Road Trip Fuel Economy Reveals Believable MPG Gauge - 2013 Lexus GS 350 Long-Term Road Test

2013 Lexus GS 350: Road Trip Fuel Economy Reveals Believable MPG Gauge

March 20, 2013

2013 Lexus GS 350

My recent road trip to Austin, Texas and back revealed a few things about the fuel economy of the 2013 Lexus GS350.

First, the EPA highway rating of 28 mpg is totally achievable so long as you're not taking full advantage of the 75 and 80 mph speed limits found across Texas, New Mexico and Arizona. One tank we've already talked about came in at 30.0 mpg and the last 337-mile tank I burned across western Arizona and the breadth of California worked out to 28.6 mpg when I officially called it a trip and made my last top-off at the Shell station just around the corner from my house.

But most of the journey did take place on high-speed interstates at 80 mph, so several tanks occupied the 25-to-26 mpg space. The trip average was therefore 26.4 mpg over 2,929 miles of data. Not bad at all considering the pace.

These figures come from direct calculations using miles driven and gallons pumped at each fill up. We do this routinely because in-car mpg gauges can rarely be trusted not to overstate a car's performance. Conspiracy theorists and engineers familiar with the ways that carmakers respond to things like the JD Power Initial Quality Surveys suspect that some such displays routinely err on the high side (within the tolerance, of course) to make it appear to the customer that their car is doing better than it actually is.

A few months back we started recording a long-term car's in-car mpg gauge readout at every gas stop to track this issue, so I was able to compare my calculated results with the GS 350's on-board MPG computer data to see if it can be believed.

It can.

Our GS350 understated every single one of the eight tanks that made up this road trip. The difference wasn't large, and it varied between 0.1 and 1.8 mpg, but it always came in low. There wasn't a single exaggeration. My trip average was 26.4 mpg while the Lexus' on-board trip average was 25.5 mpg.

You could take issue with any such conclusion made after a single fill because of the tank-to-tank variation in the way different gas station nozzles click off. But eight straight tanks means it's a real thing.

Why believe the hand calculations at the fuel pump over an in-car meter? Gas station nozzles have flow meters that are regularly calibrated by Weights and Measures to prevent cheating. Anyone who's seen Office Space knows that small differences can add up to real money. In-car MPG meters are not supported by direct fuel flow measurement.

Fuel flow rate is estimated by multiplying the amount of fuel that SHOULD be dispensed by a given injector pulse at a given throttle opening and engine load by the number of engine revolutions. But these factors aren't tracked and summed each-by-each. Occasional readings are taken and assumptions are made about what happened in between.

You'd think cars would know exactly how much fuel moved through their pipes over a given number of miles, but they generally don't. The assumption aspect behind in-car MPG meters leaves room for creative use of the tolerances involved. By erring consistently on the conservative side the Lexus GS 350 doesn't seem bent on over-hyping its own performance. I appreciate that.

Dan Edmunds, Director of Vehicle Testing @ 13,176 miles

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2013 Lexus GS 350 Research