Effect of tyre sizes on final drive, in gear speeds and performance?

[JB21]

After my money shift at Oulton, I'm looking for ways to make sure it doesn't happen again. I've looked at the TRD shifter, custom shifters even a different gearbox. However to easiest way would be to stay out of 5th gear as this has always been the weak point on my roadster. 4th to 5th and 5th to 4th at high RPM's needs high concentration as the gate with these 2 gears is very temperamental, and sometimes jams.

I'm now looking to replace the rear 15" wheels with 16"s. Currently running 225/45/15 AR1's or very similar sized slicks and this gives me a 4th gear top speed of 104mph. At Oulton my top speed was just 108mph in 5th gear, so all I need is an extra 4mph in 4th.

If my calculations are correct I would need to change from a 225/45/15 to just a 225/45/16, this would give a 4.4% increase in Revs/Mile which should equate to 108mph in 4th gear, does this sound correct?

Dont think this change will have much of an effect on the final drive ratio and ultimately performance given the OE FL size rear was 215/45/16, which is just a 1.5% increase on Revs/Mile using a 225/45/16.

Thoughts?

[Dev]

I haven't checked your math however the profile of the tire is tied to the percentage of the width so the wider tire with the same profile being 45 would be taller and therefore have a different percentage for revs per mile. How much more I do not know but the calculator would tell you.

[JB21]

I haven't checked your math however the profile of the tire is tied to the percentage of the width so the wider tire with the same profile being 45 would be taller and therefore have a different percentage for revs per mile. How much more I do not know but the calculator would tell you.

[Dev]

Dont think this change will have much of an effect on the final drive ratio and ultimately performance given the OE FL size rear was 215/45/16, which is just a 1.5% increase on Revs/Mile using a 225/45/16.

Thoughts?

 I was commenting on this part.  If we know the difference between the PFL tire size vs FL percentage then I assume there is a difference there that might add up. 

[JB21]

Dont think this change will have much of an effect on the final drive ratio and ultimately performance given the OE FL size rear was 215/45/16, which is just a 1.5% increase on Revs/Mile using a 225/45/16.

Thoughts?

 I was commenting on this part.  If we know the difference between the PFL tire size vs FL percentage then I assume there is a difference there that might add up. 

2.4% differnece PFL to FL, 874 v 892 REV/MILE

[Dev]

Better yet the difference between  225/45/15 vs 215/45/16 and see what you get.

[JB21]

Better yet the difference between  225/45/15 vs 215/45/16 and see what you get.

2.8%

[Dev]

Better yet the difference between  225/45/15 vs 215/45/16 and see what you get.

2.8%

 Im getting 4.3 if we add the difference between the 2 16" sizes which is 1.5.

Close enough.

[Roj]

I've looked at the TRD shifter, custom shifters...

Considered this?

[JB21]

I've looked at the TRD shifter, custom shifters...

Considered this?

Indeed but whats the total cost delivered with duties at VAT, near a grand?

[Roj]

I've looked at the TRD shifter, custom shifters...

Considered this?

Indeed but whats the total cost delivered with duties at VAT, near a grand?

Around £700.

TRD one is about the same, isn't adjustable and is pretty much unobtainable. Compared to adjustable shifters for other cars, like CAE, the RZR's a good bit cheaper.

[tatieu]

If my calculations are correct I would need to change from a 225/45/15 to just a 225/45/16, this would give a 4.4% increase in Revs/Mile which should equate to 108mph in 4th gear, does this sound correct?

Dont think this change will have much of an effect on the final drive ratio and ultimately performance given the OE FL size rear was 215/45/16, which is just a 1.5% increase on Revs/Mile using a 225/45/16.

Hi, I had 205/45/16 on the rear with AD08R and this year, I change for 225/45/16 with R888R. The increasing of the size increase the top speed but it reduce the acceleration.
With this setup, I loose 0.9s compare to last year.
For your request to increase the speed, is it the good choice, but you can loose a little acceleration.
The other possibilities is to change the tyres to 225/50/15 and you can keep your wheels.
For me, the best setup is yours, with 225/45/15 but it's depends of each use.

[JB21]

If my calculations are correct I would need to change from a 225/45/15 to just a 225/45/16, this would give a 4.4% increase in Revs/Mile which should equate to 108mph in 4th gear, does this sound correct?

Dont think this change will have much of an effect on the final drive ratio and ultimately performance given the OE FL size rear was 215/45/16, which is just a 1.5% increase on Revs/Mile using a 225/45/16.

Hi, I had 205/45/16 on the rear with AD08R and this year, I change for 225/45/16 with R888R. The increasing of the size increase the top speed but it reduce the acceleration.
With this setup, I loose 0.9s compare to last year.
For your request to increase the speed, is it the good choice, but you can loose a little acceleration.
The other possibilities is to change the tyres to 225/50/15 and you can keep your wheels.
For me, the best setup is yours, with 225/45/15 but it's depends of each use.

Thanks for the reply. I have since realized that a 225/50/15 would be my best option to increase in gear speeds. What you say you lost 0.9s is this a lap of a track? If so which one?

[tatieu]

I think my gain or no gain is not clear.
Before my best laps in 1min00, now in 1min09s.
More larger, but in first, the diameter.