Top Speed

A stock Miata is drag limited; you can't hit the rev limiter in high gear while driving.

(yes, I've tried. Mine topped out around 120 indicated. 115 indicated with the top down, on another occasion, though the car had some breathing improvements since)

"Lanny Chambers" wrote

There are a bunch of Mazda engineers who would be unhappy to hear this. Actually the Miata is an excellent car from an aeordynamic viewpoint. It's quite stable at high speed and in crosswinds, both of which are tough to do with a three-box body style. The air dam makes a noticeable difference in stability at highway speeds, and the car's comparatively low power is not the fault of the aerodynamicists.

Note that Mazda was a fore-runner in the field of computational fluid dynamics, and their racing cars of the late 1980s were among the first designed using CFD techniques. Maybe the Miata was finalized before this lab was available, but they did put a lot of work into this area of the car's design...

Most cars with a 5 speed manual gearbox are designed to reach maximum power (typically close to redline revs) in 4th gear at a speed which requires that maximum power. If 4th were any higher, they wouldn't have enough power to go as fast, if it were any lower they would run out of revs. 5th is typically higher for reduced fuel economy.

The same applies to most 4 speed autos, with top speed in third and better economy in 4th.

Make up your mind--if adding an airdam improves stability, then it's clear that a stock Miata needs help. (The airdam was allegedly supposed to be standard, but it proved too low and fragile to survive the abuse of average drivers.) The resistance to crosswinds is probably more a function of low sail area and high roll stiffness; crosswinds have much less effect with the top and windows down.

The Miata body shape was designed in a studio. IIRC, Bob Hall said there were never any plans for wind tunnel testing. Nobody really cared about aerodynamics, as long as the car was cute and wasn't outright dangerous.The only aero benefit is the low frontal area, a coincidence of styling decisions. Pop up the headlights, and even that goes out the window.

Theory is always interesting, but it's no match for documented history.

Except that its Cd sucks. It's pretty good for a convertible, but that's a big caveat.

If you normally drive 95+, anyway :-)

I think not. "Proper" sportscars are designed to be able to *just* hit maximum power in top gear (4th in a 4-speed, 5th in a 5-speed, 6th in a

6-speed) i.e. the speed which actually requires that much power to overcome total drag at that speed as you say. In fact what's happening here is that the overall reduction of the gearbox and final drive is resulting in a finite amount of thrust being available at the driven wheels and the car stops accelerating when the total drag exactly matches the thrust at the wheels. Making the gearing taller (i.e. less reduction) means that the speed at which drag excedes available thrust occurs lower down the engine's power curve so you are using less of the available power. A family car WILL most likely be designed so as to be over-geared in top gear as you describe, for fuel consumption, engine wear and NVH reasons. Virtually all motorcycles are intended to hit max power revs in top gear, supersports bikes will often actually be undergeared in top gear so they'll rev out past maximum power to sacrifice theoretical top speed for acceleration at lower speeds. My family car (Renault Scenic 1.8 16V) will rev out to something like 900rpm below max power at 118mph in 5th gear, the Honda CBR600F4i that I commute to work on is allegedly good for more than 155mph which is well over 1000rpm past maximum power in 6th gear. Due to the very close-ratio 'box it will go almost half this speed in 1st gear.

SteveM

2001. 1.6 133mph downhill :) 124mph top up 110 top down (it's so loud you can't hear your own thoughts, let alone radio, phone or co-driver)

"Most cars" and "Proper sportscars" are probably mutually exclusive groups.

For racing, certainly any gear above that which allows top speed is wasted, but the very fact that people are reporting that their Miatas / MX5s do not run out to redline revs in top gear demonstrates that those cars are not geared for maximum speed in top gear.

Nothing a little boost won't fix. :-)

True, very true...

The 6-speed Miatas should reach top speed in 5th gear at redline. The

5-speed miatas reach top speed in 5th gear at the top of the power curve.

In 6th, yes. It should reach the same speed in fifth at redline. Theoretically, anyway.

Actually on a flat road I reach a higher speed in 5th rather than in 6th. I guess this is because of the higher rpm. In 5th I reach the high 120's at