DeusExMaxima

iTrader: (1)

I regularly drive from So Cal to Nor Cal and I get a stated 30 mpg on my OEM fuel computer. I drive about the same speed each time and i take the same route. Since the last trip, I got 20" rims which are heavier 15 lbs each maybe), I installed my RLM bumper, and I made a lower trey to go under the bumper and part of the engine compartment. Also, I was carrying more weigh in the form of my daughter, her luggage, and my bowling ball (total of about 170 lbs). This time I got 20 mpg!!!! That's a difference of 33%!!! Could it be the extra weight? Is the engine running rich? O2 sensors??? Any idea as to what would cause a sudden drop in mileage like this?

twentyeggs

Quote:

Originally Posted by DeusExMaxima

I regularly drive from So Cal to Nor Cal and I get a stated 30 mpg on my OEM fuel computer. I drive about the same speed each time and i take the same route. Since the last trip, I got 20" rims which are heavier 15 lbs each maybe), I installed my RLM bumper, and I made a lower trey to go under the bumper and part of the engine compartment. Also, I was carrying more weigh in the form of my daughter, her luggage, and my bowling ball (total of about 170 lbs). This time I got 20 mpg!!!! That's a difference of 33%!!! Could it be the extra weight? Is the engine running rich? O2 sensors??? Any idea as to what would cause a sudden drop in mileage like this?

an extra 200 lbs will take a little bit of gas mileage away, maybe 1-2 mpg on highway, and 4-10 on city. think inertia.

but if i were to guess, its the larger rims that took away your gas mileage.

usually, larger rims means less wheel revolutions per engine stroke = lower rpm and better gas mileage, but it also means your engine is working harder to turn a bigger wheel which kills gas mileage.

ill explain it another way, it would make sense that with a bigger wheel you are covering more ground per engine revolution right? but.. even tho you are covering a greater distance per revolution of the tire wheel assembly, your car is exerting a constant torque, and the way that torque is calculated, it means your car is exerting the same amount of force over a greater perpendicular distance (or moment), which means that it actually has to work harder and expend more fuel than with the original setup.

think of it on a larger scale, say 100" rims (i know this is impossible but bear with me) think about the extra energy it will take to get moving with a wheel that large on a stock gearing ratio. also, the larger size wheel the slower acceleration..

DeusExMaxima

iTrader: (1)

I thought about the possibility of the heavier rims. But with respect to inertia, it seems to me that mileage would be affected more in the city, than in the highway because of the stops and starts in the city. I believe it is more than the heavier rims but i could be wrong. Has anyone with heavier rims seen a drop in mileage?

twentyeggs

exactly, with more weight in the car your gas mileage suffers more in the city than on the highway. but again, its not the weight that is cutting your mileage by 33% its the 20" rims you have.

DeusExMaxima

iTrader: (1)

I suppose I could experiment. I could drive my car as is, with no passengers and see if my mileage is still down. If it is still low, then we can eliminate the passenger and carrying weight as a problem area. I could also swap my lighter rims and see if the mileage returns to normal. If that doesnt work then it is probably an engine problem. I cannot imagine that my bumper or undertray would adversely affect the mileage. To the contrary, it should have increased the gas mileage.

twentyeggs

......................it isnt your engine, it isn't your bumper, and its not your passenger with baggage... you are losing gas mileage because of your new rims. not because they are heavier, but because they are larger. your 20" rims are hurting your gas mileage.

your larger 20" rims are the primary reason for a decline in your gas mileage.

twentyeggs

if you don't believe me do this... take a 5 mile drive on the freeway with no passenger and nothing in your car. document your mileage.

go home and swap our your rims for the factory rims, find 60lbs of stuff and throw it in your trunk (this is to match the weight of your 15lbs each heavier 20" rims) take a drive on the same stretch of road.

make sure you do this when there is no traffic.

what you will see is you get worse gas mileage with your 20" rims, and because you took into account the extra 60 lbs for your second test run that eliminates the weight variable. leaving the only difference in the size of the wheel.

what people don't understand when they go out and buy different sized rims in our cars are not made for them, so it is going to mess with the fuel economy, your performance (slower acceleration) and the speedometer. so your 800 dollar rims are really costing your many many times that, in fuel costs over time.

joecurr

You can't just say this is due to the diameter of the wheel without taking into account the tire size he has now and the tire size he had before. If he chose the correct tire size for his 20" wheels, then the diameter of the tire should almost be the same, which would mean it's not the diameter that's the issue. A 245/35/20 tire is 0.27% larger than the OEM 245/45/18 tire, which would hardly be the cause of decreased gas mileage.

twentyeggs

well i did think about that but i took the liberty to assume he didn't take into account his tire size because what else could take away that much mpg, were talking about a drastic change in mpg that just so happened right after he switched rims... it isn't the extra 170lbs he has, with highway driving i couldn't see any more than 1-2 mpg last with the extra weight.

to the OP what size tire do you have on your rims?

