why am i getting worse gas mileage?

[fishnhuntnboy]

My truck normally gets 16 mpg and even in the winter, it may drop 1 or 2 miles but nothing enourmous. Then about oh 3 weeks ago my truck dropped down to 13 mpg and idk why. I just put new tires on and they were one size bigger, instead of 70R16 i put 75R16's on. Now i kno this shouldnt affect my mileage much and it never did. Could all this be from something else that im not catching or is it the tires? Also I just switched to synthetic oil about 4000 miles ago, would this affect it at all? Also what is something to do to improve mileage? Like doing stuff to the truck at all or anything.

thanks for any info

[spearchucker]

Wouldn't bigger tires make your odometer a little off? If they are bigger your odometer will read less miles than what you are really traveling. Maybe you aren't getting worse mileage?

[Spearing Machine]

When I put new tire on my 2006 Ford Fusion this fall they were the same size just with good tread on them my car lost on average 2-4 mpg. This was calculated only on my 5 hour trip of all highway miles traveling from MN to WI. Also when winter hits I always have lost 2-4 mpg as well from the colder weather and leaving it run to warm up. I guess what I am getting at is I am not really surprised you lost just a couple of MPG.

[CAMAN]

You say you just switched to synthetic. Did you flush all the old oil out? Some synthetics and conventional oils don't mix and could cause issues. One thing it could form is a cottage cheese like substance that has lost alot of its lubricaton value and doesn't go through an oil pump real well. The resulting friction could cause the drop in mpg you are experiencing.

[Ralph Wiggum]

You say you just switched to synthetic. Did you flush all the old oil out? Some synthetics and conventional oils don't mix and could cause issues. One thing it could form is a cottage cheese like substance that has lost alot of its lubricaton value and doesn't go through an oil pump real well. The resulting friction could cause the drop in mpg you are experiencing.

I'm pretty sure that all modern synthetics are compatible with conventional oils.

[TruthWalleyes]

Put a GPS in your vehicle - You're probably going faster than your speedometer is going. I've got one size smaller tire on my vehicle and my speedo is off by 4mph when i'm going 65 my speedo says 69.

Also, with the increase in tire size, are you seeing a slight increase in your RPM's due to more power required to turn a bigger wheel?

[spearchucker]

Put a GPS in your vehicle - You're probably going faster than your speedometer is going. I've got one size smaller tire on my vehicle and my speedo is off by 4mph when i'm going 65 my speedo says 69.

Also, with the increase in tire size, are you seeing a slight increase in your RPM's due to more power required to turn a bigger wheel?

Wouldn't you be running less RPM with bigger tires? You are going to be going faster than your speedo is telling you. If you are running more RPM at highway speed your tranny is slipping.

[Jeremy airjer W]

I'm pretty sure that all modern synthetics are compatible with conventional oils.

agreed, this may have been the case as synthetics first came onto the market but there are no issues switching to or switching back and forth, or topping off with either.

[TruthWalleyes]

Think about your bicycle when you switch gears from the small sprocket to the larger sprocket - does it take more effort to turn the pedals when your going off the larger sprocket...Yes it does take more effort.

Think about driving through a bunch of snow, or water puddle, it will increase the torque necessary to turn that tire at the same speed.

If your vehicle was light as a feather, then in theory you would run less RPM's with bigger tires.

Edit: I was probably wrong about increased RPM's, but think i am right about an increased load (torque) requiring more gas under the same RPM's.

[CAMAN]

agreed, this may have been the case as synthetics first came onto the market but there are no issues switching to or switching back and forth, or topping off with either.

I don't believe synthetics were brand new when it happened to me 4 years ago....

[FishLocker]

I have 2 oxygen sensors. Both went out this winter. I lost 4 MPG because the engine is not fine tuning itself. You may want to check your OH-2 sensor.

Had to change to taller side wall tires on my wifes car in order to get the snow tire I wanted on it...now she is going 68 when her speedometer is reading 65. Did not get it checked with original tires so cannot tell you how much it actually changed (more or less than 3 mph) from the original tires.

[pureinsanity]

I'm pretty sure that all modern synthetics are compatible with conventional oils.

That is what everyone says. But I would still flush it. They say you can put certain anti freezes in cars, but the fact of the matter is I end up doing a radiator flush to get all the crud and gunk out because someone listened to someone that heard something online in a forum.

[pureinsanity]

I will still top off ams oil with ams oil. Just like I will still fill my anti freeze with OEM Antifreeze because ive seen what the stuff you get at an auto parts store does to the system.

[BobT]

Wouldn't bigger tires make your odometer a little off? If they are bigger your odometer will read less miles than what you are really traveling. Maybe you aren't getting worse mileage?

This is certainly one possibility. How are you measuring your mileage. If you're using the odometer and make changes that affect the actual miles driven then you are not using accurate data.

