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Bigger tyres, less mpg. Why ?

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fugz

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Discussion starter · #1 ·
Hi All.
Today I have upgraded from stock 215/60/17 to Geo's 225/65/17. I have heard that from others experience that mpg goes down, I haven't had them long enough to see this yet. What confuses me is that if I increase the tyre diameter the wheels rotate less per mile therefore the engine presumably is doing less work and mpg should go up, so why does mpg go down or is this just the evic telling lies ?.
By the way, I really like the look and feel of the new tyres. What an improvement to the Continentals in all this snow we have over here at the moment.
 
#2 ·
well, if the odometer is measuring number of rotations and calculating distance travelled, then the computer will think you've gone less distance than you actually have with bigger tires, reducing the reported milage shown on the dash. I don't believe it's the actual milage shown on the EVIC.

My EVIC definitely reports worse milage after putting on the bigger tires.
 
#3 ·
More surface area = more friction to overcome. More rubber = more weight to move.

Probably pure BS, but that's my guess.
 
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#4 ·
It is not less work for the engine it is more work. A larger diameter tire is (slightly) harder to rotate. And all the other stuff the other guys mentioned too. It all adds up.
 
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#5 ·
Bigger tires, typically mean more weight even with a larger diameter meaning less rotations per mile the engine and transmission and or transfer-case still have to work harder. also the axle's and brakes can wear out little faster but if its only couple inches and wheels are kept the same it wont make much of a difference. mabye 2-3mpg for town but not much difference on highway at all.

i had a 1990 toyota 4x4 with 22re originally had 29inch tires i put some 31-10.50-r15 and mpg only went down like 3mpg but was little better on highway if i wasn't speeding.
 
#7 ·
The wider tire means an increase in frontal area. Plus, a slightly higher stance means that more of the tire vertically is exposed, so a little more frontal area, again. That's the bad news, mileage wise. The frontal area effect goes up with the square of the speed.

The good news is slightly lower engine and drivetrain rpms per mile. But, without telling the computer that you have taller tires, it doesn't know that you are going farther per tire revolution. Your 225/65/17 tires are about 5% taller than your 215/60/17 tires. So you can add 5% to the EVIC reported mileage.

If you are using the loud pedal more because you like using your new tires, that will reduce gas mileage also.
 
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#9 ·
It's pretty well all been said already, but it basically boils down to tire weight and torque. A slight increase in tire diameter and/or weight requires an exponential increase in force to get that tire moving. I can't remember the specifics, but it's really remarkable how much extra energy is required to turn a tire which is only slightly larger in diameter. Conversely, an exponential amount of force is required to slow that tire down, hence the increased brake wear.

However, once that tire is moving, as in highway driving, it is possible to see an increase in MPG because you are moving further for each revolution of the engine. I just switched up to a 29.3" diameter tire and for my usual drive to work (which is 90% freeway driving) my EVIC reads 3.5% worse MPG, however my tire circumference is 4.5% larger, so I'm actually getting 1% better MPG.
 
#10 ·
fugz said:
Hi All.
Today I have upgraded from stock 215/60/17 to Geo's 225/65/17. I have heard that from others experience that mpg goes down, I haven't had them long enough to see this yet. What confuses me is that if I increase the tyre diameter the wheels rotate less per mile therefore the engine presumably is doing less work and mpg should go up, so why does mpg go down or is this just the evic telling lies ?.
By the way, I really like the look and feel of the new tyres. What an improvement to the Continentals in all this snow we have over here at the moment.
Click to expand...
You'll need to use a correction factor for the larger tire. 225/65R17's are going to put your speedometer off by 1.8% (based on the Miata Tire Calculator). So at 60 mph you actually travel 61.1 miles. When you divide your miles traveled by the gallons used, add 1.8% more miles. So let's say you fill the tank and zero the trip odometer then travel 250 miles before the next fill-up.

250 X .018 = 4.5 + 250 = 254.5 actual miles.

250.0 mi / 11 gallons = 22.7 mpg
254.5 mi / 11 gallons = 23.1 mpg

May not seem like much but it all adds up over the long run (especially on the average of the EVIC).

This is of course assuming your old tire was exactly 28" and the new one is exactly 28.5". Tires wear and get smaller, and manufacturers can have slightly different tire sizes when new. So if your old tires were really worn out (assuming when it was new, it had 10/32nds of tread depth and they were replaced with 2/32nds remaining, or 8/32nds worth of wear), maybe they were 27.5" in diameter and the new ones are actually 28.6", now we have a correction factor between old and new of about 3.8%.

250 X .038 = 9.5 + 250 = 259.5 actual miles
259.5 mi / 11 gallons = 23.5 mpg

Makes it seem like you lost almost 1 mpg just from the new tires, when in fact you slowly gained .5 mpg from the old tires wearing, then reset yourself .5 mpg in the other direction with the new ones.
 
Discussion starter · #11 ·
Thanks for all the replies. I didn't realise it could be so complicated.
I just hope I am not doing much damage to the car with the increase in tyre size, maybe only faster wear and tear, hope it doesn't affect the warranty
 
#12 ·
If you have a gps you can get the actual distance on a large trip (over 200 k). Fill up at the start, initiate the gps, follow the gps and then gas up at the end of the trip. Record the numbers and calculate. This will give you a really accurate mpg.
 
#13 ·
fugz said:
Thanks for all the replies. I didn't realise it could be so complicated.
I just hope I am not doing much damage to the car with the increase in tyre size, maybe only faster wear and tear, hope it doesn't affect the warranty
Click to expand...
Those tires would be a marginal increase in weight and size. Nothing to worry about damage wise.
 
