Mileage

Joined
May 4, 2008
Messages
355
Location
Point Judith Area, RI
I know, it's already been brought up before....

Basic division is about as far as my math skills go -

Mileage on the way up (with about 20/30 miles of city driving, and heavy traffic)
157 Miles on ODO, took 13.8 Gallons to fill tank

Stand by for mileage/gallons of fuel for return trip....

Both ways, holding a steady 2500 RPM, running NEW 33x10.5s, 3.73 stockers

Some one be kind and do the math for me - please?

- jake
 

2mbb

SILVER Star
 
 
Joined
Sep 1, 2003
Messages
4,693
Location
WC, CA
157mi/13.8gal = 11.37 mpg
You also need to do a correction for the larger than stock tire size. the odometer measures rotations of the drive shaft, but doesn't know what size the tires are (or if you have changed your differential gearing). Since the 33" tire is larger than stock, you actually traveled more miles than the odometer says. To correct the mileage, you need to multiply the mileage by the new tire diameter and divide by the original tire diameter. Assuming the stock tire diameteris 28", then:

Actual Miles = 157*33/28=185 miles

Therefore the acutal mpg = 185mi/13.8gal = 13.4mpg

Because your tires are larger, you are also travelling faster than what it says on your speedometer--your actual speed is 18% higher than your measured speed. If your speedometer say's 55mph you are really travelling 65mph. You can thank me in advance for the speeding ticket I'm saving you from...
 
Joined
Nov 8, 2006
Messages
482
Location
Sugarloaf, CO
i got about 13.5ish mpg from golden colorado to just north of green river utah one time also on 33s and corrected... so that sounds about right...
 

ylexot

 
Joined
Jun 25, 2006
Messages
574
Location
Columbus, OH
Mathematically that correction factor is right. Multiplying mileage by the ratio of new and old diameters will give you actual miles driven. (We are really interested in circumference, more specifically tire revolutions per mile, but because pi is both on the top and bottom of the ratio they cancel.)

However on the road I noticed that this math didn't work out on my rigs. Assuming the Speedometer checks on the freeway are measured out correctly, my stock FJ60 with 31" always came up 10% off. Doing the math, it should be 30.7/28.5=7.7%. Even after 40 miles on the freeway, it was exactly 10% low. I got bored watching after 40 miles, but I have repeated the experiment several times on different road trips and different stretches of freeway with the same result. My FJ62 with the same tires read was nearly spot on, only off by maybe 1 or 2%.

I don't know if the POs modified the speedometers/odometers on these Cruisers or not. Regardless, I recommend measuring the error on your odometer vehicle by vehicle.
 
Joined
May 4, 2008
Messages
355
Location
Point Judith Area, RI
New Numbers

Return trip

8.916 Gallons @ 123 Miles entirely Highway

2mbb....already used the 'speedo correction' excuse with local law enforcement - the officer understood, in fact - he thought the truck was pretty cool.

Thanks for the help

- jake
 
Joined
May 4, 2008
Messages
355
Location
Point Judith Area, RI
answer?

I think I just answered my own question using 2mbb's formula

Here goes -

123 ODO miles x 33 = 4059

3444 / 28 = 144.9

144.9 / 8.9 = 16.2?

Is that correct?
 

2mbb

SILVER Star
 
 
Joined
Sep 1, 2003
Messages
4,693
Location
WC, CA
the correction factor used for the odometer would be the same correction factor used for the speedometer--assuming the speedometer is an accurate reflection of the odometer. I think the odometer actually measures the number of times the speedo cable turns. The speedometer (somehow) measures the rate of spinning.

anyway, my point is, if you have access to a GPS you can easily determine the ratio between the acutal speed (from GPS) and the measured speed (speedometer). You can then use this ratio to correct your odometer. Or you can find one of the freeway speedometer checks mentioned above, and use it to confirm the accuracy of your odometer.

The "theorectical" correction factor based on tire diameters given above is not precise because a tire does not generally measure out at it's given size, and then the rolling diameter is also influenced by inflation pressure and the total vehicle weight.

My approach is not to worry about it too much. I do a calculation from time to time, but that is mostly used as a trigger for me to do some preventitive maintenance--time for valve adjustment--or an indicator that something might be wrong. You won't win many MPG bragging contests by coming to the table with a 2F. If that's your thing, you should look into a diesel conversion, or work on your poker face and just lie about it.
 
Joined
Nov 8, 2006
Messages
482
Location
Sugarloaf, CO
well one time in nevada i reset my trip meter at a mile marker... no stops or detours, as the trip meter rolled over 100 miles, i had travelled between 113 and 114 mile marker signs... and i used that for my correction for my 33s (it was slightly later in the same trip that i measure the 13.5 ish mpg)
 
Top Bottom