Piston Slap: Bogging, Tripminding, Economizing

Joel writes:

I have a manual transmission car and am traveling on the highway at a speed that has my RPMs low for mpg. Now say I come to a hill, and my car can hold its speed and rpms, but I have to floor it or nearly floor it to keep my speed. I could downshift, where my engine rpms would go up, and I wouldn’t have to give as much gas through the pedal. My question is, is downshifting using more gas than keeping it in the same gear?

Now, I do realize that this is pretty much an OCD kind of question, but then again, if the answer is rather large, it would be good to know. And, if the answer is negligible, that would be cool too. So, if you know the answer, or is the kind of thing to put on the website, I’d love to know the answer. Thanks in advance.

FYI: the cars in question are a 2001 Honda Civic and a 1985 Volvo 245, and a 2007 Honda Fit, and all are sticks. That said, I even converted the wife to driving a stick, the Fit is hers!

Sajeev Answers:

I answered this question once, well before there was a Truth About Cars. Years ago I installed a somewhat desirable, vacuum fluorescent display’d Ford “Tripminder” computer in lieu of the bean-counted clock in my 1988 Mercury Cougar. Proud of my accomplishment, I religiously monitored my instantaneous fuel economy to ultimately answer your question. And the answer was most inconclusive.

You have three factors in play: gearbox ratios, engine torque, and long term durability. Let’s stick with that final point, because going uphill at low rpms with high throttle effort isn’t a good idea for the engine. The extra load can have a negative impact on any number of bearings, shafts, etc. And while you might not notice any problems today…

Then there’s engine torque: if you have a gutsy V8 or turbo diesel at the helm, the motor is far less likely to “bog” at lower rpms when you hit the throttle. But most importantly, it’s game over when the motor bogs, downshift to save your engine all that unnecessary stress.

Finally there’s gearing: wide ratio or short ratio gearboxes in particular. When I did my fuel economy tests on my (wide ratio) four cog automatic-motivated Cougar, the rpm jump out of overdrive was quite significant. And mileage went down considerably compared to nursing the big V8 up a crest in top gear. After I made the switch to a six-speed stick, the rpm jump from 6-to-5 was far smaller, keeping the rpms low while avoiding the “bog” was no big deal.

Grain Of Salt Note: Houston is a relatively flat city, so my Tripminder’s calculations have little to no relevance in places with real terrain changes. No driving condition is the same, so I’d recommend this: avoid engine bog at all costs, downshifting accordingly with no regard to the trivial change it might have on fuel economy.

(Send your queries to mehta@ttac.com)