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Jan 20, 2012 15:19:32 GMT
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After a bit of advice on gearing my mums Yaris. Now it currently sits on 175/65/14s so the first plan is to change wheels/tyres. Now I would like to drop the engine revs about 400RPM, taking it from 3000RPM @ 60mph to a more civilised 2500(ish)RPM By increasing the rolling radius I think this should be possible but am unsure just how big an increase to look at.     Thanks |
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Some days you just need to take a grinder to an inanimate object, just to make your day a tiny bit better!!
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Onne
Part of things
Posts: 822
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Jan 20, 2012 15:38:43 GMT
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Maybe use one of the online tyre calculators? They tell the difference in speed.
Speedo will be out though, probably a lot, and it'll under read too
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1990 Mercedes W126 300SE
1997 Mercedes W140 S320L
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RobinJI
Posted a lot
"Driven by the irony that only being shackled to the road could ever I be free"
Posts: 2,995
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Jan 20, 2012 16:13:10 GMT
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I think that's probably too big a difference to manage just by changing the wheels. You're current wheels have a circumference of 1832mm and diameter of 583mm, and you'll be looking to increase the circumference by a 5th to get the gearing you're after. This means you're wanting a circumference of ~2198mm, and diameter of ~700mm.
that's a 117mm difference in diameter, so 59mm each side, which is quite a lot for the arches to swallow.
To manage that with easily available tyres doesn't seem to be possible without going to at least a 225 width, although you could get close enough with a 215/65R16 (2550rpm @ 60) or a 215/60R17 (2536rpm @ 60). The best you could manage with your 14s would be a 185/80, which would give you 2685rpm at 60.
What's the reason for wanting to drop the revs? 3000rpm at 60 sounds pretty normal for a small engined hatch back. If it's noisy sound deadening will help a lot more then dropping 500rpm. If it's for economy then I don't think you'd improve things massively, the engine will struggle more accelerating and around town, so unless you do all your miles at speed, then you may not find any real gain from it, plus it will make the car noticeably slower pulling away. Ideally what you want is just a longer 4th and 5th, with the same lower gears as your current box, could you fit a box from a Corrola or something?
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Jan 20, 2012 17:12:44 GMT
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I I think I would struggle to get 215/65/16s in the arches, as the MR2 runs 225/55/16s and there is no room.
Think 195/205 would be the max width really.
Honestly didnt think the ratio would be that high leaves a little room for thought as 2 and half inch is a little much to try compensate in wheels/tyres.
IMO its revving to high, and if anything a little less when trying to pull away would help.
Think I may suggest trying a 15/65 if I can find some
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Some days you just need to take a grinder to an inanimate object, just to make your day a tiny bit better!!
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Jan 20, 2012 17:22:13 GMT
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It's a shopping car with a small engine, so it seems quite reasonable to me. My car does 4,000rpm at 60, and that's quite happy sitting at 70-odd all day. It does only have 4 gears though. Even my Dad's Audi is in the region of 3000rpm at 60. I honestly wouldn't worry about it. |
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Jan 20, 2012 17:50:45 GMT
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3k at 60? thats absolutely bang on the money for a small car. Actually a bit tall compared to a lot of things I've owned.
Putting wagon wheels on to get your gearing closer to an executive cruiser would jolly pain in the backside all sorts of things up.
1: Speedo would be miles out.
2: Acceleration on the motorway would be awful. You'd need to drop into 4th all the time (there is a reason the gearing is set as it is - If the car could take a higher final drive and still perform OK, it would have come from the factory like that)
3: The car would be 2 inches further from the ground and would look strange, and probably drive quite funny.
4: Wind resistance would be increased due to more air getting under the vehicle, probably counteracting any improvement from the engine spinning more slowly.
You'd end up with a stange looking, sluggish car with tyres that catch the arches all the time. It might be slightly quieter at speed, but it would use just as much fuel.
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Last Edit: Jan 20, 2012 17:52:48 GMT by cobblers
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Jan 20, 2012 20:22:19 GMT
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Its a Yaris Verso they don't get much stranger looking lol
I think it would be worth a punt on some 15s TBF but staying at 55s if they fit correctly.
It could do with gearing up on motorways aswell really as its a VVTI
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Some days you just need to take a grinder to an inanimate object, just to make your day a tiny bit better!!
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benjy_b
Yorkshire and The Humber
Posts: 409
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Jan 21, 2012 14:59:02 GMT
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My daily drive is a Smart Fortwo, on which I have reduced the rolling radius with different wheels.
My car does 3800 rpm @ 70mph.
Most modern engines are very efficient at high rpm, hence why they are sent out the factury with low gearing. Are you wishing to change your rolling radius to improve economy? If so, I would hazard a guess that what you plan to do would actually decrease economy. As your engine would be put under more load at lower rpm, therefore having to compensate by giving it more gas and in turn using more fuel.
... and breath...
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2005 Subaru Forester 2.5XT
1999 BMW E36 318i Touring with OM605 Mercedes Engine
1996 Lada Riva with Honda S2000 Engine
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