Top speed discussion of 2.5L

[Chibana]

I can't find the thread now, but there was a discussion on calculating the top speed of the 2.5L car based on Car & Driver's 133mph top speed for the 2.0L automatic car. I calculated the top speed of the 2.5L based on similar weight and the 184hp of the 2.5L, and verified the results with a website calculator (139mph). However, I failed to notice that the 133mph top speed of the 2.0L automatic is redline-limited. So if the gear ratios for the 2.0 and 2.5L automatic transmissions are the same, the top speed of the 2.5L is actually slower, as it has a slightly lower redline. Not that it matters much to me. It's an academic question for me, since I don't think I've ever even hit 100mph in my 2014 Mazda3.

 

[ceric]

Chibana,
I don't think redlines matter in this case. Peak hp at 5700rpm way below redline.
Wind drag is proportional to V^2. (V for velocity). Tire friction is proportional to V.
Mulitplying both big factors, you are looking at top speed close to V^3 vs hp.
So, indeed, it computes to about 138.8mph for 2.5G.

 

[10spokess]

You also have to factor in tire size and diameter.

 

[Chibana]

Chibana,
I don't think redlines matter in this case. Peak hp at 5700rpm way below redline.
Wind drag is proportional to V^2. (V for velocity). Tire friction is proportional to V.
Mulitplying both big factors, you are looking at top speed close to V^3 vs hp.
So, indeed, it computes to about 138.8mph for 2.5G.

That would only be relevant if the car doesn't make enough power past that power peak to continue to gain speed. In any particular gear, engine speed correlates to one particular vehicle speed. As long as the car makes enough power past that power peak (at higher RPM) to overcome friction in moving parts (including tires) and air resistance, the car will continue to gain speed. If you had another gear to shift up into, then you would do that just past the power peak. _Car & Driver_ lists the redline-limited stop speed of 133mph in their measured results, which I would take to assume that means they actually tested the top speed and banged into the rev limiter at 133mph, probably in 6th gear. They're one of the few car magazines that actually measures top speeds of cars in full reviews, I believe.

The question now is whether the 2.5L auto transmission has the same gear ratios as the 2.0L auto. It might have slightly taller gearing since it makes more torque. I did some quick looking on Mazda's site and didn't see gear ratios listed. Maybe I'll have to do some more Google-ing sometime.

 

[Chibana]

You also have to factor in tire size and diameter.

I suspect that the 2.0L and 2.5L, 16" and 18" wheels and tires all have the same overall diameter, if for no other reason than to keep parts and programming simpler for Mazda and their suppliers.