What's happening to my top speed? (x-post from main forum)

  • Thread starter JakeC7R
  • 5 comments
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Texas
I don't really tune myself. I just take other tunes posted here and tweak them to the PP requirements that I want. In the process of doing this, transmission setups don't always translate to having more or less power than was initially intended.

Because of this, I've been having some issues with the top speed on some of my cars. With default gears, the car will typically redline and hold its top speed over a long distance (such as the Mulsane Straight at Le Mans). However once changes get made, I've been having an issue with cars redlineing, hitting top speed, and then suddenly losing 5-7 MPH before accelerating again to redline. This repeats until I let off the gas for a corner.

So why is this happening? Is it because the top gear is unreachable for the car at certain final gears?

And is there something I can change to avoid this issue or would it be better to just set up the transmission from scratch for whatever I need rather than adjusting the final gear of existing tunes?
 
Usually, changing power parts will change redline, lowering the final drive gear will fix this issue. Most tunes are set for a specific track or tracks, going to a shorter track may need more accel and less top speed, rasing final gear fixes that, lower final gear more top speed, slower acceleration. Even if you build trans from scratch.
 
So is it best to just adjust the final gear to redline at the very end of a given track's longest straight, before the braking zone?

Thanks for the reply. Love your tunes.
 
So is it best to just adjust the final gear to redline at the very end of a given track's longest straight, before the braking zone?

Thanks for the reply. Love your tunes.
Yes, if you look at Motor City Hami's tunes, he always says "adjust here for longest straight" next to the final gear setting. I sometimes do also but usually forget that part. Adjust for the longest straight or just a tick more, some cars do better with power before redline......C3 Stingray, Shelby GT350, 'Cuda are all examples where power drops before redline. You can see this in the power/torque graph on the left of the settings page.
 
Some general building guidelines include ensuring that the transmission is the last item you install, after all power parts have been added (inc. any power limiting). Any changes to power output will result in a change to gear ratios. A few BHP one way or the other will likely not affect ratios, but much more than that will.

Provided you can get the posted ratios to match your car, then all you need to do to change the top speed is to adjust the Final Drive setting. If you require a higher top speed, lower Final Drive, if you require a lower top speed, then increase Final Drive.

Unfortunately, we're missing one of the settings we used to have in GT5 that would give you an approximate Top Speed when you moved the Final Drive slider. This means that you will have to take the car to whichever track you desire and actually drive the car to see what top speed your chosen Final Drive will achieve. All very tedious...

{Cy}
 
Any changes to power output will result in a change to gear ratios. A few BHP one way or the other will likely not affect ratios, but much more than that will.
When you install/uninstall parts that alter the rev range you get this result.
The actual range of the individual gears are adjusted to "fit" the new rev range. Gran Turismo has been like this since I can remember.

Parts that affect rev range: Engine stages, ECU, exhaust upgrades (not manifold), and catalytic converter.
 

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