4000rpm in 6th what speed to you get?
 

Before I had my engine replaced,
with my winter tires (17's) at 4000rpm in 6th I was at a speed saying
134-137km/hr
With my stock 18's I was
130km/hr dead on

Got my new motor in today and Im at
120km/hr dead on

WHY???? That dosnt sound correct to me?
I understand the diff. from 17" to 18" rims
but just changing the engine???

Let the new engine break in first

Are you an AUTO?
If you're a manual, breaking in is irrelevant to rpm/road speed ratio. That is fixed as determined by transmission.

My car is standard

Wheel size would not likely have too big of a difference because smaller rims typically run thicker tires. ~135 is proper, thats about 80mph. This is what you should be at, perhaps you looked at it wrong? Shot in the sky but check your invoice and see exactly what they did. Then again, I can't see anything changing that factor, its all in the gearing nothing motor.

Well that's why I asked the question because to me it does not make sence

My 17's to 18's makes sence because both sets are 45's so there the 17s are a bit smaller.

I document everything about my car in a booklet I keep, So I know exactly what rpm's I was running at, mainly because I use this pretty much on highway only.

I'll let the engine break in, but I just dont see it making a difference, I was thinking there might be something different about it or something, but shouldnt it only have to do with the gears of the transmission?

My car is running so far really really nice, its so much smoother just seems a little higher rpm to maintain the same speed. Thought it was interesting

I know 3750 rpm in 6th is 74 mph so i'd say about 77-79 for 4k rpms.

yeah 4k is 79-80 mph not sure the conversion to kph

Would a sensor foul-up cause this problem? Perhaps the readout is wrong. I'm not familiar with the rx8 wiring so I can't say for sure. Maybe try to find out how fast you are actually going, have a friend drive alongside you or something

Also, tires will change diameters although you shouldn't be seeing anywhere near that kind of dispairity.

the reading on your speedo is not influenced by tyre size - the ECU is programed for the stock tyre size .
Actual speed at a set rpm will differ with different tyre sizes but not the speedo readout ......

could be something up with the rpm pickup .....