Need a math wizard to check my math (1 Viewer)

[Copenhagen1]

I am going to start recording my fuel consumption here....

Fuelly | Share and Compare Your MPG

I am doing this because I have ordered a set of DT headers and would like to get a baseline before install to see if they make any difference. I can record this from my iphone at the pump and it should be fairly simple.

My 100 has BFG AT 285/75-16 on it. Tirerack specs say this size revolves 634/mile. Tirerack specs also say that the factory Michelins 275/70-16 revolves 666/mile.

When I fill up my truck at the pump and reset my tripmeter, shouldn't I multiply the reading on my trip x 1.05? For example, I fill up my tank and read my tripmeter from last time. The trip says 200, but because of the oversize tires, I would multiply 200 x 1.05 (5%) to equal 210 miles actually driven? Then report 210 miles and ?? gallons to fuelly.com. Am I thinking right?

 

[Kigiin]

Or you could just ignore the tire size impact both pre-headers and post-headers - would be apples-to-apples comparison. Might not get exactly MPGs, but you would get a relative comparison.

 

[Jacket]

Seems good to me. With stock gearing and larger tires you should drive more miles than what your odometer shows.

Edit: I'm no math wiz, but I have taken at least 7 different Calculus classes in my life....

 

[OregonLC]

your math assumes the ODO is correct on factory tires. this is not necessarily the case. for accurate and repeatable results regardless of tires or wear use GPS.

 

[e9999]

yes, if your tires are bigger you need to multiply by the diameter ratio (as in 1.05) to get the correct distance travelled.

Yes, the speedo can be incorrect (it should be slightly overestimating speed for most vehicles) although IIRC mine was surprisingly close.

GPS isn't perfect either but if you use it to calibrate things on a straight road it should be better than on a curvy one, and likely plenty good for simple estimates.

 

[inkpot]

Or you could just ignore the tire size impact both pre-headers and post-headers - would be apples-to-apples comparison. Might not get exactly MPGs, but you would get a relative comparison.

I would go with this idea. Leave the 1.05 factor out of the math. Why make it more complicated that it needs to be? If your not in a MPG race with all of the 100's on MUD, but you are only comparing your rig before headers to your rig after headers, with no other changes. Keep it simple. Use only your odometer readings and your gas pump readings. If ya want to get crazy technical, you will have to call the state to see when they have verified the metering on every gas pump you go to!!!! John

 

[snapaxle]

There are going to be too many minute variables that are not really going to be statistically important enough to worry about. Tire size being one of them. Just use the same station (preferably the same pump) for a few fill ups and record the average miles per tank. Then do the same thing after the header install. For some reason I doubt the headers are going to show a significant change.

 

[Copenhagen1]

Thanks for the replies. I guess I really never heard that my math was incorrect.

I can only assume that the speedometer/odometer was correct when it was engineered to work with the factory size tires. I have to use that as a baseline. I guess I know for sure and without fail that the tires on there now are 5% bigger than the OEM tires.

I understand that I am probably splitting hairs here but people use this fuelly website to determine real world mpg's on vehicles they may be interested in purchasing. There are only 4 - 2001 LC's in the average so I want to be as close as possible to real world mpg's. I dont want mine to skew the numbers because I have bigger tires on there.

If, for example, my odometer shows 250 miles and it takes me 17.9 gallons to fill up, then my mpg's are 13.9. With the tire size adjustment, I actually travelled 262 miles on the same amount of gas, then my mpg's go to 14.6.

If I was keeping this average on a notebook in the glovebox, I would just use the hard numbers before and after the headers. Since these numbers are "published" on the web, I think I should account for the 5% difference in miles travelled.

 

[EdsCruisin]

One of the more complicating factors is speed. My 285's cause my speedo to be off more at higher speeds. i.e. at about 70 "true" mph, it is reading 65, and at 25 "true" mph, it is reading 24. So how far you are going and the mpg is relative to the type of driving you do and the road gain your brand of tires provide at various stages of their lives.

I know that doesn't seem to make sense, but I have tested it out again and again with GPS, radar, etc. A percentage difference is a ratio - so 1.05 times speed will result in a different speed differential at every speed.

 

[OregonLC]

the other (and bigger) factor in your accuracy isn't splitting hairs with the odo, but rather having consistent operation cycles. hopefully you have months of data before the headers from which to compare (and then will need months post-install to also compare). maybe months isn't correct for you if you drive a lot (I'm maybe 500 mi/month). oxygenated fuels, % of time wheeling, and tire pressures will also make a significant difference.

 

[9toys]

when you hear the exhaust and feel the increased power with the headers you will probably drive faster and invalidate the entire process, anyway.

 

[inkpot]

when you hear the exhaust and feel the increased power with the headers you will probably drive faster and invalidate the entire process, anyway.

Agreed. Perceived performance may skew your driving habits. Also, you will be comparing a modified rig to the others that are stock. Comparable? Maybe, maybe not. John

 

[OregonLC]

when you hear the exhaust and feel the increased power with the headers you will probably drive faster and invalidate the entire process, anyway.

i hate to burst your bubble, but...

 

[Copenhagen1]

the other (and bigger) factor in your accuracy isn't splitting hairs with the odo, but rather having consistent operation cycles. hopefully you have months of data before the headers from which to compare (and then will need months post-install to also compare). maybe months isn't correct for you if you drive a lot (I'm maybe 500 mi/month). oxygenated fuels, % of time wheeling, and tire pressures will also make a significant difference.

Man I wish I was only driving 500/month. I am closer to 2000/month. I probably fill up 7 to 10 times per month. I never off road except for ranch roads so I dont change the tire pressures. Mine should be fairly consistent.

when you hear the exhaust and feel the increased power with the headers you will probably drive faster and invalidate the entire process, anyway.

I am buying the headers strictly to quiet down my exhaust. I am so sick of starting my nice truck in a parking lot full of people and trying to "idle" out of the parking lot because that damn ticking noise makes it sound like a piece of crap. If I get throttle response gains or better mpg's as a result I will be pleasantly surprised. As long as it fixes the ticking sound I will be pleased.

EdsCruisin.... I agree with you about speed and I think we are saying exactly the same thing. If you set your cruise control at 25mph and did not touch it for exactly one hour, then you would have travelled 26.25 miles, not just 25. If you set your cruise control at 70mph and left it alone for an hour then you would have actually travelled 73.5 miles. Same principle.

Thanks for everyones input. I am sure not driving this tank because I am highly worried about mpg's. I just think monitoring mpg's is just another indicator of overall health of the vehicle.

 

[Gottagetone]

Is Fuzz getting an average of 13.7 mpg? 2000 Toyota Land Cruiser MPG Reports | Fuelly

 

[OregonLC]

Is Fuzz getting an average of 13.7 mpg? 2000 Toyota Land Cruiser MPG Reports | Fuelly

Did you catch that the other one was a Diesel!

 

[TheFuzz]

Is Fuzz getting an average of 13.7 mpg? 2000 Toyota Land Cruiser MPG Reports | Fuelly

I was using fuelly back when I had stock tires...now I have an App on my phone that I just punch the numbers into using the tripmeter instead of the odometer. Its easier than logging onto the fuelly website on my phone at the gas station. I haven't logged a fill up on fuelly for quite some time now.

 

[Gottagetone]

Did you catch that the other one was a Diesel!

Yep, just for fun I have been looking for 105 diesel, found one stateside but they wanted bookoo bucks. there is a guy here in Austin that put a diesel in his 80 series...pretty cool rig!