Freeway cruising speed RPM

zwh02 · Post by **zwh02** » 25 Dec 2019, 02:42

Just recently got a new set of tires on my 97 850 base sedan (automatic). I previously had 195/65/15 sized tires but I went with the stock size 195/60 for my new set.

Naturally, I knew I would likely see a slight increase in engine speed with the smaller ratio sized tire.

Now I am just a little under 3k rpm while cruising @ 65mph on the freeway, I find I'm anticipating it to shift into another final (higher) gear to lower the engine speed. I'm often sticking to the slow lane staying @ 60mph for the slower (quieter) engine speed. I also worry about the increased fuel consumption with the higher engine speed but this is stock configuration after all..

Is this typical gearing/rpm range for a stock tire(d) auto 850?

callahanoffroad · Post by **callahanoffroad** » 25 Dec 2019, 04:41

So I run 196/65/15's as well. The circumference difference is 3.62% which is basically nothing.

When you run the 65's if your speedo says "100 mph" you were actually doing 103.62mph.

When you run the 60's when your speedo says 100mph, you are actually doing 100mph.

So what's happening is that your speedometer is actually correct again.

As for the shifting point of an automatic that has to do with engine speed, throttle position, and some kind of unholy voodoo magic. I have a 5-speed so I'm not entirely familiar with the automatic transmission's. That being said though the car should not be driving any differently than it was on the 65's. The car never knew that you had outfitted different tires. So to the car 65mph with 65's is the same rpm and speed display with the 60's. You could track the actual vehicle speed with a gps based speedometer. But the car doesn't know and never did.

Now I'll also tell you that there is no power in these non-turbo motors under 3,000 rpms. If you're going over hills and such the car HAS to downshift and increase engine speed to keep the road speed.

Hope that answers your question.

Merry Christmas

befarrer · Post by **befarrer** » 26 Dec 2019, 21:24

I likely have different gearing, but I'm doing 2500RPM at 65MPH, but I have a 98 V70 LPT AWD. (not XC or R)