Sudden Drop In Gas Milage

[By_Design]

Hey AZ friends, I recently had a bad driveshaft support bearing, my mechanic replaced the entire driveshaft with a 3rd party unit. Immediately following, my gas milage dropped, used to get ~400mi to a tank, I now get ~300mi.

I contacted my trusty mechanic and he said it has nothing to do with it, but I can’t help but think it, or something he did was the cause, with the sudden change I saw after picking it up. Does anyone know if this could be the case? Like the shaft is bad or was over-tightened (I have no idea) or something? Any other thoughts on what could cause a sudden drop like that? Thanks in advance for any thoughts/answers, sorry for the wordy post,

By_Design

[Turbo510]

Sounds like something coincidental happened. The mixture might have gone richer for instance; check the coolant temperature as reported by the CTS to the ECU to see if it seems reasonable, especially when fully warmed up.

[EuroxS4]

Winter fuel is what happened possibly.

[Kevin C]

A wag at the math. If you are using 25% more fuel and you. Assume that at cruse on average you are doing 30 mph, 400 miles ~ that's 13 hours of drive time. 13 gallons per fill-up a wag is one gallon per hour. That's about 120000 BTU per hour. If your fuel economy dropped 25% then something increased your consumption by that amount. That puts the waste heat at about 30,000 BTU's. Since you only get about 40% if the gasoline energy converted to output power, the parasitic drag would need to be 40% of the waste heat (with cold start and part throttle 20 to 30% may be more realistic). For a WAG lets say 35%. So something has to waste 35% of 30,000 BTU per hour as heat. That's abut 2600 watts of heat. As a unit of measure figure about two 120V portable heats worth of energy being dissipated by the driveshaft friction. For that to happen the driveshaft is going to get quite hot. Like burn the grease out hot.

Another way to look at the losses is your drop in economy is more than what you would get from running the AC. A typical AC system draw 2 to 5 hp... Yea that's a lot of heat. While it possible that there is more drag I don't think it can account for that much of a change.

[DPDISXR4Ti]

First, we need actual MPG data (from several re-fills), rather than approximate "miles per tankful". As already discussed, there's lots of moving parts to fuel efficiency. I would agree that it's highly unlikely the driveshaft change directly caused a problem. It would be melted already if it was causing a 25% drop in fuel mileage. :)

[Puddin Tane]

There's a way you could investigate your suspicion, assuming you have a way to rapidly raise the car, e.g. ramps: take it out for a good hard spin and when you get back, back right onto the ramps and leap in under there and feel the driveshaft close to the support bearing, or use a non-contact IR thermometer to take its temperature.

As for the possibility of a bearing problem causing so much parasitic loss, this past summer I rehabbed my front drive axles because the boot was blown on the right inner and that corner was making classic bad bearing noise. Figured while I'm in there I may as well do the wheel bearings too, just in case. Once and done. No known issues with those, though. Before this I was getting 28 MPG; after the bearings wore in, 32 MPG. I'm surprised, because the old wheel bearings weren't misbehaving AT ALL. No slop, no roughness, the car would roll on the slightest gradient (but now it rolls so easy it's ridiculous). The needle bearings inside that bad inner joint still rolled smoothly and freely, so I don't see how that could be the MPG thief. Maybe that right wheel bearing was bad.

So IMO (FWIW) your whole drivetrain is a worthy suspect, depending on your mileage. Mine was at 165K. Those wheel bearings were original. A Quattro drivetrain is so friggin complicated. A little drag here, a little there... It could add up to a lot.