Converting mileage calculations-help?

[RangerX]

Hey, I need help finding a formula to find out my true mpg. I checked 4Lo and rangerstation sites, and either it's not there, or I'm not enough of a math wiz to recognize what I need.

I went 298 miles on 20.8 gallons. My speedo (and odo) are correct for 33" tires. I now run 37" tires.

Anyone?

 

[Robert]

Since your 37" tires are 12.12% taller than your 33" tires you traveled 12.12% farther than 298 miles. This means you actually drove 334.12 miles. 20.8 gallons into 334.12 miles equals 16.06 MPG. Just multiply your indicated miles by 1.1212 and then divide your gallons into it for your MPG. Alternatively, you could replace your speedometer gear and save yourself a step.

 

[RangerX]

Thanks, Robert!

 

[Henry]

Originally posted by Robert
Since your 37" tires are 12.12% taller than your 33" tires you traveled 12.12% farther than 298 miles. This means you actually drove 334.12 miles. 20.8 gallons into 334.12 miles equals 16.06 MPG. Just multiply your indicated miles by 1.1212 and then divide your gallons into it for your MPG. Alternatively, you could replace your speedometer gear and save yourself a step.

I'm going to have to differ with this. The height of the tire is 12.12% taller; however, the distance travelled is in relation to the circumference of the tire, not strictly the height and I don't think the relationship will stay at 12.12%. There's a tire size calculator site (Tire Size Calculator) but it only works with the "standard" size specifications and I don't know how to convert from the 31x10.5 type spec. to the standard.

 

[Robert]

Uhmm, no. The circumferance is still 12.12% longer with the 37" tire versus the 33" tire. Circumferance = PI X Diameter. Since PI is a constant (3.1415927) and the given diameters are constants, nothing will vary since none of the calculations involve squaring or square roots. The circumferance of a tire is the actual distance traveled for every revolution of the tire. For instance the 37" tires have a circumferance of 116.2389299". The 33" tires have a circumferance of 103.6725591". 116.2389299 / 103.6725591 = 1.1212121212. Still a 12% difference. Now in the case of volume or area, then no the tire doesn't hold 12% more air or have 12% more area. If you laid the tires flat on the ground and measured just the surface area of them, the 37" tire has 25% more surface area (disregarding the area of the rim).

 

[FitzMatt]

Originally posted by Robert
Uhmm, no. The circumferance is still 12.12% longer with the 37" tire versus the 33" tire. Circumferance = PI X Diameter. Since PI is a constant (3.1415927) and the given diameters are constants, nothing will vary since none of the calculations involve squaring or square roots. The circumferance of a tire is the actual distance traveled for every revolution of the tire. For instance the 37" tires have a circumferance of 116.2389299". The 33" tires have a circumferance of 103.6725591". 116.2389299 / 103.6725591 = 1.1212121212. Still a 12% difference. Now in the case of volume or area, then no the tire doesn't hold 12% more air or have 12% more area. If you laid the tires flat on the ground and measured just the surface area of them, the 37" tire has 25% more surface area (disregarding the area of the rim).

Circumference = 2 x PI x r

Matt

 

[Henry]

Ok, thanks Robert, I stand corrected!

 

[Henry]

Originally posted by FitzMatt


Circumference = 2 x PI x r

Matt

Actually the diameter is 2 x r, so Robert was correct too. (And now I have 100 posts!!!!!)