Lack of power on M13A engine

For about 4-5 months, I noticed that the engine doesn't have much power. (M13A)

I am not sure if it came after I switched my street tires (13.5kg each, INSA A/T) with a simex profile (about 18kg/each). But still, it shouldn't be much difference.

The consumption is about 17-19L/100km (HUGE, it's like I'm driving a hummer)
The acceleration is very very slow. Only in the 1st gear I'm feeling some power, all others seem very sluggish.
Beside the ABS sensor, there's no error on the diagnosis.

An important thing: even when going down on the road (steep roads), the car slows down - even if it's on neutral !!! (and this is not an engine issue anymore)

What's been changed/investigated until now:
- changed the spark plugs (from old NGK to brand new Denso)
- the engine compression is ok for a 18y old car
- new camshaft sensor
- new crankshaft sensor (very soon)

What should I investigate next (no particular order):
- catalytic converter
- ignition coil
- ignition wires
- lambda sensor

Thank you!

Welcome to the forum. It sounds like you are on the right track looking at the catalytic converter as they are prone to disintegrating and or becoming blocked. It would certainty explain the lack of power and heavy fuel usage. When you changed the tyres did you buy remoulded ones as they tend to be significantly larger for the same size as a new tyre which has a negative effect on the gearing. Also going from a road pattern to even a moderate mud pattern can cost upwards of 5 mpg and the more extreme the pattern is the worse the consumption gets.

Thank you Lambert for the warm welcome.

I was expecting a lower mpg, but a friend has 12-13L/100km (22mpg), but I'm at 15mpg (19/100).

Initially the tyres were 205/70/R15 insa turbo ranger A/T (13.5kg) and now I'm at 195/80/R15 insa turbo special track (17kg/each).

Both of them are remoulded tyres.
So, the new tyres have a lower footprint compared to the old ones.

I'll take a day to make a test with my friend's tyres, just swap them, but I really think it's not the right direction to go.

Is there a way to check the catalytic converter without going to a mechanic?

An important thing: even when going down on the road (steep roads), the car slows down - even if it's on neutral !!! (and this is not an engine issue anymore)

This sounds as though you have a seized brake caliper, check for heat but DO NOT TOUCH the brake disc after a run.

Hello Ipop and a BigJimny welcome from the Cotswold Hills!

Sounds like your brakes are binding if you are coasting downhill in neutral and finding you are slowing down,
Also is there oil in the diff? Does the diff' or any of the wheels feel excessively hot? Compare the temperatures of each wheel but don't burn your hand in the process

I missed the not being able to freely roll bit so yeah check the brakes for stuck pistons and caliper slides. Also the difference between a 205,70 and a 195,80 isn't just the width of the tread but the overall height is going to be noticeably more on the narrower tyre especially as they are remoulded. As for checking the catalytic converter if you are able to remove it you should be able to hear if the contents rattle and if there are any particles that can shake out but an after market cat isn't massively expensive compared to living with 15 mpg!