Or my favorite



Here's my buddy sliding off of the rock, and the real
angle this rock sits at, my pic makes it look like an
easy climb,  but he never could get on it with welded
front diff and a locker in the rear on 38 Swampers 

According to my calculator....
- P215-75-15...............27.7" diameter.........7.24' circumfrence
- 33-10.50-15..............33.0" diameter.........8.635' circumfrence

You travel 19% farther per indicated mile on the 33's than on the 215's

You don't have to work out the circumference, its all directly proportional. 33" is 19% bigger than 27.7" (33/27.7 = 1.19 or 119%)

Lets make this real fun for the days that you have absolutely nothing better to do ( such as watching the grass grow)

Put on the small wheels.  put truck on straight away flat road with no traffic to bother you, such as a parking lot.

put a chalk mark on the sidewall of a rear tire where it points straight down at the road.

continue the mark onto the road as a start point.

roll the truck forward with wife or girlfriend (but not both) walking along the side of the truck.

count ten revolutions and stop with the chalk mark pointing straight down again.

mark the road at that point.

measure the distance from start to end in inches.

Write it down.

change to larger wheels.

repeat process of marking and measuring.

write that down

Take wife or girlfriend out for a beer

while having beer do calculations (I hope you remembered to bring the calculator )

take number of inches in your first measurement.

divide by ten.  thats your rolling distance per revolution accurized by a factor of ten.

take number of inches in a mile and divide by your rolling distance per revolution figure.

(I'm not going to give the inches per mile figure here that would make it too easy and take all the fun out of this)

result is your number of revolutions per mile.

take the measurement of your second tires distance and divide by ten to get your second accurized rolling revolution figure.

take the revolutions per mile from the first calculation and use that to multiply the second rolling distance figure from your larger tires.

the result is your true distance traveled on the larger tire.

now heres where the wife or girlfriend part really gets important!

Say to her "honey I know you are much smarter than me cuz I failed math in school while you got straight A's cuz I was daydreaming about four wheeling all the time in school"

then get her to figure out the percentage of difference in travel distance and calculate the mileage difference.  She can then do all the other math to help you figure out true distance covered, true speed, gas mileage, etc.   

Then go mow the grass or whatever to pay her back and keep her happy cuz you owe it to her!

by the way this only works if you know the true mileage you were getting on the small tires.

Now wasnt this fun?

that's a lot of work to come up with the same percentage you would get by just dividing the 2 tire sizes ...

using the numbers rhinoman used ... 19% means you have gone 19% farther on the 33" than you woulda on the 27.7". If your speedo was accurate with 27.7", then with the 33" you're going 19% faster. Similarly, you would have gone 19% farther on the odometer. To figure gas mileage, take the odometer reading multiply it by 1.19 (this adds the 19%) and then divide by fuel quantity used.

Lets make this real fun for the days that you have absolutely nothing better to do ( such as watching the grass grow)

Put on the small wheels.  put truck on straight away flat road with no traffic to bother you, such as a parking lot.

put a chalk mark on the sidewall of a rear tire where it points straight down at the road.

continue the mark onto the road as a start point.

roll the truck forward with wife or girlfriend (but not both) walking along the side of the truck.

count ten revolutions and stop with the chalk mark pointing straight down again.

mark the road at that point.

measure the distance from start to end in inches.

Write it down.

change to larger wheels.

repeat process of marking and measuring.

write that down

Take wife or girlfriend out for a beer

while having beer do calculations (I hope you remembered to bring the calculator )

take number of inches in your first measurement.

divide by ten.  thats your rolling distance per revolution accurized by a factor of ten.

take number of inches in a mile and divide by your rolling distance per revolution figure.

(I'm not going to give the inches per mile figure here that would make it too easy and take all the fun out of this)

result is your number of revolutions per mile.

take the measurement of your second tires distance and divide by ten to get your second accurized rolling revolution figure.

take the revolutions per mile from the first calculation and use that to multiply the second rolling distance figure from your larger tires.

the result is your true distance traveled on the larger tire.

now heres where the wife or girlfriend part really gets important!

