Fuel Mileage Correction

Greaser · **Posted:** Wed Mar 23, 2011 7:32 pm

I know that there is an issue with doing correct fuel mileage calculation (like the odometer is off by a percentage or something), does anyone know what years this affects and what the correction is?

ATXKJ · **Posted:** Wed Mar 23, 2011 7:36 pm

use a GPS and calculate your own.

in general - 245/75/16s made my speedometer dead on - but odometer was still off a bit
(didn't pay too much attention)

265/75 make it ~4-5% slow

stock was way optimistic

however in terms of mileage - I'm only comparing with myself - so as long as I'm consistent it doesn't really matter.

Greaser · **Posted:** Wed Mar 23, 2011 7:41 pm

Yeah I notice I'm normally about 2 mph slower than what the speedometer says (which probably irritates everyone riding around me), everything is stock

LibertyCRD · **Posted:** Wed Mar 23, 2011 8:27 pm

With my tires, my '05 is about 4.5% too slow on the odometer, and about 3% too slow on the speedometer. It's odd that they are off by different amounts, but they are according to GPS testing.

bhc04 · **Posted:** Wed Mar 23, 2011 8:39 pm

My 06 CRD since new, odometer has always read 3 m.p.h. faster than actual speed compared to 2 different GPS's. Tire size 235/70/16 Stock tire size 225/75/16. Comparing the 2 different tire sizes, according to charts, in 1 mile the 70 series tire turns 9 more revolutions in that mile than the 75 series. Milage between the 2? Insignificant.

irollgen4s · **Posted:** Wed Mar 23, 2011 9:53 pm

I know through GPS that my speedometer is dead nuts now with 265-70-16's. But i cannot figure out how to see if my odometer is off. Mile markers orr what? :SOMBRERO:

4sefed · **Posted:** Wed Mar 23, 2011 10:03 pm

i've had a buddy follow me and tell me what my speed was based on his speedo.... mine was bang on. So I figure my indicated speed is correct with the stock size tires, yet my TomTom says I'm 6 km/h slower than my indicated. so like 3.5ish MPH slower...

very strange I think.

OldSkull · **Posted:** Wed Mar 23, 2011 11:41 pm

My GPS indicate 60 mph but my speedometer show 62mph, to compensate my 2mph off reading I need bigger donut

Let's use the mathematical constant π (Pi)

The stock tire are 225/75/16 so 225x75% = 168,75mm or 6.6437"x2+ rim size (16")= 29,29 High

29.29 x π (3,14159265) = 92 inches of diameter since 1 mile = 5280 feets or 63360 inches

my wheel are doing 688.69 rotation every mile or 41321.74 rotation per hours at 60mph but my speedometer

indicate 62mph or 42698.78 rotation per hours a 1377.03 rotation error so I need tire capable of doing

39645 at 60mph or 660.75 rotation per mile so a mile= 63360inch/660.75= 96.89 wheel diameter/π = 30.52 High

245/75/16 and 215/85/16 are close to this result just need to lift the jeep and fit them :wink:

Joe Romas · **Posted:** Thu Mar 24, 2011 8:00 am

With stock size tires when my tom-tom says I'm going 65 MPH the spedometer says 69 mph. :shock:

Some brands of gps's have miles traveled reading but my tom-tom does not seem to have that feature so I can't easilly verify the odometer

If/when I get another GPS I'll look for that feature :SOMBRERO:

Jay ne Ohio · **Joined:** Sun Dec 12, 2010 5:49 pm **Posts:** 216

Speedometer on my 2006 will read 70 mph while my GPS says that I am going 67.

Odometer will read 10 miles when I have actually traveled 9 miles according to the interstate mile markers.

I don't think I have ever taken my 2005 on the interstate, so I don't have a good idea on the odometer. But the speedometer will read about 1 mph more than the gps at 55mph.

papaindigo · **Posted:** Thu Mar 24, 2011 10:06 am

Assuming a stock vehicle the speedometer & odometer readings in the US are legally supposed to be accurate to within 5% of the actual speed or distance traveled. That's just a max variation and some stock vehicles are dead on and some are at the high end of the range for a variety of reasons. It's not safe to assume that 2 tires of the same nominal size have an identical diameter given different manufactures or different tire pressure. Further tire wear over time muddles the issue; given a nominal tread depth new of .3125" vs a worn depth of .0625"; if I am doing my math correctly a tire that's 30" in diameter new becomes 29.5" (tread wear comes off both sides so to speak) a small but measureable change. Even if one assumes road mile markers are accurate, they rarely are for short distances, over a distance of say 50 miles things as simple as lane changes can impact the difference between the odometer reading and the mile markers. My CRD correction for full consumption is 1.0173 X hand calculated mileage based on a 50+ mile GPS run while stoutdog's correction on a 06 with new Michelin tires (stock size I think) is slightly different. I don't know about my speedometer. Bottom line is there is no "universal" correction factor. I know of only 2 approaches the expensive one which is to get your speedo/odo professionally calibrated and the cheap one if you have or can borrow a GPS and to a test run.

Then you get into the accuracy of the pump reading between different pumps and the consistency of your fillup level which is a whole different can of worms. I'd go on but...

Fuel Mileage Correction

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