i was averaging between 21-25 mpg per tank, before and after my switch to synthetic about 60k miles ago. almost always 23point something. then i notice it's getting steadily worse, all the way down to 18.331 mpg last week. i go in for a change since it's been 7k since my last one (since switching to synthetics, i've been running extended changes). there i find that my mobile 1 10-30 is darker than normal, and that i'm a quart low
now, i've been super **** about keeping up my fluids, all of them, since i drive so many miles. this change was rather sudden and with no memorable events/catastrophies in the recent past, it's a bit confusing. anyhow, i took a trip down to san diego this weekend for a concert and when i filled up again last night, i calculated 23.723 for the trip. (mind you, i was flying on the freeway there and back, but experienced considerable traffic while in san diego).

ok, so my question is this. the only thing i can think of is my belts, i had both replaced about a month ago. could it be possible that they were overtensioned causing this mileage change? if so, how come it rebounded so quickly? i've run the same service interval (6k-70) between oil changes for about 18 monthes now

 

oh yeah. i had them catch some of my oil midstream for blackstone to check out. i've been driving around with the bottle for a while so i figure now's a good time to see what's up. anyone know what i should be looking for when i get the results back??

thanks

 

Wow... Hmmm... I gave my engine 3k to break in and have had Mobil One in it ever since, have 129k on it now. I have averaged 21-25 for the entire life of the car and never burn/lose a drop of oil. I'd find the quart you were low!!

One question (I've done dumber things), you said you "go in" somewhere to get the oil changed. Is there *any way* that they missed a quart putting oil in? I still doubt being a quart low would affect mileage that dramatically, but it would take something pretty substantial to drop 4-5 in one tank.

Just go through a checklist of what might cause the car to run rich... (MAF, O2, etc...) If that happened to me I'd pull my plugs out and see what they look like, they may not be bad but they might tell a story!

Good luck...

 

wow... when i drove from TN to NY, we didnt have to refuel for almost 420 miles until the light came on... and that was all highway... thats no joke either...

 

My highest before light is 420 also, but I have never even remotely driven for mileage

 

oops. not a quart, a half quart. still a lot tho.
and i always go to the same place for my changes and fluid flushes.

 

oil color does not tell your its life. only oil report can tell you if your oil is working for you. my new mobil one gets dark after running it for a week...but after a 3500-mile drain, the TBN of the oil is at a high 12.

 

You musta been drivin EPA style to do that.

 

I can't PROVE it, but I think the Max likes to eat synthetic oil.

I never had to add oil UNTIL I switched to synthetic. Now I've added 1/4-1/3qt each time after about 1-2K miles.

 

I put in royal 5w-30 and generally have to add a half quart at about 3k. The say that i should only really change oil after 6-7k that is a long time but that is what every body says and my car works fine. I generaly get about 23 mph

 

so you guys don't think the belts have anything to do with it?
just kinda strange. hopefully the oil analysis will tell me something.

 

Same place doesn't always equal same tech.

 

my gfriend went 18000 miles between oil changes in here altima when she was in high school and was only 1 quart low ... course the car only lasted 96,000 miles ... I had to drill it into her ....3-4k . now she has a TRD tacoma and romps it weekly ... I think she enjoys killing autos

 

It's rare for most women (there are some out there) to give a **** about cars.

 

My GF in high school, now wife, did around 20K miles on the factory fill oil before changing it on her Sentra. I just met her at that point and she said she hadn't changed the oil until then and only because she happened to work at a dealership and they offered to change it for free. Otherwise, she didn't know she was supposed to change it.

BTW, we still drive it and it has about 150K miles on it, but it's restricted to around town trips only.

 

I used to purchase millions of gallons oil for an intl. quick lube chain. If you're going to go syn go Mobil 1. If anything your mileage should have gone up with syn. since it decreases internal overall friction due to the shape of the syn. molecules.
Belts will have nothing to do with mileage. Neither will being a quart low.
One thing you didn't mention was tire pressure. Low pressures will definatley result in lower mileage. Hey, pump those babies up to around 60psi and watch your gas mileage climb. Now don't really do that that was just to make a point.
Additionally, weather also can effect your gas mileage cool temps will make your engine more effecient as opposed to 100 degree days.
All in all gas mileage is a moot point .... when the tank is empty your choice is rather limited afterall!

 

thanks
i mentioned the belts cuz i thought the engine crank turned the compressor and accessory belts as well, putting a load on the engine, albeit a slight one. if a belt was overtight, wouldn't it increase the amount of work and decrease efficiency? i figure the load may be miniscule compared to the power of the engine, but if the load was insignificant, how would a udp give any gains?



or am i just overthinking this??

 

My understanding is an UDP frees up horsepower by spinning other accessories slower. So I'm thinking, same load, different speed.