
Wider heavier tires take more force to move, lessens mpg.
Underinflated tires tend to give lesser mpg than harder, more firmly inflated ones.
So really, there are a LOT of factors that influence an observed fuel mileage reading in a car, and a 2% change here, 7% there, it all adds up and COULD make a difference.
Our next lesson, sounds like you need, is how to read DOT required labeling of tires. On the sidewall, near where the size is stamped by the rim, you see a code, usually inside an oval indent with a couple numbers. THAT'S THE MANUFACTURE DATE OF THE TIRE. - first 2 numbers are week, second two are year.
If the number are 1213 the tire was made 12th week of 2013. If it says 4405 that's the 44th week of 2005.
Tires should be replaced every 5 to 7 years on average based on climate (some are ok longer, depends on how the rubber is aging, etc, but that's the general rule)
(As an aside, what size tires ARE you running??)
There's 3 numbers to modern radial tires. You may see something like 195/75/14 for a stock/original tire size.
First number is the width - in this case 195 mm.
Second number is called aspect ratio, meaning, what percentage of the width of the tire is the sidewall in height - in this case 75%
Third number is the size of the rim - basically the hole in the middle although that hole is slightly smaller to account for bead surface on the recessed lip.
All together they give you the diameter of the tire, which, multiplied by pi (3.14 is close enough) tells you how much road surface is covered by one rotation of the tire.
Let's give you a unrealistic/exaggerated example to demonstrate why this matters for your mpg question.
Say a tire is 10 inches tall. One revolution of that tire covers (10×3.14) approximately 31.4 inches on the ground.
Say you have a 20 in tall tire. Well, one revolution of THAT tire (20x3.14) covers approximately 62.8 inches on the ground.
When your odometer gives a speed, or counts a mile (where you're getting your mpg from) it uses a little color coded gear based on your rear axle ratio AND your factory tire size to account for how many times a minute the tires are spinning around. It converts that for you into mph and miles on the odometer.
Using a larger tire than stock causes your odometer and speedometer to read LESS than you're really going. Likewise, using a smaller tire than stock causes the odometer and speedometer to read MORE than you're really going. (Unless you swap your speedometer reduction gear on the side of the trans by the speedo cable)
Make sense so far?
So, size of tire makes a difference.
ALSO, type or rubber makes a differecent both in traction and mpg. As tires age, rubber hardens (leads to cracks, worse grip, etc). Snow tires use softer rubber compounds to resist cold and have the sipping work better, but, wear out faster. All season tires have firmer rubber to last longer. Sport tires have softer rubber but not as soft as snow tires to get better grip at expense of longevity. That's why the mileage ratings are worse on snow/summer vs all season, and why you see ads for fuel maximizing tires with better rubber compounds.
Wider heavier tires take more force to move, lessens mpg.
Underinflated tires tend to give lesser mpg than harder, more firmly inflated ones.
So really, there are a LOT of factors that influence an observed fuel mileage reading in a car, and a 2% change here, 7% there, it all adds up and COULD make a difference.
Our next lesson, sounds like you need, is how to read DOT required labeling of tires. On the sidewall, near where the size is stamped by the rim, you see a code, usually inside an oval indent with a couple numbers. THAT'S THE MANUFACTURE DATE OF THE TIRE. - first 2 numbers are week, second two are year.
If the number are 1213 the tire was made 12th week of 2013. If it says 4405 that's the 44th week of 2005.
Tires should be replaced every 5 to 7 years on average based on climate (some are ok longer, depends on how the rubber is aging, etc, but that's the general rule)
My 502 gets 8 mpg @ 165 mph and 5,200 RPM continuous, so I don't think its the tires, unless they are square.
That seems like an unburnt gas problem
That seems like an unburnt gas problem
How long have you had the car? Have you ever worked on the carburetor? Is the carburetor stock? Perhaps it needs tuned or rebuilt.
That is steady state 165. Slowing down to 120 for corners and accelerating back to 165 45-50 times over an open road race like Big Bend its closer to 6 mpg... but that is absolutely matting it at the apex of every corner.
You sure you mean miles per gallon and not gallons per mile?
I find the best way to determine economy is in smiles per gallon.Alright, let me clear up a few things. If I had been the one to take the rims to the mounting appointment, I could/would have checked the dates on the tire, but since I had an acquaintance drop them off I didn't get the chance before they were paid for. Second, I'm not expecting inflating my tires to dramatically change my fuel economy, my carb hasn't been touched any deeper than the mixture screws since it left the factory floor, I was just curious if the tire change was a contributing factor at all. They were also a parts bin special so no 2 tires on the car are the same manufacturer, one of them is actually a really high-end looking Japanese sport/soft compound tire and the other 3 are just generic cheapo all seasons.
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