I swapped the engine/tranny form my old FD into my new RZ. I know the RZ has a 4.3 rear end, and I think this is the reason for the speedo being incorrect, but what can I do to solve it?

 

anyone have any insight for me?

 

How do you know it's incorrect? You have your original 4.10 final drive so that's not an issue. The JDM trans does have a different 5th gear ratio in the trans.

 

I swapped the engine and trans from a JDM 92 FD with a 4.10 rear end into a 92 JDM FD RZ with a 4.30 rear end.

I know because I hit my governor (115 mph) half way through the back straight at my track whereas before I would not even hit it at the end of the straight.

Also, I drive 90 mph (according to my speedo) on the highway and get passed by granny in a Buick.

 

The rear end difference is only 4.9%. At 90 that is 4.4 mph. At 115 that is 5.6 mph. That's not that much.

 

Gotcha, I misunderstood what you were saying.

Not saying this is the 'best' answer, but if you run 275/40-17 on your drive (rear) wheels, that'll get you back to a stock type ratio, mathematically speaking. You'll have a 25.7 inch rolling diameter rather than the stock 24.9 inch.

 

I did that same calculation but thought I must be erroneous in my math. There must be another cause then.

any ideas how a speedo can be wrong?

 

The quickest way to get an estimate (without being clocked by a cop) is to use a gps unit that display current velocity/speed and do a comparison with your dash and see how much you are off. It should be only a couple of mph +/- off from your actual.

 

Have a buddy pace along side you and see what speed his/her car is reporting verses yours. Then you can figure out if the variance is something tire/gearing related or if the speed sensor is jacked.