Battery and fuel economy

[ylexot]

I figure this might be of interest to a few. It is a bit of a happy head scratcher for me.

My aged battery was sufficiently worn out in my that it failed a battery test at the dealer and became unreliable to start the car.

Replaced the battery with an Autozone special a couple weeks ago. Then the next 3 tanks of gas averaged 17.6 mpg. I can hardly believe it. The best I ever got was 16 driving down hill from the continental divide with a tail wind. For the last several years, typically I get between 11 and 15mpg depending on driving conditions.

Only theories I have is less load on the alternator with a better battery and/or tighter contacts on the cables. (didn't replace the terminals). Anyone else seen results like this? Better explanations?

 

[duggy]

The alternator doesn't take a measurable "load" in terms of gas mileage. It only spins slower or faster as the engine revs, so that theory is out.

I don't really have a good explanation for this, but how many tanks have you had through it and how are you calculating your MPG?

 

[re_guderian]

You reset the ECU when the battery got disconnected to swap. I would expect a return the your regularly scheduled abysmal MPG soon. Either that or you went off winter blend...

 

[ylexot]

The ECU reset makes some sense. It drives fine. No real noticeable change in power climbimg hills or passing a slow poke.

It would be fab if this furm economy stays.

I dont think it is winter mix as i have mileage data going back 2+ years.

Method for mpg calc: trip odometer ×1.1 (tire modification) / gallons pumped to full. I try to use the same pump, same station. A little vsriation from not getting back to the same fill level is to be expected, but i havent seen a 20% swing that couldnt be explained by something: dragging a trailer, all town driving, etc.

 

[bmw635]

The law of average will catch up to you once ECU is seasoned.

 

[Sandroad]

The alternator doesn't take a measurable "load" in terms of gas mileage. It only spins slower or faster as the engine revs, so that theory is out.

I don't really have a good explanation for this, but how many tanks have you had through it and how are you calculating your MPG?

I don't know if changes in vehicle electrical power needs are enough to have a measurable impact on mpg, but the load on the alternator and therefore on the engine vary with electrical demand, not just rpm.

 

[ylexot]

The electric load definitely puts a mechanical load on the generator/alt. Ain't nuthin' free. Im not sure how much of a drain ut puts on the system. Can't imagine a bad battery being 20% of the load to move the rig through traffic tho.

So if it is the ECU, why is it programmed to make crumby mileage. I can pretty much live with 17 mpg, but 14-15 stinks. Perhaps irrational, but 340mile range on a tank beats the pants off of 280.

 

[AMGAndrew]

Ecu adaptation codes all got reset when you changed the battery and it's in the process of relearning. Over the years it's adapted the MAF, timing, injectors, ect, to keep everything working in harmony and sometimes that means adding fuel trim and pulling timing because running a little rich air/fuel ratio is safe for the engine. I'd venture to to guess your engine is running a little leaner and a little meaner (lean is mean!) and that will return better fuel efficency, for now. A couple more tanks of fuel and you'll be right back to "normal".

And no, you'll probably not notice the "power increase". We're probably talking fairly small % of power and still propelling a 5,500lb 4wd bank vault.

Keep us posted as I'm genuinely curious how long adaptation will take. And trust me, I'm hoping we're all wrong and you keep banging out 17mpg because thats freaking awesome.

 

[Buu100]

I would be lucky to hit 12 around town.

 

[re_guderian]

10% is a generous compensation factor unless you're running 315's. With 295's, I'm around 6% on the GPS.

 

[ylexot]

Update: The ECU reset theory is winning, unfortunately. My last tank was down to 15.7. It looks like MPG is settling back down to the normal abysmal mileage. (I just got a '79 F350 dumper, so I think abysmal mileage is going to take on new meaning for me.)

Above @AMGAndrew commented that 'lean is mean', does this mean that running a lean mix is long term hard on the engine? I'm sure the geeks at Toyota know what they are doing, but I'm wondering that if the performance was acceptable to me with a leaner mix, is advisable, and then possible, to reprogram the fuel map to be leaner all the time? I drove an FJ62 for a long time, I'm ok with a less power.

Regarding the 10% bonus for bigger tires, 17.6mpg is probably generous. However, 10% correction is a linear factor so the 20% fuel efficiency gain is solid regardless of 10, 6, 4, 0% over rate. When I calculated the correction factor for tire size, I came up with 4% for 285s. However, I consistently get a 10% tire correction factor when using interstate odometer checks, checking every 10th mile. My dad and I took a trip this weekend and his GPS says 4% too, so I'm inclined to trust that. Yet, I can't explain the discrepancy between odometer checks and GPS.