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The gearing for my 02 st4s is to high for my liking. The stock gears were changed to 15/42 . At 80mph it revs at about 4200rpm. Is it possible to gear it so at 80mph the rpms would be at 5000rpms or 5500rpms? Thanks
 

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I'm sure you could shorten the gearing even more, but might again need a new chain. 15/42 is probably the most popular setup here. Consider also that your speedometer is probably reading fast so that 80 mph indicated at 4200 RPM is probably only around 72mph. If you run a actual 80 mph you might actually be close to the desired 5000 rpm.
 

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liftline said:
The gearing for my 02 st4s is to high for my liking. The stock gears were changed to 15/42 . At 80mph it revs at about 4200rpm. Is it possible to gear it so at 80mph the rpms would be at 5000rpms or 5500rpms? Thanks
If you have a 180/55 or a 170/60 sized rear tire, you will be going 71 mph (actual) at 4200 in 6th gear. If you want to spin faster, just shift to 5th! Then you will be spinning @ 5300 rpm's which is what you said you wanted to achieve in the first place.
 

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kentamcolin said:
Consider also that your speedometer is probably reading fast so that 80 mph indicated at 4200 RPM is probably only around 72mph. If you run a actual 80 mph you might actually be close to the desired 5000 rpm.
Ditto this. These speedos typically read about 10% high, although someone recently posted about an ecu recall which has led to a more accurate speedometer. I would check your speedo with a stopwatch or GPS before deciding on a sprocket change.

The largest rear sprocket I have heard of being used is Mark Turbo's 15/43T combination.
 

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Bill_Anderson said:
The largest rear sprocket I have heard of being used is Mark Turbo's 15/43T combination.
I've been running 15/43 for about 26,000 miles now and it's the perfect combination for me. I ran 15/42 before this.

With 15/43, I find that 85mph is 5,500rpm in 6th gear. Around town, I can actually use the gears more, which I like! I find the ST4s handles very smooth with 15/43. 15/42 was OK, but 15/43 is noticeably better to me.
 

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Turbo said:
With 15/43, I find that 85mph is 5,500rpm in 6th gear.
If your tire is the same diameter as OEM, 85mph (actual) will correspond to 5,137 rpm (assuming 15/43 sprockets and 6th gear). Knowing the diameter of the tire it's possible to calculate the rpm's at a given speed. The difference between 15/42 and 15/43 is actually quite minimal. For example, in 6th gear a 43 tooth sprocket and 5,137rpm's will get you to 85mph. Switching to a 42 tooth sprocket will only get you to 87 mph at the same rpm's, exactly two mph faster.

If you run a 170/60 then you are geared ever so slightly taller because the tire has a slightly larger circumference and 85mph will correspond to 5,130 rpms. Of course manufacturers have a certain amount of leeway when it comes to stamping a tire size on. I merely mentioned this to show that the differences between these two common tire sizes are relatively insignificant.

I've been pretty happy with the 15/42 combo but am considering switching to 15/41 next time to reduce rpm's slightly and let me get more use out of 5 gear on the open road and also the lower gears when on slower roads. Just a personal preference, everybody tends to find their own happy medium.
 

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Try the Gearing Commander

Mike said:
If your tire is the same diameter as OEM, 85mph (actual) will correspond to 5,137 rpm (assuming 15/43 sprockets and 6th gear). Knowing the diameter of the tire it's possible to calculate the rpm's at a given speed. The difference between 15/42 and 15/43 is actually quite minimal. For example, in 6th gear a 43 tooth sprocket and 5,137rpm's will get you to 85mph. Switching to a 42 tooth sprocket will only get you to 87 mph at the same rpm's, exactly two mph faster.

If you run a 170/60 then you are geared ever so slightly taller because the tire has a slightly larger circumference and 85mph will correspond to 5,130 rpms. Of course manufacturers have a certain amount of leeway when it comes to stamping a tire size on. I merely mentioned this to show that the differences between these two common tire sizes are relatively insignificant.

I've been pretty happy with the 15/42 combo but am considering switching to 15/41 next time to reduce rpm's slightly and let me get more use out of 5 gear on the open road and also the lower gears when on slower roads. Just a personal preference, everybody tends to find their own happy medium.
Hi,

Try the Gearing Commander.
It helps you calculate these things quite easily (speed at what RPM and RPM at what speed, effect of sprocket changes and effect on chain and sprocket distance)

Regards, Jean
 
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