Timing belt vs mpg?

[SSSVX]

Just got my timing belt and allll drive belts changed!
Question:
If your timing belt and drive belts are too tight or may
be the timing off a bit? Would these decrease the mpg?

But the car runs great!! The timing seems good.
No more belt whining noises from its first start!
The tensions also seem okay now.

I just really can't figure out why my mpg average drop like 2mpg.

Provided right after belts changed, I had a fresh oil changed, too.
Should have been better mpg with fresh oil.
The only thing I think of, I got 5w20 instead of 5w30 from last time.
Same brand, synthetic blend.
Thinner oil should have better fuel economy too.

Well, it's been like one thousand miles driving after belts changed
average dropping like 2mpg. why?
I know when I'm driving hard! But, I have like very consistent
mpg driving for more than one hundred thousands miles.
The testing after timing belt/drive belts changed are under:
same driving behaviors, same driving conditions, same brand
name of gas, same gas pump station at same outside temp,
of course same driver just me

I'm just a lil bit lost so far, why dropping.....
I usually got like 2mpg improved even after oil changed.

[Blacky]

Fresh oil has no affect on mileage. Thicker oil could have a tiny influence. The timing belt tensioner should take care of the tension on the belt. If your timing was off say by one tooth, the car would run poorly and mileage would take a dive.

[SoCal LS-L]

Oil as thin as 5w20 increases economy on NEW engines. On a higher mileage boxster, you may be losing a small amount of compression due to the much thinner oil film on the cylinders. This means you would have to step on the gas slightly more to maintain the same speed or acceleration as before, reducing your gas mileage. The same thin oil may also be getting burned more in the cylinders since more would pass by the rings, tainting the O2 sensors just enough for a false oxygen reading.

[svxistentialist]

Originally Posted by SoCal LS-L

Oil as thin as 5w20 increases economy on NEW engines. On a higher mileage boxster, you may be losing a small amount of compression due to the much thinner oil film on the cylinders. This means you would have to step on the gas slightly more to maintain the same speed or acceleration as before, reducing your gas mileage. The same thin oil may also be getting burned more in the cylinders since more would pass by the rings, tainting the O2 sensors just enough for a false oxygen reading.

I also do not believe the timing belt per se could affect fuel use, unless it was a tooth off.

The last point above is much more likely. If your consumption has disimproved I would be inclined to look at things like a possible dirty MAF sensor or a failing [but maybe not gone yet] O2 sensor. These things affect how the engine runs and would be more likely to affect your fuel.

Another possible fault might be worn clutches in your gearbox if it has high miles on it. Plate slippage = more friction heat and more fuel used per trip.

A third thing to look at is your brakes. Are any of your brakes dragging? Does any one wheel feel hotter than the others after a trip? This would cause higher fuel use also.

Joe

[SSSVX]

of all the miles I've driven like 300+k from different cars or my SVX or
my friend's car, I've noticed that the next few tanks of gas are usually
better mpg after the fresh oil changed. my friend has same feelings as me.
so that's our experiences.

hm sorry forgot to mention this is not my SVX H6 3.3 engines. it's my daily
drive car's I4 1.6 dohc engine. the car is 2006 it's still like new but has
hit 60k as due for the belts changed. it's 5 speed. the car still runs pretty
good after timing belt changed. it seems to have good pick up too. the car
doesn't run poorly or anything. it's impossible of its maf or o2 sensor or
sticky caliper or anything. as it's still not at that old age to worry about
those things. that's why it made me lost. on my last dd, i have consistent
around 21-22mpg of 70k miles free trouble miles. for the past 40k or more,
i've been getting consistent mpg on this car. but it drops like avg 2mpg after
the timing belt changed. why?

why? i was even thinking if i didn't buy the factory belts so the aftermarket
belts has different material or rolling resistance??

sorry forgot to mention it's not our eg33s again. it may have to be in the not-exactly-svx.
as i think we have people know different stuff here and i'd rather to ask here than other
forum. my SVX also has pretty consistent mpg for me throughout the years. i only used
synthetic blend 10W-30 for my SVXi all year. this car is factory recommend using 5w-20.
I only used 5w-20 for this oil change, for the past many oil changes I always used 5w-30
because 5w-20 is usually not available in stores.

tonight, i finally noticed some difference to my car. i don't know much about the timing stuff.
how bad is considered off too much? but when you drive this car, you know it runs and sounds good.
i have noticed my RPM speed is different at the same speed. is it considered as advanced timing or
retard timing? any education here?
before i changed my timing belt, from all over the time, on fifth gear when i was driving 60mph,
the RPM was at 3000rpm. but right now after the timing belt changed, the RPM was 2800rpm @ 60mph.
and when i drive it at 3000rpm, the speedometer is at 62-63mph. this is the difference i have found out.
any opinions?

[svxistentialist]

Originally Posted by SSSVX

tonight, i finally noticed some difference to my car. i don't know much about the timing stuff.
how bad is considered off too much? but when you drive this car, you know it runs and sounds good.
i have noticed my RPM speed is different at the same speed. is it considered as advanced timing or
retard timing? any education here?
before i changed my timing belt, from all over the time, on fifth gear when i was driving 60mph,
the RPM was at 3000rpm. but right now after the timing belt changed, the RPM was 2800rpm @ 60mph.
and when i drive it at 3000rpm, the speedometer is at 62-63mph. this is the difference i have found out.
any opinions?

If the timing belt is correct for your car, the correct length and number of ribs etc, and I presume it is, then change to a new timing belt could not cause this change to your car's running.

What you are observing here is a change to your car's overall final drive. The only thing I could imagine causing this would be if you had put on four new tires, or possibly two new tires on the drive axle if it is a FWD or RWD car.

Different makes of tires of the exact same profile can be taller or shorter, which means a different rolling radius, and so different final drive.

If you think about it, even if buying new tires of the exact same make and profile the new tires will be taller than the old ones because of the deeper tread. However it is unlikely that the exact same make of tires could cause the difference in rolling radius that would cause a 200 rpm drop.

For that my guess is you have put on different tires, same profile, but they are taller. This could be affecting your final drive ratio. Another thought: if you asked the dealer to replace tires, did they use the correct profile? Might they have used say 65 profile when correct would be 60 profile?

This is what I would look at.

Joe

[SoCal LS-L]

Agreed, since its a manual transmission theres nothing that would change the engine speed at ANY rolling speed besides tire diameter, since you havent swapped out any differential or transmission gearing.

As for the MPG drop...... how are you averaging your MPG? Is it per tank? Or going by gallons used/miles driven since your belt changed? This makes a difference in math, as if you are using the latter, you are also factoring in the computers relearn duration (IF of course you disconnected the battery while doing your timing belt) and mileage at that time would suffer. It is also possible your computer relearned a different optimal burn this time around with older O2's, injector spray patterns, etc. and adjusted your A/F mixture differently.

Thats all I can think of for now.....