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1997 Chevy Blazer

I have a 1997 Chevy Blazer with 292,000+ miles on it and it still runs pretty good considering. My question is this - when driving down the highway, sometimes at 60 mph my tach runs around 1800 rpm’s and a reading of about 22 mpg. Driving under the same road conditions - grade, wind…- at other times at 60 mph the tach will register 2100 rpm @ 17 mpg. Anyone know what would cause the difference here?



Thanks!



Jeff

It takes more gas to develop enough power at that speed with a head wind, so the stuff goes through faster.

The only reason for the tach to register higher RPM with a head wind would have to be that the engine kicks into a lower gear, which would also explain the higher fuel consumption. I’m assuming you are using cruise control, which normally would make the engine downshift to maintain the same speed with a heavier load on the engine.

What if I’m going the same direction, speed, head wind and so on? All variables are the same but the difference in rpm’s & mpg’s.

Keep in mind that those MPG indicators sometimes aren’t all that accurate. A better way to do it is to fill the tank, keep track of how many miles you drive until the next fillup, then divide miles driven by gallons used.

Don’t put too much faith in the computerized doohickey.

Are you sure the lock-up torque converter was engaged in both directions?

Tester

That sounds like when the overdrive kicks in for lower rpm and better gas mileage vs the car remaining in 3rd to apply extra torque needed. No way I know to force it into overdrive, but the 3 on the shift keeps it in 3rd gear for situations where it may switch back and forth to overdrive and third too much possibly causing a problem, as the manual indicates in those situations where constant up and downshifting occurs it is preferred.

It sounds like this ol’ buggy is, at 60 mph, at the “cusp” of shifting into overdrive. Variations in things like temperature and barometric pressure could be enough to make the difference.

Remember that there is a relationship between the barometric pressure and the amount of oxygen the engine can breath in with each intake stroke. Drag racers constantly check atmospheric conditions at the track and adjust their vehicles accordingly. Yes, it makes the difference between losing and breaking the track record.

You’re asking about a small variation at a specific speed on a 12 year old vehicle with 292,000 miles on it. Small differences can reflect themselves as what you’re seeing.