yep if they are the same ratio it will throw your speedo out...

they should increase grip slightly and make your fuel economy worse....

assumming you have reached a level 0 tune (i.e. everything up to snuff but un modified) then increasing power depends on a few factors:-

1. getting a better spark (wil make a difference - but unless coupled with other mods probably not massive)- adding ground wires is a cheap way to maximise what you have

2. getting more air in the engine. essentiall fre flow filter (carefull this may be at the expense of engine life) removing restrictions from the air intake, turning the boost up. most volvo turbo's are capable of producing MUCH more boost that is used - carefull again overdo it and your engine is scrap.

3. ease the flow of exhasut gas. this is perhaps the easiest and most productive. basically it wont give you more power (well it might but not much!) but it WILL reduce your spool up time so the power comes in faster (i.e its usable!) custom exhausts do this - but they need to be put together by people who REALLY know what they are doing you can as easily sutract power as add it

4. add more fuel. this can only (sensibly) be done by remaping the EMU and may require replacing the injectors - then you have to increase the airflow (tweek the turbo)....


for bang for the buck your BEST bet is to make sure what you have is really really right in the first place....if you havent already done these do them first...

1. replace rotor arm & distributor cap (volvo genuine!)
2. replace ht leads (all of them!) at least volvo genuine
3. replace spark plugs
4. replace fuel filter
5. replace theromstat & coolant
6. replace transmission fluid
7. replace brake fluid
8. service and grease brakes
9. repalce airfilter with new standard one
10. clean radiator/intercooler with hose from inside and out (airline is good too!)
11. remove & clean out all turbo air ducting and the inside of the intercooler
12. remove battery clean contacts and replace in car (get a new one if its over 4 years old)
13 check belt tightness
14. check engine mounts - replace ifat all dodgy
15. clean out throtle body - especially palte nad vacume tubes
16. clean PCV tubes
17. runa dose of injector cleaner through the car
18. check all earths
19. remove all uneccessary weight from the car

now i bet that makes a difference, probably a few mpg too! and you need to do it ALL before even thinking about performance modifications