Ok so I have heard that the speedo on older rx-7's is incorrect at high speeds, but how incorrect are they? Last saturday night i was driving around on I-90 east and decided to just gun it accross the I-90 bridge so right before I enter the tunnel I drop it into to third let it wind all the way up to 8K shift to 4th and as i'm heading down the bridge at 7500rpm the speedo reads a bit more than 140mph! I have an 88 vert with full racing beat exhaust and apex'i dual n1's out back and just went over 100,000 miles and some fresh oil and i keep the car and engine in tip top shape. So do you guys think that the speedo was correct. or incorrect ..also remember I was going down onto I-90 also.

 

130-140 is possible for an n/a, but at that speed I'd say it could be possible for it to be off about 5-10mph, I think Mazda factory specs show that being 5mph off at 60mph is still within spec, heard that somewhere.

 

Look it up in the 2nd gen section. Whenever this stuff comes up in there and someone claims going over 130 in an n/a, they're called lyers and told that they're wrong. If you look, people have busted out the calculators to figure out the mathematical possible top speed. As far as I remember, they said 4th gear will top you out at around 128 mph (stock), and then everyone with an n/a knows that you shift to 5th and start losing speed.

 

I swear it was 135, because I followed that debate.

 

Something many of you might not know.

The 88 vert, which he has, is the only 5speed FC to come equiped with a 3.9 rear diff. Compare that with the 4.1 that every other 5 speed FC came with and you can understand why his speedo would read that high in the top of 4th.

So, was he going 140? Probably more like 130-135. But the point is that the 88 vert (modded) can pull to that # on the speedo in 4th gear.

 

In my old GTU with a street port, I had a hell of a time hitting 130. Plus I think I hit 130 in fourth and as soon as I hit fifth the car couldn't hold that speed.

However you choose a great place for a top speed run. I go that route every so often in the middle of the night to test the waters of the 3rd gen.

 

in my 2nd gen, sport model using the standard gear ratios and rear end ratio and stock tire circumfrence i calculated about 138-140 in 4th at 8000 rpm. with stock size tires.

also when my car was dynoed, which had mph reading also, and it was at 134 mph at about 7650 rpm.

ive gotten my car up to 135 in 4th before, and i was very close to 8000 rpm..... 5th is way too long in the NA ,so you can go faster over reving in 4th than in 5th....

 

thanks guys but i dunno if this would help or not but my vert also has turbo II tranny just thought i would put that in.