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Gear ratios & RPMs

So, I’m looking at new cars and comparing their RPMs at a given speed when the car is in its final, overdrive, gear. Things don’t seem to quite add up. My current Saturn SW2 has a fourth gear (auto, overdrive) ratio of 0.72, tire circumference 76.02" and at 60 mph spins at 2500 RPMs. I test drove an 09 Focus with manual transmission, 5th gear ratio of 0.77 and a tire circumference of 76". At 60 mph the Focus spins at 2300 RPMs, or just a hair below. This doesn’t add up. Why the difference?



An extended, philosophical question: I’ve always had the impression that a taller final gear ratio was a good thing, i.e. the engine spins more slowly at high speeds, better mileage, fewer revolutions over the life of the car, etc. Should this be a consideration when shopping for a new car, i.e. how fast the engine spins at highways speeds?



Adding to my confusion, I test drove a Honda Fit. The Fit has an auto 5th gear ratio of 0.55 (quite tall) and with tires with a 75.4" circumference the engine spins just a hair above 2000 RPMs at 60 mph. Should I lean towards the Honda? But, the Honda is rated at about the same (or slightly lower) mileage than the Focus despite having the taller 5th gear. Even more baffling is the fact that the manual Fit’s 5th gear ratio is NOT 0.55 as would be expected but 0.72. Honda claims the auto gets better mileage but Consumer Reports claims the manual does, despite having a shorter 5th gear.



What gives?

You’re leaving one thing out - the axle (differential) ratio. Three things are involved: transmission ratio, axle ratio, and tire circumference. I wouldn’t base a purchase on this, if you like the car and the fuel economy, that is. Mileage is a complicated result of a number of variables, and the different tests between the EPA and CR will give different results.

Ditto to what Texases said. You’re missing a gear.

I also agree that this means nothing. Power curves, wind resistance, how efficiently the gear ratios and the power curves are matched up, power train resistance, vehicle weight, lots of other stuff goes into gas mileage.

Testing protocols can also vary. As can atmospheric pressuer, temperature, and particle count during testing. And, of course there’s the test drivers…or test equipment (EPA uses lab equipment, CR uses test tracks).

Buy what you like. Look up a few data sources, get a feel for what to expect, and then enjoy your ride.

What the others said, plus:

Tachometer needles are not precision instruments.

The engineers have worked for weeks and months on this stuff, using resources unavailable to anyone else. Don’t try to second guess them, you will lose.

Thanks for the good advice and comments!

well, this discussion is old now but I want to express my opinion. I think sometimes the final ratio is too high. I bought a '94 Dodge pickup new and at 1750 rpm it would do 70 mph, but was short on power. I have a 96 Dodge van with the same engine and trans and final drive but has smaller tires and it has more power in overdrive.

Don’t base your buying on gearing numbers.

I totally agree.

I would also add, just encase I missed it, speedometers are seldom very accurate either.

Gear ratios issues only make sense if they offer more than one with the same exact engine and car. I doubt if you will see this anywhere other than on a truck, or a car with a towing package.