Look, this might be hard to explain, I’ve had this game since launch and this issue i’ve been having with rpm’s (or more simply, acceleration), has only been happening on and off for a month or so.
Stay with me here, whats seems to be happening (more frequently it seems) is the rpm’s will not climb at a normal rate, ill start a race with a road car, in manual, and it will go fine until some random point when the rpm’s give out, i can see it on the tachometer dial too! it stutters on the lower gears, does not increase smoothly then on the higher gears it literally takes FOREVER to increase acceleration, even though im full throttle. For me, This happens with all cars (seems like it only happens in manual, but im not sure) except the Lotus F1 car (which is so good may I add) This became frightfully noticeable when playing in the Daytona track, driving Mercedes Black edition, Imagine fighting for second on the final lap only to have the rpm’s start to stutter and increase very sluggishly on one of the straights and being overtaken quickly, after the previous lap where i blasted through this part and the RPM’s were increasing normally, it’s just makes no sense unless this is some kind of result of the simulation damage mod which I use every time (but I dont hit things)
In short this is a game breaking bug (for me) while it seems to happen frequently it does not happen every time (which is confusing I know) but prevents me from enjoying Forza 6 when it does
Perhaps you are running out of fuel? There’s a little white line in the on screen tach - right below the gear number. That white line is your fuel gauge. Is it full (or close to it) when this problem occurs?
I’m only guessing here but have you tried dragging your final drive slider ( tuning / gears ) to ensure that 6th gear isn’t actually set beyond the car’s theoretical top speed because I have noticed that an awful lot of default setups seem to be a bit out - even more so when you upgrade car parts. This may account for 5th and 6th gear been a bit sluggish. Also, it’s sometimes a good strategy to tweak the individual timings to ensure acceleration doesn’t ‘deaden’ during shifting. I found for class d, if you tweak the timings to ensure the rpm’s are constantly above 6500 when shifting then acceleration is clean all the way to 6th gear. It sounds kinda complicated but it is actually very straightforward. I use the alps track (test drive) because i can set timings on the flat parts (smooth shifting) for standard tracks and then drag the ‘final drive’ left to find 144mph down the big hill to ensure my Celica doesn’t max out, effectively creating 2 baseline gear tunes in one go.
But like i said, only guessing.
If your not crashing or hitting anything , It sounds to me like you might be downshifting to aggressively and damaging your engine that way. A damaged engine is the only thing that causes the rpms to stutter that I’m aware of, turn simulation damage off and see how you go