Is it possible to convert the 6th gear of the 6 speed transmission of a mazda miata to a taller gear for better fuel economy? If so can you tell me the gears sizes that would work
“Possible?” Given sufficient time and money (mostly money), it’s likely to be possible, but it won’t be cheap and it won’t be easy.
I’d stick with the ratios chosen by the Mazda engineers.
Changing the rear-end gears or the complete assembly is a much better and cheaper option. Investigate what YOUR final drive ratio is and what is available…
How do the figures work out? (what I mean is how long would it take for the better mpg to pay for the modification?)
Don’t others think like this or is it “get me 2mpg better at any cost”?
If mileage were my major concern I wouldn’t buy a sports car.
I agree with the other replies about cost vs benefit.
Suppose a taller gear would give you a 2 mpg increase. If you figure out the number of miles you drive/year, your current mileage, and the current cost of gas, you’ll be able to determine what the savings would be per year.
Suppose a change like that would save you $100/year (yes it’s optimistic). Then any discussions for what a conversion would cost can quickly be put into perspective.
I suspect it will take many many years for the conversion to pay for itself.
In addition to being costly (whatever the route taken) I doubt you would save any gas at all. 6 gears let Mazda put a pretty optimized set of ratios in there.
It would be a waste of time and money. You would never regain the money spent in fuel savings.
Thanks for everyone’s response. After hearing what you all said I’ve decided not to do so. Will have to find out what the rear end ratio is. Maybe I can do something there to improve. I will let you all know what I find out.
MPG figures are pretty much a male “bragging rights” issue like a few other things.
None of my friends would dare brag about MPG. You’d be sitting beerless on the front porch in a timeout… 
My V10 F250 gets 6.5mpg during 4x4 snowy city driving. I get 10mpg with my Miata, while on track…how’s that for bragging
Miata info and time killing… http://www.miata.net/
If it’s a new one, here’s the info from MazdaUSA’s site:
manual:
3.82 - 1st
2.26 - 2nd
1.64 - 3rd
1.18 - 4th
1.00 - 5th
0.79 - 6th
3.60 - reverse
4.10 - final drive
Automatic:
3.54 - 1st
2.06 - 2nd
1.40 - 3rd
1.00 - 4th
0.71 - 5th
0.58 - 6th
3.17 - reverse
4.10 - final drive
both are rated 21/28 city/highway
Thanks bscar, that is the information needed… A 4:10 rear gear, even with a .79 overdrive 6th means the gearing is optimized for PERFORMANCE not mileage. The OP should move towards 3:50 gearing which, with Japanese differentials, might be next to impossible. Gasoline is cheap. Custom gear-sets are VERY expensive. Good Luck.
That’s acceptable!
I’ve got a few cars that can generate a breeze from the gas gauge when romped on. 