Changing gear ratio on manual transmission
Just wondering if anyone has or looked into regearing the transmission.
Ive driven 7500km on my new micra. I love the car, great in the city. But mileage on the highway sucks. 3000 rpm at 100km/h, thats rather high. I tend to be doing 120-140+ a fair bit. So its becoming a thirsty car.
I find first gear a wee bit low I usually end up shifting by 20km/h and often end up skipping 3rd or 4th depending of my driving. And usually in 5th by the time I am doing 80 on highway....even 50 in town. Ive never really had to downshift to pass anyone.
I am seriously considering ripping my transmission out. Put a slightly taller first gear. 5th gear (or something close to the current gear) would then become 4th, id get a taller 5th gear highway cruising. 2nd and 3rd id adjust to fit nicely with the new ratio of the other gear.
Im not worried about voiding the warranty, ill provably end up voiding it pretty soon anyways. Im not one to pay over priced mechanics to do my maintenance.
Any thought?
swapping 5th gear or final drive
Changing individual gear ratios within the transmission is a very big job. (Eg. 1st gear).
It's better to look at what other cars used the same transmission with different ratios, or the same spacing on the input & output shafts (with different 5th gear ratios).
Changing just 5th gear...
This can be relatively easy... if the gears are OUTSIDE the transmisson case under a stamped steel cover (like most FWD cars I've owned).
I know of Saturn owners who have done a 5th gear swap for lower highway RPM & better fuel economy:
---> Saturn S-Series 5th Gear Swap (images in this thread are broken, but the text is still informative).
The trick is finding a car with compatible gearing as a source of the parts. (Saturn had several different engines & transmissions that were all interchangeable).
Unfortunately, the last time I checked (2015), the Versa sedan & Versa Note (same HR16DE engine as Micra) transmission was identical to the Micra's, so there's nothing helpful there.
If no alternative 5th gearsets are out there (I'd be surprised if that's the case), you could have a set machined. I know of a couple of people who went that route, but it's very expensive. (Over $1k).
Changing the final drive...
I did this in my Firefly, by swapping the entire transaxle (same gearing 1-5, just taller final drive) from a 4-cylinder Suzuki Swift. When you change the final drive, you change performance in ALL gears, relative to stock. Note: this will hurt acceleration, if that's a concern of yours.
---> Project 'nerd gear': taller tranny transplant nets +5.2% MPG
I think the easiest route for someone interested in doing this would be researching 5th gear ratios and measuring the diameters & distance between the input & output shafts on other Nissan 4-cylinder FWD cars. EG. Sentra, Stanza, Altima were more powerful, so 5th gear ratio is probably taller (numerically lower). If you find something that's identical to the measurements on the Micra trans, then swapping just the 5th gearset may be doable.
Juke uses a 1.6 turbo, so the transmission bellhousing is probably the same as the Micra's. Not sure about the internal measurements though.
As Alpha points out, you can swap larger diameter tires, but the difference won't be as big as a gearing change. Also, larger diameter tires raises the ride height, which also increases aerodynamic drag (thus bringing fuel economy back down). You could use lowering springs to get it back to normal, or lower.
I do hope someone investigates this, so when I eventually get a Micra, all the answers will be here. :)