Is it just me or do most drivers notice how high the rpms are at cruising speed on a xj6? My lady friend has a Chevy Impala and her rpms at the same speed are about 400-500 less than my xj6. Her car also has a 4 speed auto.

I was wondering if the converter lock up was working properly. What kind of rpms should I be seeing say at 50 or 60mph?

Is the rear end ratio responsible for the higher rpms?

 

Hi

At that speed my 4.0 sovereign 1995 sits at just over 2000rpm

 

MPH = (RPM X Tire OD)/(FDR X 336)

Where FDR = Final Drive Ratio and accounts for Axle ratio as well as what gear your transmission is in (including overdrive)
Tire OD is in inches
336 is a constant that accounts for our friend "Pi" from the circumference equation, as well as the factors necessary to move from inches/min to Miles/Hour.

For my '95, just a tad over 2500 @74mph: https://www.jaguarforums.com/forum/o...81/#post955557

What's going on, here? Engine RPM divided by all applied gearing gives you tire revolutions per minute. So back in the good ol' days, before automatic overdrive transmissions, this would devolve to rear axle ratio in "high" gear or "drive."
Tire circumference (Pi * OD) gives you the distance the car travels in one tire revolution (Assuming no spinning!)
By now, we know how many inches the car is traveling per minute and the rest is just unit conversion to something more readily recognized, like MPH.
For example, 26" tire, 3.54 rear axle, 70% Overdrive and 2500 rpm yields (2500 X 26) / (3.54 X 0.7 X 336) = 78.1 mph

 

At 50mph my car runs about 1850rpm.

 

Many of the US cars of the 80's and 90's were desperatly chasing better fuel economy figures, so put some very tall gears in them. As an example, a friend of mine has an 85 Ford LTD station wagon with the 5.8. It has a 2.73 rear axle and a 0.67 OD. 70 mph is about 1650 rpm and it's a dog to drive. The XJ6 has a 3.58 rear axle as a comparison.