#14 ·
fugz said:
Thanks for all the replies. I didn't realise it could be so complicated.
I just hope I am not doing much damage to the car with the increase in tyre size, maybe only faster wear and tear, hope it doesn't affect the warranty
Click to expand...
todde702 said:
Those tires would be a marginal increase in weight and size. Nothing to worry about damage wise.
Click to expand...
I agree. Nothing to worry about. In fact, bigger tires mean more fun especially for off-roaders
 
#15 ·
Taller gearing does not always mean better fuel economy. In fact, usually the opposite in the city.

OEM Tire weight 23 pounds
Replacement weight 31 pounds

That's a lot more weight to turn which has a lot more impact than adding 32 pounds inside the cabin. It takes more torque to turn and stop turning the wheels/tires now. So, yes, expect more fuel used, accelerated brake wear, and accelerated bushings/ball joints wear. How much? Hard to estimate.
 
#16 ·
check the number of revs per mile on your new tire vs the OE one. if you have a difference of, lets say, 5%, then to calculate your fuel mileage, take your miles travelled X 1.05 to get a more realistic distance travelled. THEN calculate your MPG's. that should get you much closer.

also, keep in mind that your speedo will be off the same percentage. for instance, if you're travelling at 70mph by your speedo, you're probably really going 73.5 (at the same 5% difference). :)
 
#17 · (Edited)
Bigger Radius Tires vs MPG Gains

Any vehicle on which a larger radius, larger circumference, larger diameter (all diff ways to say same thing), is installed the odometer will produce less numbers. That's because the larger tire rotates less number of times per mile.

Sure, we can argue various points: more torque required (although not much more), larger radius tire generally wider causes more wind resistance (although not by much), larger radius tire weighs more (although not by much), etc.

The bottom line is that the mechanics of the components driving the odometer, trip meter will turn less per mile. Even with a super powerful motor that will over come added wind resistance, torque problem, added weight - the same is true - less turns of tires to tick up the odometer.

So, every one of you who ignore this fact in your calculations will compute inaccurately.

A GPS is the only way to truly know how many mile you drove in a test drive after new larger tire installation. Also, the other difference factors, weight, wind resistance, torque - require a vehicle to be driven several thousand miles IGNORING THE ODOMETER and focusing on only a GPS mileage number to calculate the actual mileage accurately.

All we have to do is accentuate, exaggerate tire circumference (or radius or diameter) size to demonstrate my point - more tire circumference, radius, diameter is less odometer. You car isn't lying to you, just confused - unless it has a way to punch in the tire size and it can adjust its computations.

Put a set of tires on a vehicle that is 20 feet of circumference - 20 ft around the tire. This will cut down on odometer reported mileage every time big time. Fuel efficiency will differ between vehicles. This is of course an exaggeration. If we increase tire diameter by 3 inches then we must travel a good number of miles to realize the drop in odometer readings.

But, suppose you up-size the tires on your vehicle without decreasing fuel consumption for 500 GPS confirmed miles. The odometer, trip meter will show less miles. So please ignore the odometer and on-board computer for computations of accuracy and recognise that bigger tires produce less revolutions per mile yielding less miles at the odometer.

He who has no GPS, and uses the odometer for mileage readings after up-sizing tire circumference size to compute is lying to themselves.
 
#18 ·
treads21 said:
Any vehicle on which a larger radius, larger circumference, larger diameter (all diff ways to say same thing), is installed the odometer will produce less numbers. That's because the larger tire rotates less number of times per mile.

Sure, we can argue various points: more torque required (although not much more), larger radius tire generally wider causes more wind resistance (although not by much), larger radius tire weighs more (although not by much), etc.

The bottom line is that the mechanics of the components driving the odometer, trip meter will turn less per mile. Even with a super powerful motor that will over come added wind resistance, torque problem, added weight - the same is true - less turns of tires to tick up the odometer.

So, every one of you who ignore this fact in your calculations will compute inaccurately.

A GPS is the only way to truly know how many mile you drove in a test drive after new larger tire installation. Also, the other difference factors, weight, wind resistance, torque - require a vehicle to be driven several thousand miles IGNORING THE ODOMETER and focusing on only a GPS mileage number to calculate the actual mileage accurately.

All we have to do is accentuate, exaggerate tire circumference (or radius or diameter) size to demonstrate my point - more tire circumference, radius, diameter is less odometer. You car isn't lying to you, just confused - unless it has a way to punch in the tire size and it can adjust its computations.

Put a set of tires on a vehicle that is 20 feet of circumference - 20 ft around the tire. This will cut down on odometer reported mileage every time big time. Fuel efficiency will differ between vehicles. This is of course an exaggeration. If we increase tire diameter by 3 inches then we must travel a good number of miles to realize the drop in odometer readings.

But, suppose you up-size the tires on your vehicle without decreasing fuel consumption for 500 GPS confirmed miles. The odometer, trip meter will show less miles. So please ignore the odometer and on-board computer for computations of accuracy and recognise that bigger tires produce less revolutions per mile yielding less miles at the odometer.

He who has no GPS, and uses the odometer for mileage readings after up-sizing tire circumference size to compute is lying to themselves.
Click to expand...
This is 100% correct and true!
 
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#20 ·
When we did the math 3 1/2 years ago we also tried to duplicate the math with GPS. Seemed dead-on to us.

Went from 215/65/17s to 235/65/17 s in a HT tread. HT vs AT does make a difference.
Our multiple is 1.05. It's in the math!

The mileage didn't change enough to make a case either way. You don't change your driving style or routes with a size change, you're just trying to compensate for your speedo and odo. Add 2 mph for 215s --> 235s.
 
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