Citroen XM Forum

At least my V6 auto revs few hundred rpm less than my brothers V6 manual in highway speeds and they both are early examples.

I have an XM with ES9 and manual transmission and a Xantia with ES9 and automatic transmission. At 130 km/h the XM engine is turning about 200 RPM more than the Xantia.

I think that all of the gears on the XM are too short, and it would be improved by installing the intermediate gear set from a diesel C5. An easier change would be to just swap the 5th gear cogs which can be done without removing the transmission.

All the Xm automatics run a slightly longer top gear to compensate for the additional losses: approximately 1km/1000 revs.
Overall, the manual will give better performance and fuel economy, but the autos can be sometimes better when on long distance cruising in top gear with the torque converter locked up.

Xm were designed in the days when the gear ratios were designed to keep the engine in it's power band, with top speed being at peak power in top gear.