TreizeRXH

I'm reading this thread and it's got me confused a bit. You're accounting for rim size and weight as a factor for the drastic reduction in gas mileage. Should, for arguments sake, he have 20' rims that are light weight (say, less than factory) wouldn't that help since, even though you have a rim that has more of a diameter to push, it's light weight and would theoretically require less energy??

(of course I do realize his rims are heavier, but I'm just trying to grasp the physics of this...something seems off)

twentyeggs

well yea, the lighter rims would help with MPG, but the larger diameter hits MPG more than weight does..

so say his rim were 15 lbs each lighter that oem... he will be 60 lbs lighter and will gain (for arguments sake) 1 mpg, the larger wheel diameter is still taking away 10 mpg, so he ends up with 9 mpg total that he loses.

mpg is very fickle, infact going up a grade of only 5%. can hurt gas economy by a lot! even strong wind blowing at you can kill economy.

there was a mythbusters episode on drafting a truck for fuel economy. they got up to 60% better fuel economy basically because the truck took away wind resistance. that would give our cars like 45MPG.

when we are talking about putting any strain on the engine we are talking big changes. a larger wheel will do this.

joecurr

You are doing it again. Just because the wheel size is larger doesn't mean the total (tire on wheel) diameter is larger. I'm sure when TreizeRXH asked his question, he is talking about keeping the total diameter the same but changing the wheel from 18" and say 20lbs to 20" and say 15lbs. The issue with this is that the weight may have moved further away from the center of the wheel which may negate the fact the 20" wheel is lighter and might actually perform the same (don't hold me to that, it's been years since I've used this type of physics, but think about figure skaters spinning with their arms out, and as they bring their arms in they spin faster). The issue is NOT the diameter of the wheel.

DeusExMaxima

iTrader: (1)

The size of the tires slips my mind but I recall that the diameter is very slightly LARGER than factory which should bring down the rpms slightly at any given speed - 255 35 20 is what I recall the size of the tires (I didnt drive it to work today so I cant check at the moment).

taz394

iTrader: (2)

its the rims i had some 20's and took them off within a few days

Bad gas mileage .... and don't get me started on how sluggish the car felt

it was like i was running only on 2 cylinders

twentyeggs

i know many people who did the same thing..

joecurr

255/35 on what width wheel? Also, the weight of the wheel DOES play a role in this, it shouldn't be neglected. A 255/35/20 compared to 245/45/18 is 1.26% larger. That is HARDLY the cause of decreased gas mileage. IMHO it is a combination of everything you stated initially. I highly doubt the RLM bumper was designed for aerodynamics, so drag was probably increased. The wheels more than likely weigh a lot more than your original wheels, and a mass of the weight was probably moved away from the center of the wheels. You were carrying a heavier load. All of those things play a role in gas mileage and the performance of the car.

BadBlackMaxSL

iTrader: (1)

Deus, If anyone would know the answer to this I thought it would be you. You autocross in your Max and you put on smaller diameter tires because it takes less power to accelerate with all of the science behind unsrung weight, Rotational mass, inertia, etc. which means better efficiency of the drivetrain. Thats why you don't see race cars, F1, Nascar, LeMans, etc riding on 20's.

rohanmax14

I had 20" rims for 5 yrs on my max n I get 20 mpg. I thought somethin wrong with the engine, but nope that my 20" rim lol. Im thinking off putting back my stock 17" wheels on the max maybe, so I can get all my peformance back until next year I get 20" vossen cvcs 3 wheel (hope).

DeusExMaxima

iTrader: (1)

I suspected the rims may have an effect on the mileage but I didnt think it would affect it so drastically. Since I did some other mods to my car, I wanted to get everyones opinion.

BadBlackMaxSL

iTrader: (1)

I know, but I have always known you to be on top of these kind of questions, not to mention the aerodynamics of the Max to squeeze the most out of it. Still love your black and red theme- Timeless. Later.

