sojah

iTrader: (1)

alright so i put 20's on my car that came with 16's i am just estimating that when standing side by side that the 20'' mounted is 2'' taller than the 16'' mounted, therefore i wanted to know how much farther i am traveling per wheel rotation. I used pi (3.14159265) * Diameter (D) to get 50.265" traveled on stock and 56.548" traveled on the 20's. This is an increase of 12.22% traveled distance per rotation.

I am not sure how the odo registers total distance traveled but i am guessing that it is based off of tire rotations. if this is correct, then with 20's, my car is traveling 12.22% farther per rotation so can i then say that on a tank of fuel if the odo registers 360 miles that i am actually traveling 12.22% farther than the odo is registering based off of the 20's??? Therefore instead of traveling 360 miles on a tank of fuel for example i am actually traveling like 405+ miles. If this is true instead of getting like 22 mpg i am getting 2.5 better mpg pushing it from 22 to around 24.5 mpg...!!!!

how does this sound??

jcat_350

actually not necessarily true. haha i hate to burst your bubble but you are actually by increasing the diameter of the wheels causing your speedometer to register you going slower than you actually are. Regardless, your fuel consumption is more based on the RPM of the engine. Any upshot in distance you see from the higher diameter wheels will be offset by their higher weight/rolling mass, thus negating any MPG increase you might see.

sojah

iTrader: (1)

so prove it! give me the science or numbers behind it. i just sort of proved my theory and now i am waiting for someone else to prove me wrong or prove something different. you say so this means that if it says I'm going 60 then I'm really going 67.2 mph according to the 12.22% increase figured out by the math above.

i am calculating the mpg math myself with my cell calculator at the pump when i fill up and yesterday it came out to 364 (miles driven on a tank) \ 16.75 (gallons of gas purchased to top off) = 21.75 and since you admitted that i am actually going faster than the car registers, than that means you admit that the car is traveling farther than the odo registers, no???

therefore the "real" mpg has to be greater than my mpg calculated by my cell calculator.

so jcat_gs3 you also said so does this mean that if i'm wearing a blue shirt that my mpg's will go up or down??

Kyle Harty

iTrader: (17)

Go here to figure out the differance in speedometer and odometer readings and actual speed or differance based on your tire size. http://www.1010tires.com/TireSizeCal...?action=submit

My 285/30/20 rears put the speedometer off about 3.8% (slower) from the stock 16's according to their calculator.

When I fill up, I add 3.8% to the miles before I divide by gallons used since I actually traveled 3.8% farther than the odometer registered.

I think you're correct that your actual mileage will be slightly better than what you calculate strictly using the trip odometer.

What jcat is argueing is a completely differant point, you're talking about how to calculate your mileage, he's talking about how you're mileage will be affected by the 20's. Like he said, the extra weight and rolling resistance of larger wheels will increase your fuel consumption and probably more than offset any gains you make from the slightly increased wheel diameter. For me, when I want to go 60, my speedo is reading about 58 so if I drive at 58 on the speedo (60 actual speed) the RPM's are slightly lower than when I was going 60 on stocks. I don't think it's enough of a difference to really mattter, but I think that side of things is what jcat is looking at.

All that being said, I figure my mileage like you were stating by adjusting the miles on the trip odometer to account for the tire diameter, but your not going to be off by 12% unless you've got some huge tires on there, 2.5-4% is more realistic. It won't make as big a difference as you're hoping for but it will make some. Using that method, my mileage didn't go down significantly when I switched to 20's, I'd expected a more noticeable drop but I'm still in the same general range as I was before.

jcat_350

haha i'll go in order so we can keep this organized.

1. i can't exactly prove it, because there is no real 'formula' for how weight effects your mpg. I do know that with 80lbs worth of subwoofers in my trunk my mpg dropped by about 1-2. your adjustment for speed sounds about right since going from 15" to 17" on my old maxima threw the speedo off by 4mph, so going from 16 or 17" to 20", that 7.2 mph sounds about right.

2. the problem with the whole extra mileage thing is that it really doesn't take into account startup and time at idle. Every time you start your car uses about a minute's worth of gas at idle. In order to figure out your fuel consumption at idle, you need to know your pump's flow rate in gallons per hour, and divide that by the constant RPM at idle, then compare that to a miles per gallon figure in 1st gear, so that no manipulation of engine rev is occurring, and then compare that mpg figure to the corresponding flow rate. see why they hire people with engineering degrees? haha.

