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260E (300E 2.6) differential ratioWhat is (or what should be) the differential ratio on a 260E(or, in the USA, 300E 2.6) model(W124 series)?
On various sites I've met ratios of 3.07:1 or 3.27:1, on another site even 3.92:1. I'm first and foremost interested in 260E with manual transmission.~Thank you Nautilus |

Not sure about '90, but in '88 it was the same as my car - 3.27:1 for both manual and automatic transmissions. You can verify with the tach reading at 65 MPH:
(65/60)837(3.27)(0.8) = 2372 (manual) 2372/0.8 = 2965 (auto) 837 is the revs/mile spec for the OE 195/65R-15 tires, 0.8 is the top gear ratio for a five-speed. If you you use another check speed, say 70, then replace the first term with 70/60, At 60 this term becomes one, so it can be ignored. If an automatic just drop the last term (0.8)because top gear is 1:1. The speedo/tach relationship will not change if different revs/mi tires are installed, but the speedo will be off the percent difference in revs/mi from OE calibration. You can also rearrange the above relationship and compute the axle ratio with an accurately observed top gear tach reading at any speed. With an auto make sure the reading is on level ground to minimize converter slippage. Five speed ratios are 3.86, 2.18, 1.38, 1.00, and 0.80:1. Duke |

Thanks.
I've tried to calculate the ratio in city traffic(pretty inaccurate yes, but I didn't have the time to take a ride on the highway). Just edited the post since I was wrong.... My tyres are Michelin, 205/60R-15, worn. That makes them about 0.5in less in diameter than standard. According to this source: http://www.miata.net/garage/tirecalc.html revs/mile ratio for a 195/65R-15 is 807, for a 205/60R-15 is 817, and for a 205/55R-15(roughly similar to a worn 205/60R-15) is 845. Tried the formula - it works... I can say I don't have a 3.92:1, but the difference between 3.07 and 3.27 is too small to tell with accuracy ~Nautilus |

Just calculated, using the value of 817revs/mile given by miata.net, and 4th gear(to get 1:1 ratio and simplify the formula). It's
3.27:1 :) ~Thanks Nautilus |

Duke 2.6: You said: " If an automatic just drop the last term (0.8)because top gear is 1:1. "
Is there a lock-up torque converter on those 2.6??? I don't thnik so. If not, some slippage is present making the calculation off by.....something:) :) jackD |

I don't think you have a lockup converter, but there is little converter slippage at highway speed on a level road probably no more than two percent. Also the RPM/tach relationship is fixed and will not vary with installed tire size, so use the OE revs per mile in your calculations.
Shorter tires will increase speedometer error, but will not effect the speedo/tach ratio. That is fixed by the speedometer gearing. Duke |

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