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Discussion Starter #1
Ever since I got my 1996 9000 I've felt it's a bit too high geared for general driving. I know it wafts along at 70 mph doing about 275 rpm (sic) but it's a bit awkward around the town. 3rd gear is too high, 2nd gear is a bit low
(for British town speed limits).

Well as I'm changing my wheels up to 16" 16" tyres I wondered if I could combine my new tyre purchase with a complete new set of gearbox ratios.

So instead of running the 225/50*16 I was going to use, I could use :
225/45 to give me a 3.6% reduction or
225/40 for a 7% reduction in ratios.

Given the small amount of time I spend driving long distances at motorway speeds it shouldn't make much differnce to the cars fuel economy.

I know it will throw up an error on the speedo but could that be corrected by attaching another speedo face plate re-aligned to suit or marking the tacho bezel to suit. ie 30mph in 3rd & 60/70 in 5th.

Any thoughts.
 

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So instead of running the 225/50*16 I was going to use, I could use :
225/45 to give me a 3.6% reduction or
225/40 for a 7% reduction in ratios.[/b]
By rights for these lower profile tyres you would need a wider alloy, 7j or possibly 7.5J

225/50 is the widest/lowest profile you can safely fit to a 6.5J rim.

Also, with the lower profiles you might get easier alloy damage from kerbs and pot holes.

Andrew

PS, 70mph at 275 rpm!!!

Assuming you meant 2750, then that's low! 70mph at 2350rpm in the Aero.
 

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Not sure how you would compensate the speedo. That's potentially quite an error and the 9000 speedos were only calibrated for two different tyre diameters, I think - the equivalent of 205/55/16 for CS and later CDs or 205/50/16 for earlier cars. I don't know whether the speedos are interchangeable between instrument panels as the instrument panel ties in with the engine ECU and the SCC/clock, etc.

As for the ratios, I have tried changing the ratios "properly" on my '96 Aero by changing the final drive. Standard ratio is 3.61:1 (yours should be the same) First I tried 4.05:1, which is from the 2.0LPT cars and is 10% lower. This was really too low. The car was easy to drive round town but much of the usefulness of the massive torque band was lost. I now have a 3.8:1 ratio fitted and I have no idea which model that was originally used on. This is 5% down on standard and is a great balance for a car with bags of torque. Town driving is still pretty easy although the gears are not quite as intuitive as on my 2.0LPT (with the 4.05 ratio).

What effect are you hoping for by fitting 225s and are you using standard Saab 16" rims (6.5J x 16) or wider aftermarket ones?
 

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Discussion Starter #4
Thanks for the info chaps.

For BillJ - I wasn't going to try to recalibrate the speedo -I was just basically going to attach another fascia infront of the existing one- but rotated by a few degrees .

Andrew - I never considered the rim width as a factor in choosing lower profile tyres. I always thought that the tyre width was the governing factor.

Anyway, once again I learn.

I'll stick to 225/50 on Super Aero rims. Pity though, because I can get some really good 225/40 tyres dead cheap.
 

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Originally posted by whitesaab:
[qb]For BillJ - I wasn't going to try to recalibrate the speedo -I was just basically going to attach another fascia infront of the existing one- but rotated by  a few degrees .[/qb][/b]
This wouldn't achieve the effect you need. Instead of offsetting all the marks by the same amount, you would actually need a scale where the marks were farther apart.
 

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Discussion Starter #6
Couldn't I just tilt my head over to one side?

That may explain why you see so many old folks driving with their head on a tilt. They have obviously put on extra low profile tyres & they're compensating for speedo inaccuracies.
 
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