3. depending on what color blue the extra weight of the dye in the shirt will affect mpg haha nah, i should have elaborated more. Miles per gallon is kind of a deceptive unit term because it's more dependent on fuel consumption based upon how much fuel the engine is flowing and burning at once moreso than it is the miles you travel per gallon of gas used. You can travel a mile at idle, and not use a lot of gas, or you could be flooring it trying to break 150 and cover the same mile using a ton of gas.

basically by increasing the diameter 12% or so, you increased your speed by 12% and your distance traveled 12%. However, I can almost guarantee that your 20's are more than 12% heavier in WEIGHT, and also much much more than 12% greater in ROLLING MASS. the difference is that weight is the object's mass with the force of gravity acting on it. You can see that because of the nature of weight, the extra resistance to movement would cause more drag on the drivetrain. The rolling mass takes effect most significantly when leaving a stoplight, meaning it takes much more energy for the engine to get the wheels moving from a standstill, upping your fuel consumption. However, people fail to realize that this also takes place any time you're increasing your speed. Even on the highway, going from say 65-80 to pass someone, it's going to effect your consumption.

in short: just be glad the car is averaging over 20mpg, you learn to appreciate this like i did after coming from an ML430 awd that got maybe 18 or 19 on the highway any increase in distance traveled is probably offset by extra weight, rolling mass, and extra energy required to get the wheels moving.

jcat_350

you're absolutely right, i was more exploring why it'll probably make almost no difference when calculating the mpg. i, too thought that 12% was big but when looking at the difference in diameter from 50-56, considering 5" would be 10%, that doesn't seem far off.

Kyle Harty

iTrader: (17)

I think you've done a great job of explaining alot of that, the extra weight of the wheels is a killer, especially since that weight is farther away from the center of the wheel now too with the 20's, making it even harder to get moving.

My rear wheels and tires weigh around 9-10 lbs. more than 16's and the fronts about 5-6 more, my mileage and acceleration were noticeably affected but not by as much as I thought they would be going to 20's.

sojah

iTrader: (1)

well as much as i like your very detailed explanation about fuel consumption, and you are correct that they are much heavier, stock was like 43 lbs if my memory is correct and the lowenharts in the rear with tire and everything was like 63 or 73lbs or something like that.

but regardless yesterday dividing the odo trip reading of 364/ 16.75 gallons and getting roughly 21.75 mpg's based off of this calculation. what would you say is a "real" world figure for my "real" mpg rating? if its 21.75 mpg with the trip odo reading and this is supposedly skewed by about 12% then what?? what would you say is the real mpg?? i am very curious...

also my wifes gets about 38 mpg average on mixed driving and also clowns me for my **** poor saving of the economy with my low mpg's.

CK6Speed

iTrader: (1)

If you are averaging over 20 MPG with your 20" rims even before your adjusted calculations, I'd be very happy. On my stock 17" rims I'm only averaging about 16 MPG. However, or city driving is basically stop and go in bumper to bumper traffic which kills the MPGs

jcat_350

like i said man, if you're anywhere near 20mpg with 20's on, count your blessings haha. I'd say thats a realistic figure given the balance between extra distance traveled and added weight/rolling mass.

sojah

iTrader: (1)

nice web site KYLE_HARTY it was very informative. basically 7% loss of speed with that tire calculator when i added the big rims to my gs4 back in march of 05. this whole time i've been driving 7% faster and 7% farther than i thought (well summer months at least on the 20's), i have also been getting about 7% better mpg than i thought. this puts it at like 23 instead of 21 mpg... 23's not so bad. i think i'll sleep better tonight... haaaaaaaaaaaaarrrrrrrrrrrrrrr!

MJImport

iTrader: (1)

I didn't see it mentioned, but the tire patch also increases going to a larger outer diameter, increasing rolling friction.

Kyle Harty

iTrader: (17)

Not bad! I generally average somewhere between 21 and 23, that's with a good amount of highway driving and not too much playing around.

Just curious, what's your tire size to be off 7%?

I put one of my stock 16's on a bathroom scale (not necessarily the most accurate) and got 48 lbs. I should've weighed my L-Sportlines on the same scale but shipping weights were 58 lbs for the rears and 54 for the fronts with a little packing material. You gotta figure that alot of that weight has been shifted farther away from the center too which requires more effort to get going and to stop.

sojah

iTrader: (1)

i have 235x35x20 in the front and 275x30x20 in the rear, and had 225/50R16 with less than full tread if you get my 'drift'. j/k

i had it dynoed with the heavy rims on and it was 250 hp/ tq and i know it is like 275 with the stocks on. i wonder if it would be like 305 with some super lightweight 17's or even 18's on. and apparently everyone is shocked to see how low on the dyno it really is. i do loose a lot of power from like 0-45 or so but no so much in the high end... i actually think the drivability is so much safer and better feeling with the 10'' wide 275's in the rear. every winter when i switch back to the stockers it feels like i'm on some roller blades or something,,, all light and not so sturdy feeling over like 65 mph so i prefer the tank feel with the heavy wheels/ tires with a little loss in performance over the stocks for sure.

the next time around with a performance car i will go for lightweight but i started out vip style and yeah... but i would still like the big vip sedan with the big rims too. i'm just stuck in the middle with this current gs4 setup.
99 gs400

Kyle Harty

iTrader: (17)

Someone correct me if I'm wrong (it's been known to happen once in a while) but I believe the stock tire size is 225/55/16 rather than 225/50. It sounds like someone had put the wrong size tires on their at some point; you'd want to figure the difference from the 225/55.

I like the feel of mine alot better with the DF210's and 20's as well. 99% of the time I don't drive hard enough to notice the loss of a little acceleration.