Say to her "honey I know you are much smarter than me cuz I failed math in school while you got straight A's cuz I was daydreaming about four wheeling all the time in school"

then get her to figure out the percentage of difference in travel distance and calculate the mileage difference.  She can then do all the other math to help you figure out true distance covered, true speed, gas mileage, etc.   

Then go mow the grass or whatever to pay her back and keep her happy cuz you owe it to her!

by the way this only works if you know the true mileage you were getting on the small tires.

Now wasnt this fun?

Ah but you forgot that the tyre swells with speed so you need to repeat the chalk mark test at lots of different speeds. Then to work out how far you travelled you have to calculate the distance travelled for each speed at which you drove and for how long you drove at that speed.

Ah but you forgot that the tyre swells with speed so you need to repeat the chalk mark test at lots of different speeds. Then to work out how far you travelled you have to calculate the distance travelled for each speed at which you drove and for how long you drove at that speed.

Yes, but to figure out this test at different speeds, wont I need a much faster wife or girlfriend?

Quote from: Rhinoman on August 19, 2005, 09:21:15 AM

Ah but you forgot that the tyre swells with speed so you need to repeat the chalk mark test at lots of different speeds. Then to work out how far you travelled you have to calculate the distance travelled for each speed at which you drove and for how long you drove at that speed.

Yes, but to figure out this test at different speeds, wont I need a much faster wife or girlfriend?

Oh yeah, a little running is required  . There should have been a couple of smilies after that statement but all I got was an error message. If they're really quick then they can also draw a chalk line across the tyre and check your pressures at the same time 

Well, I just got in from the driveway where I swapped the Swampers back on. 2 MPG just wasn't worth it to me. I'll un-modify Silver and give him the stockers. He can be my runabout rig...

Ok my turn......if you have a handheld GPS, or can buy or borrow one.....use the trip odometer feature on it and get REAL numbers instead of calculations. A GPS will tell you how far you have gone just like your odometer on your rig. It will also tell you how far your speedo is off...and is fun for lot's of other stuff too   

If that isn't an option......do what they said   

I really thought my idea was pretty clever!  Probably would have been real neat to see it on video to boot!

guy marking tires $35
guy convincing girlfriend to help $50
guy and girl doing calculations in a bar over a few beers $100
Girlfriend running alongside truck counting tire revolutions  PRICELESS
for everything else theres Master Card

I went from 205's to 235's and used radar to check for accurracy.  on 205's I was dead on for speed and now on 235's I'm going 2 miles per hour faster at 45mph indicated than I was on the smaller tires.  (By the way the variance in speed increases with higher speeds)

So now when I get 22 miles per gallon average I'm actually doing about 5% better, so really about 23 MPG! ! !

Boy this has been a very educational thread for all of us gear heads and fat tire muddy trail runnin wheelers! ! !

Well, I figure that once the tires are rolling, the bigger tires are easier to keep rolling and roll over road imperfections easier than smaller tires, so the bigger tires get better mileage on the open road. They get worse mileage when climbing up hill or accelerating from a stop though... I'm sticking with the 33's though. They SO much better on there! LOL!

all I know is that I changed from 205's to 235's and then my wife in her corrolla was driving in front of me one night ..... 10 kms an hour diff at 100 kms/h was alot more than the 4 kms/h that the calculator on one site told me....and whether I drive nice or hard is a difference of 400kms a tank uncorrected or 300kms a tank uncorrected.....?

..... 10 kms an hour diff at 100 kms/h was alot more than the 4 kms/h that the calculator on one site told me

Are you working on actual tyre diameters or just what the makers claim? Tyre diameters vary quite a bit from what they should be and even more when rim width is different from the one the tyre manufacturers took their measurements on.

I have found this thread very interesting. So when I'm running a set of 205/60/r15s (my free set of winter tires) what exactly is it doing to my rig? Sorry, I'm a little slow on the uptake...

steveo

You will read faster than you are really going

Sit a little lower

Handle a little better

Zag