What sends the signal to the speedo, a sensor on the wheels, or trans, or? I want to put larger diameter tires on my 780, either 205-65-15 or 205-70-15 insted of the 60 series, to give a softer ride. Will the larger diameter tires cause the speedo to read incorrectly?

I have an '89 740, and my speedometer reads fast even with oversize tires (195/65/15 in place of 185/65/15). Check for speedo error now - find a road with mile markers, hold to 60 mph, and see how long it takes to go a mile. Oversize tires may actually correct speedo error.

Mark -

Try this site: www.miata.net/garage/tirecalc.html for the speedo error. You will need to know the tire size that's stock for your car. On a 240 that info is on an info sticker on the rear edge of the front passenger door.

Good Luck,

Bob

The slotted wheel on the differential carrier provides the electrical signal to the speedometer.

The speedometer stops at 85 mph, is that in all 242's ranging from the

DL to the turbo's? I saw a picture of this guys 1983 242 GLT, and you can see the top speed is 85. Isn't there one that surpasses the number? please don't tell me the top speed on a 242 TI is only 85mph.

OH... has anybody heard about those speed limiters in the new cars?

It keeps the cars from reaching its full maximum speed. Example, your car says 100mph, but the car will stop at 85 and then start to violently shake or something? I saw it on the news here in california, it was a Ford!

85 MPH spedos were the norm on EVERY car in the US from the late '70s to early '90s. It has nothing at all to do with a car's top speed! Since drivers subconsciously (in many cases - UNconsciously) settled into a speed where the speedo needle pointed straight up, usually 60MPH, the maximum was dropped so the centre reading was a little over 45...slowing traffic...saving gas.

Those speed limiters are usually on light trucks and SUVs. It has to do with maximum allowable zoom on the tires the factory mounted. Cutout hits around 92MPH and engine restores around 89.

Now to the guy's original question: The pre-1990 Bertones (780)came with larger diameter tires (205/60-15 at 24.8" dia) than the 760 it was built from (which used 195/60-15). The speedometer calibration is different than all the other 200, 700, 1990-91 780s, 900 cars which came with 24.25" diameter shoes. Besides making a noticable jump in speed readings, if your 780 is built like mine, the taller 75 series tires WILL hit the bumper on turns. If you insist on getting off the 60 series at the expense of handling, the closest size you can go to is 205/65-15 touring at almost 7% larger. The smallest 75 series tire is 13% larger than standard.