My gas MPG is way down lately

[sublime187]

In the last couple weeks, and this coincides with a new tire change, my gas is down from about 18 to 16 over the last 3 or so fill ups. New fuel filter about 8k ago, oil change 1k ago, OEM air filter is a little dirty at about 8k but not terrible. My new tires are the exact same size as the old ones (225/55-17). Bg44k'd about 8k ago and chevron with techron cleaner a couple times since then. Nothing but 93 octane. Even before I changed out the fuel and air filter I was getting 18mpg or more. What gives? Winter gas? New tires? I drive the same as always, with more than half of driving on the highway.

[maxnix]

Could be the change in winter gas formulation. 10% ethanol will do that on its own. But you must use Tier 1 Premium, regardless.

If KS and injectors check out and there is no code, run Iso-HEET every tonk or two and BG44K or Redline SL1 before every oil change.

Also, the extra time to warm the engine to operating temperature for winter cold starts should not be discounted.
Modified by maxnix at 7:43 PM 11/28/2008

[sublime187]

Forgot to mention I run iso heet once a month at least. I just realized my tires are inflated to 35 psi instead of 38 psi as they were before. I guess I sort of answered my own question here.

[youngq45r]

Do you really think that 3psi would make that much of a differience?

[maxnix]

Probably not at that level, but everything is cumulative, so it contributed a few tenths.

[youngq45r]

how much of a change would the 4.08 make?

[goody90q45]

.......My new tires are the exact same size as the old ones (225/55-17)........

You went with the same size tire but was it the same manufacturer? Or a softer tire? ZR rated? Is there a mileage warranty difference (less on the new tires)? I also see a 1-2 mpg difference when we switch in CA from summer to winter gas.

[Q45tech]

4.083 results in 15.4% more rpm for a given speed compared to a 3.538.

Probably about 10% worse MPG depending on speeds and idle vs acceleration vs cruise duty cycle but it could be 15% depending if 100% highway.

[youngq45r]

wow. That's not as bad as i thought it would be. Nice. Sorry to thread jackthanks

[dusred]

Any other info about the Q? How many miles?

When the AC is on is there a gas smell?

It is possible that there is a small fuel leak under the Plenum. On my Q I waited a year before I fixed it. At first it was very minor but eventually got worse.

It's worth checking out.

[sublime187]

84k miles and no gas smell, ever. And I did change treadware ratings from 400 to 520 (i know, i know 520 sucks...) and the psi is 4 or 5 lower... + winter gas, i figured this adds up. But my last tank i got 15.7... Wow. I will say this, when I bought the car, it had a K&N filter in it and I was getting about 19 and this was last February. Maybe I'll go back to it, I mean it makes that cool sound and all. Hahah.

[maxnix]

The winter blend non-ethanol fuel we are getting seems to have less energy, plus longer warm ups.

You really can't tell with out a road mileage test (station to station on highway only). Even then, if everything is all right, it is more a measurement of fuel formulation's BTU.

Don't get the dropping the tire pressure unless you are drving across fields.

[qsiguy]

Just simply colder weather will reduce MPG. More dense air will require more fuel to maintain AFR's.

[Q45tech]

But density changes at 1% for 11F so 10% would need 110F air change.

[maxnix]

Just simply colder weather will reduce MPG. More dense air will require more fuel to maintain AFR's.

And denser air means more power per unit voume ingested.

[qship96]

Yes, the Q just begs for those cold winter sub-zero temperature highway floggings.....the engine feels so much happier and powerful compared to summer temps- just returned from a nice 50 mile highway drive to nowhere, just felt like driving. Ambient temp display was 30-32, speedometer said 80,90,100,105....ahhh what a way to burn cheap premium fuel!

[Q45tech]

ECU calculates in weight not volume...........remember no body measures WOT MPG. It is always idle to very mild flow MPG for most people their daily drive AVERAGE RPM is < 2000 rpm.............seconds at idle vs seconds at EACH rpm.

It would be very interesting to COUNT the exact total daily number of revolutions the engine made vs miles driven..........instead of using MPG.

A mile is surely not a mile on different cars.

Pounds of fuel per hour is a much more scietific term.

[sublime187]

TDon't get the dropping the tire pressure unless you are drving across fields.

I used to fill 'em to 40psi.. When I got my new tires (225/55-17 94V), they said OEM recommendation was 32psi, I told them to fill them to 35. Would you suggest otherwise?

p.s. - I usually drive alone or with 1 passenger up front. Also, it's obviously very cold out right now. Lastly, Michigan roads have many bumps and potholes..

[maxnix]

That's probably good.

Usually most bias pressure a little higher up front to equalize handling based on load, say 38/35 psi.

[Q45tech]

Based on static weight [55/44] the front tires should be ~20% higher than the rear psi................the only case where the rear lightens up is WOT out of apex of a curve. Assumming tires are all the same and you desire an equal slip angle from each.Start with 40/32 COLD and increment equally higher. Add 1.0 psi to rear for each 60 pounds in back seat or trunk............2.0 psi has already been added for a full tank of gasoline.

Equal psi will create [add] 20% understeer based on slip angles alone.Probably a few inches extra in steering wheel rotation ready depends on G load. The higher the G load the more inches of understeer will be required.

Remember the max psi is 51 so most people way underinflate for comfort not MPG or tire wear. When you panic brake the load shifts forward near, equals, or exceeds the weight limit of tire even when inflated to full 51 psi.So if you panic brake often raise the PSI or buy tires much more often due to over stress adding up.

[sublime187]

to update, on my most recent oil change, i had the tires inflated to 38 from 35, and a brand new air filter (not oem, but it looked and felt the exact same). since that change it appears i've gained back at least 2mpg. i'm guessing the oem air filter i had before killed my mpg after it got a little dirty. when i bought this car it had a k&n, which we all know doesnt hold a spec of dirt, possibly explaining why i was getting such great mpg.

[oldmako]

My car is used on the interstate about 95% of the time. I usually drive on the backside of the clock and I set the cruise control just shy of 80. I have consistently gotten 20 mpg for the last year. But in traffic without cruise control, it drops to 18-19 depending on conditions and speed. Toss in some around towning and it instantly drops to 17ish depending on how heavy my foot is. I also keep my tires a bit soft. If they were at full whack, I'd probably get another 1.5 mpg...whoopee!

The point being (as pointed out by the others), is that minor changes add up. Tire inflation and a heavy foot probably being the largest. I don't run stock rubber on mine so I had to take that into effect due to speedo/odo error.

I check my mileage each fill up as a maintenance check than anything else. If it were to fall WAY off that would be a red flag. But I can chew up a tank full in no time if I get the chance to let it rip. If careful, I can get right at 100 miles when the tank drops to 3/4. If doing the Parnelli, I might not even see 70 miles. Big diff.