DeusExMaxima

iTrader: (1)

Thanks. Appreciate the kind words. The 20s are the opposite way I have been going since they are heavy but they help raise up the car a little. I know that my lighter rims help performance and mileage but I wondered if the heavier rims made that much of a difference. Im going to try to get my lighter rims on the car when it cools off and see how the mileage is affected and report back here.

the macedonian

damn - i expected it to be other factors but reading all of this it's crazy that the 20's are the main and only reason....

mdb007

How much wider were the tires on the 20 rims compared to the stock 18? the more tire in contact with the road the better traction but you also lose mpg. that along with weight. if you could put 205's on car would get better mpg with less rolling resistance and really ***tty handling. Also are the larger diameter wheels affecting the computer in some way? just my few thoughts

infiniti medic

Your cars computer calculates gas milage buy each mile driven. With larger diameter wheels it will throw your speedo off the ECM programmed specs. Thus milage will suffer. Ill bet your off by say, 2mph nominal. This should calculate to aprox. 22%. Check your speedo by your gps if you have 1.

infiniti medic

The cars on board computer is wildly inaccurate as a whole. Do the math at the pumps, I'll bett you come up with about 24 to 26 mpg

mdb007

Yes Nissan engineers need to go back to school. Mine always says i am getting 23.4 mpg yet when i check at pump it is more like 21.7, however it is better when doing a trip with all highway driving. I have to say family members who have had Chryslers over the years, those computers calculate mpg better.

infiniti medic

My i35's onboard computer says like 33.5 to 36 mpg at 65 mph freeway. At pump when i do the math I get about 22 to 26. I don't drive it like I stole it alot. I like my tranny. Oh and don't have 2,500 large for a new one. Lmao

xoomer.com

iTrader: (19)

most 20" are like freakin' anchors. Why Deus, why?

2000_MAXIMA_KING

iTrader: (30)

Sounds like you got some heavy *** wheels! Happened to me unfortunately most aftermarket 20" wheels aren't manufactured with performance in mind since the "big rim" crowd is in it for the "hey look at me" result. I did have some MPW 20s that were really light compared to other 20s but trying to keep the same diameter as stock or close to it with rubberband tires the ride was terrible! Even switching from some motegi 18s to stock felt way better and yielded better mpg and performance. That's why now I stay away from aftermarket rims, unless they are up their with stock or better in terms of quality and performance, which is usually when wheels get crazy expensive.

xoomer.com

iTrader: (19)

Bigger doesn't automatically mean heavier. My 19" Nismos are somehere around 19 lbs, far lighter then stock offerings.

Quality rims are lighter by default but come with a hefty price tag. From what I've seen for most people around the city the bigger the better no matter the sacrifice. I've seen rims where you couldn't pay me to "wear them"

LiquidEliteMax

Hey guys, I've been getting bad gas mileage as well. I have been averaging about 11 city ever since i changed my spark plugs and cleaned the throttle body. Do you think is my MAF that's causing this?

DeusExMaxima

iTrader: (1)

My tires are 255s so only 10mm wider. The diameter of the tires are actually slightly larger than stock so technically, all things being equal, my gas mileage should be better because the rpms at a given speed is going to be slightly less. I know these rims are quite heavy and the only reason I got them was because they look good and they raise up the car a little so the front bumper doesnt scrape so much. I suspect its the weight of the rims. What I should do is install the older lighter 18s and run it around and see if the mileage improves.

Rods03Max619

iTrader: (5)

You will gain your mileage back if you put the 18s back on had 20s and when switched to 18s gained the least of 50 miles to the tank in the city a 17-20% increase and highway way even better....

DeusExMaxima

iTrader: (1)

Problem solved. I looked under my car the other day and saw an O2 sensor wire resting on the exhaust pipe. I think it melted on the pipe and was shorting out. I mounted it high above the pipe. I then got the same good gas mileage I was getting before - high 20s to low 30s on highway and high teens to low 20s in the city. I think the city mileage is not as good as it used to be cuz of the heavier dubs but its close.

maximusberg

iTrader: (1)

at any point did u have an engine line on? the rims shouldnt take of ten miles per gallon thats way to much, it should be more like 4-8mpg overall......

DeusExMaxima

iTrader: (1)

I have a check engine light on. I need to pull the code. At least my good gas mileage is back :-)