DANMAN
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| posted on 2/5/26 at 11:32 AM |
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BEC Changing from 3.54 to 3.3 diff ratio
Hi everyone,
I have a quick question about a diff ratio change.
A while ago I changed the engine in my car from a CBR919 to a CBR1000 which has made a big difference power wise but the gearing in the new engine
means higher cruise rpm and a 1st to 2nd gear change after about 20m, 1st gear is almost a waste of time outside of taking off from a standstill.
I’m thinking about changing from the current 3.54 to a Retro Ford 3.3 instead. I know this will affect acceleration so thought I’d ask if anyone else
had done this and what difference did they experience between the two?
I will also be changing from 175/60/R13 to 205/60/R13 on the rear as well.
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Ugg10
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| posted on 2/5/26 at 06:09 PM |
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I would compare your diff and tyres size against the original bike sizes (quick google suggests final sprocket ratio of 2.6:1 and rear tyre
190/50/17).
You can also use the online calculator on subarugears to see what each gear tops out at, remembering to add in the primary drive (c. 1.6-1.7:1 iirc).
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1968 Ford Anglia 105e, 1.7 Zetec SE, Mk2 Escort Workd Cup front end, 5 link rear
Build Blog - http://Anglia1968.weebly.com
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adithorp
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| posted on 4/5/26 at 12:13 PM |
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3.3 will be fine with that motor. I run a 3.21 on my 06 R1 and 205/60/13. Its a touch high off the line but hat you loose in that you gain in fewer
gearchanges (I can hit 70 in first). I think the R1 final drive is higher than the CBR1000 (from memory). I get approx 100rpm per mph in 6th gear, so
70mph is roughly 7k rpm. I'm assuming yours I a 7 so a bit less aero than mine and you'd never push 6th to the rev limit with more than a
3.3.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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roadrunner
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| posted on 5/5/26 at 06:05 AM |
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I run a 3:14 Ford diff with a ZX10r.
It just hits 60mph in first and when in 6th gear every 10mph is at 1000rpm.
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DANMAN
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| posted on 7/5/26 at 12:56 PM |
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Thanks for the advice guys, it is much appreciated.
My gear ratio in 6th is 1.160 so not sure how that measures up compared to the R1 and ZX10.
I’m a bit worried that my acceleration will be considerably blunted by changing to the 3.3 and I wont be happy after the change but I guess it will be
made up for with a longer pull in each gear before changing.
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adithorp
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| posted on 7/5/26 at 01:35 PM |
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But you said currently "1st gear is almost a waste of time outside of taking off from a standstill." so I'm pretty sure it won't
be an issue.
The diff' will be 6.8% higher and the tyre change will add another 6.6%. If you're still not confident it'll be ok, I'd change the
tyres (easiest) and see how that feels as a first step and do the diff' once you're convinced.
[Edited on 7/5/26 by adithorp]
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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