Latest Reader Question (Oct. 22, 2017)

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Here is the latest reader Q – along with my reply!

Budd asks: After not owning a car for more than 20 years (live in NYC – of all places), we finally leased a vanilla Jetta TSI last February (why is a long story). Anyway, I have been noticing a big discrepancy between highway MPG (44!) and city MPG (low 20s). After a fill-up in central NJ this past weekend, I was playing with the cockpit controls and found that not only the Range display existed, but also AVG Consumption (MPG) and Consumption (instantaneous MPG). I have also been light-of-foot on the gas and brake, I quickly saw readings of 80-180 MPG (!!) on level and downhill highway @ 65mph with 40-50 on uphills, with it dropping to the 30s when accelerating. Congested highway or local streets saw single digits when accelerating from a stop – and this seems to be what kills the “average” MPG and “range.”

Can this be for real, or are the gauges lying?  Can an engine be tuned like this? The Jetta does seem parsimonious with the gas on the highway. Otherwise, I like the car (and the freedom of having it!) – it feels solid and thus far almost perfect.

My reply: All modern cars have transmissions with deep overdrive gearing; some have multiple overdrive gears. The idea being to reduce engine speed at highway speeds, which – along with aerodynamic bodywork and low-friction/low-rolling resistance components – reduces fuel consumption at steady-state highway speeds. Look at your tachometer with the vehicle cruising in top gear at about 65 MPH. It is probably reading just under 2,000 RPM – a little more than a fast idle.

It does not take much power to maintain highway speed; but it does take power (and uses energy) to accelerate  several thousand pounds to speed.  This si why your car uses so much more  fuel in “city” driving, which usually involves a lot of stop-and-go driving. No matter how light you are on the gas, acceleration is expensive, as far as energy consumption.

On the gauges: The “80-100” MPG figures are grossly misleading.  In a sense. The instruments in your VW are modern, electronic versions of the vacuum gauges that cars once had, back in the ’60s and ’70s. They, too, would read wildly optimistically when you backed off the gas because engine vacuum was suddenly very high. The modern, electronic versions of these gauges work on the same principle. But, they are also more sophisticated and calculate usage based on real-time data (how you are driving) and adjust their mileage and range estimates based on this.

While the “80-100” MPG thing is ridiculous, the range estimates and average MPG figs these systems give are usually pretty accurate.

. . .

Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!

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1 COMMENT

  1. You never hear about the consumption of more fuel as a vehicle starts up. And the way most urban “planning” is done today only makes it worse, as they make cars stop or go very slow far more often. If they were actually interested in saving fuel, they would make it so cars wouldn’t have to stop so unnecessarily.

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