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.

 

I know everyones comments and everything was 2017 but i got a 1995 jagaur xj6 4.0 4d sedan
i have been driving it for a while now (2-3 weeks) traded it. Guy said tranny problems not shifting.
did trans fluid changed and filter.
But when im going 50 mph rpms are at 25 i get to 60-70 mph rpms are 35 i get up yo 80-90 rpms are on 40
recently replaced ecu and problem wasnt solved.
anyone might have an idea?

 

On the inline 6 / 4.0 liter you will have the ZF4HP24 transmission

The connector on the body of the transmission ( left side without jacking ) can be removed and put a meter on the internal shift solenoids

All the shift solenoids should read the same , 30 ohms , MV1 and MV2 , might as well check MV3 and MV4

When replacing a solenoid might as well chang all while there

The solenoids will most likely not fail until the transmission is fully warmed up

The trick to the connector is you must first move the connector aft 1 / 8 inch and only then twist it

You can use large tie wraps on the connector barrel to grip it

 

 

This connector can get dirty / oiled effecting the speed sensor inside the transmission body , the speed sensor by way of the transmission ECU talk with the engine ECU as they dance together though shifting / engine regulation

Usually , a fault around idle at a stop light or shifting gears out of park from reading others