[PurpleFloyd]

Think about your bicycle when you switch gears from the small sprocket to the larger sprocket - does it take more effort to turn the pedals when your going off the larger sprocket...Yes it does take more effort.

Think about driving through a bunch of snow, or water puddle, it will increase the torque necessary to turn that tire at the same speed.

If your vehicle was light as a feather, then in theory you would run less RPM's with bigger tires.

Edit: I was probably wrong about increased RPM's, but think i am right about an increased load (torque) requiring more gas under the same RPM's.

You got the last part right. Taller tires will reduce the RPM's but they require more torque to turn them. They will reduce your gas mileage when you are accelerating in most instances and also when going up hills or in a strong head wind but they can improve you mileage when you are driving a constant speed going down the highway.

In general with bigger tires you might see a decrease in mileage according to your odometer because it will show you drove fewer miles than you actually did. The larger step up in size you take the more dramatic the difference will be.

[pureinsanity]

How are the people with fancy dancy 24" rims going to be able to pay for gas for their pimped out hoopty rides this summer?

[Scott K]

PurpleFloyd is right, the larger tires add less miles to the odometer, then you are actually traveling, so by using the odometer to calculate the MPG, you are using less miles then you traveld to get the number, that would be the biggest reason, along with being a little harder on the engine to move the bigger tires, but this shouldnt be much of an issue.

[Jeremy airjer W]

I don't believe synthetics were brand new when it happened to me 4 years ago....

I would have to believe there are other factors at play. If what you said where true I would be seeing the same results on a regular basis which is not the case.

Kind of like the preconceived notion that every time there is a problem with a car its automatically a bad computer. Or the car isn't running right and I already replaced the O2 sensors what next. How about water in the fuel. With any of these the likely hood of them actually being the problem is pretty slim and the real problem is usually something else.

I'm not saying that the problem didn't occur, I just have a hard time believing the oil was an issue. What brand of oil, what kind of car, The integrity of the motor, the service life, driving habits, etc?

I am a huge believer in not mixing coolants! Extended lifes are just that. Some work better than other. As soon as you mix them with conventional coolant the extended life properties are void and in some cases will cause additional problems if not remedied.

[skolfoppa]

Fishn- Larger tires will drop your mpg. More rubber means more weight that has to be turned. Shouldn't be "harder" on your engine, but does require more gas to be used to turn the extra weight.

There was another topic on Winter Gas Mileage ??s with many good tips to improve mileage. Check that out.

When I changed my vehicles to fully synthetic fluids, both registered drops in mpg. The 4.7l lost 2mpg. The 3.4l lost 1.5mpg.

The best way to improve gas mileage is to ease up on the skinny pedal. Skol.

[pureinsanity]

So why wouldn't oil be the same? There are several different blends of ams oil. Several different blends of conventional synthetics. If you could show me a visual chart that shows every blend is compatible with every other blend id say you are right.

I don't want to mix cheap oil with expensive oil. Just as the next guy will by pass the smaller scrubby looking gas station because they are worried about getting a bad batch of gas.

[LightningBG]

These are Q and A directly from the Valvoline site. There were similar on Mobil1's site.

Quote:

#

Is it ok to switch back and forth between regular and synthetic motor oil? I heard this causes leaks? Is this true?

Switching between synthetic and conventional oil does not cause problems. Because the oils are compatible, you can switch back and forth as often as you like.

#

Can you mix different types of motor oil? For example, synthetic and synthetic blend or regular and synthetic? Is this going to cause problems?

Mixing synthetic and conventional oils will not cause any problems. The oils are compatible with each other.

[Jeremy airjer W]

All I'm saying is i have not seen an issue with cottage cheese looking oil. I figure I have done approximately 22,000 oil changes over the years. If mixing or compatibility where an issue I would have had to run into it by now?

[shizzy]

I went from 235/75/15 to 31x10.5's on my ford ranger. I went from 20.5MPG down to about 19-19.5MPG.

I did notice a little drop in power too.

also, when my GPS reads 55 my speedo reads just under the 55MPH line.

[pureinsanity]

Is there a recommended procedure for changing to AMSOIL motor oils?

There are no special requirements; however, in older vehicles or those with high mileage, it may be advisable to use AMSOIL Engine Flush first. This will ensure that the engine is clean and free of any accumulated contaminants which might have an effect on the service life of AMSOIL Motor Oils. In all cases, it is good judgement to install a new AMSOIL Absolute Efficiency Oil Filter (EaO) every time you change oil.

[spearchucker]

I can't ever see a need for a flush unless an engine has been neglected for years and is sludged up. Even the cheapest API certified oil isn't going to cause sludge if the oil is changed at a reasonable interval and the engine is in good repair.