in my ef si, with a si tranny i get 100kmh in 5th @ 3000 ish rpm. in my dads ef si, with a dx tranny he gets 130kmh in 5th @ 3000 ish rpm. what gives could i put a dx 5th gear in my si case with 1,2,3,4 si and dx 5th gear? is there any other tranny with a higher ratio i could use?thx

Yes you can do it, no I don't know for sure if there's a taller 5th. Our reference section should have specs for the D transmission ratios.

yes you can swap gears like that. The EX/SI D series transmissions all have shorter gears, which makes them quicker. VX/DX/LX trannys all have long gears, good for higher top speeds and better gas mileage.

You can swap gears but you won't notice a gain; the 1-5 in the DX is the same as the Si. The difference is the final drive; DX is 3.888 while the Si is 4.25. If you only want to change 5th; just drop in a HF 5th gear. Also, lower revs don't always mean better MPG. I got 57mpg hwy while cruising at 4000rpm(70mph) and another user on HT and D-series.org noticed gains from switching to a shorter(numerically high) final drive ratio.

here read this I know its long but its should help you understand gear ratios in honda trannys bensebuilt