The dealer only had factory gear ratio/tire speedo gear combos listed. The other way would be to go to the Miata tire calculator website (type that into a search engine) and punch in your factory tire size compared to your new tire size and it will give you the percentage difference in size. If it says 10% larger then you could check with the dealer for a gear 10% smaller than stock. The dealer told me that a single tooth change on the gear accounts for 4 mph change. Good luck.

 

The dealer only had factory gear ratio/tire speedo gear combos listed. The other way would be to go to the Miata tire calculator website (type that into a search engine) and punch in your factory tire size compared to your new tire size and it will give you the percentage difference in size. If it says 10% larger then you could check with the dealer for a gear 10% smaller than stock. The dealer told me that a single tooth change on the gear accounts for 4 mph change. Good luck.

 

The dealer only had factory gear ratio/tire speedo gear combos listed. The other way would be to go to the Miata tire calculator website (type that into a search engine) and punch in your factory tire size compared to your new tire size and it will give you the percentage difference in size. If it says 10% larger then you could check with the dealer for a gear 10% smaller than stock. The dealer told me that a single tooth change on the gear accounts for 4 mph change. Good luck.

 

Having a problem with the "Submit Reply" button??????

J.K....... It probably didn't send the first time, so you kept pressing it........ I'll guess you did it about 11 times!

Could not resist commenting on it........

 

This is what that website gave me, the 2 sections gave 2 different %. Any ideas? Thanks, Greg


stock new
255/70/16 315/70/17

sidewall: 7.0 in 8.7 in

radius: 15 in 17.2 in

diameter: 30.1 in 34.4 in

circumf: 94.4 108.0 in

revs/mi: 671.0 587.0

speedo reading with new tire is 14.3% too slow

when reading 60 mph, actual is 68.6 mph

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
255/60-16 6.0in 14.0in 28.0in 88.1in 719 0.0%
315/70-17 8.7in 17.2in 34.4in 108.0in 587 22.5%

 

Guys at the very reputable tire shop I use advised against going from stock 285/70-16's due to the ABS sensors. What kind of adjustments need to be made to the ABS?

 

Sorry bout the excessive replies. I'll never get comfortable using this thing.