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.

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).

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.

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.

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.

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]