OK before I ask the question...…..I'm never going to sell my car, its an automatic, I never race it or traffic light drag it, its a cruiser, a GT, I love it for it's style, its rarity, the laid-back driving position and lots of other reasons :-) It's market value to me is meaningless (much like a friends DB6 that he says he will be buried in !!) and truth-be-told few of our cars are truly OEM.

 

I've thought about the possibility of changing to a different auto gearbox but got slapped down :-)

 

So.…I'm wondering about changing the diff ratio to give it longer legs. It's 3.6....something at the moment. The other family car - a BMW 1 Series - is also an automatic with a 2.47 diff ratio and as a consequence is MUCH longer legged than the 300ZX. It's a diesel so quite torquey but I'd be amazed if it was more so than the TT. So what I'm looking for is a table if one exists, or a formula that I can adopt and adapt that will give me rpm-v-mph for different diff ratios ? There's a specialist engineering firm near me that will fabricate different ratios for almost any car and I need an idea of what to ask for. I'm thinking about something roughly halfway at 3.00ish.

 

 

Other than giving it longer legs is this likely to create problems elsewhere under the laws of unintended consequences ? I realise that the speedo would need to be recalibrated to account for the change.

 

 

Thanks for any input I can get.

The diff used in the TT Z32 is a R230. This info may help:

 

https://www.300zx.co.uk/forums/showthread.php?114740-r230-diff-ratios&p=1116685&viewfull=1#post1116685

 

wondering about changing the diff ratio to give it longer legs.

 

what I'm looking for is a table if one exists, or a formula that I can adopt and adapt that will give me rpm-v-mph for different diff ratios ?

 

There's a specialist engineering firm near me that will fabricate different ratios for almost any car and I need an idea of what to ask for.

 

Other than giving it longer legs is this likely to create problems elsewhere under the laws of unintended consequences ?

 

Well that is an odd one for an automatic zed given how long legged it is as standard, are you sure your gearbox is achieving torque lock-up ? reason I mention that is I found on average ( wheel size dependant ) that in top gear 3+overdrive + torque lock-up ( feels like 5 changes ) 1k rpm equates to 29/30 mph, manuals on the other hand are a fair chunk lower around the 25mph for 1krpm ( 5th gear) and the trend for fitting non turbo diffs to manual tt`s to increase acceleration drops that to around 21 mph for 1k rpm way too noisy and unsettled for a long motorway drive in what should be a GT car, I get you looking for the opposite though.

 

 

So to answer your question, yes there will be consequences, the torque curve of the engine and the auto-box have been pre-programmed, there is a degree of live data but no torque vectoring on a 30 year old car I am afraid, so you could potentially end up with a pretty lifeless car in top gear on the motorway which could leave the box unsettled, fitting larger wheels may be a compromise.

 

If your unhappy with how it feels, I would first find a long strech of road, and pull away with a fully warmed up car and drive up to at least 60mph and count the perceivable changes in the transmission, you should feel five, if any less you have your answer.

 

 

Jeff

Hi Jeff

 

Just thought I’d follow up on this.......

 

Long drive up to Jim’s today. 1000rpm = 27mph. It’s a bit lower than I’d like. Still hope to explore alternative diff ratios.....depending on cost of course !