There are no silly questions here , lots of stupid mistakes are made by not asking them.
There are various sites that will show you the difference in wheel diameter between the various sized tyres and what the effect will be on the speedo reading.
This is just one of them
https://www.tyresave.co.uk/tyre-size-calculator/ and another
https://www.tyremen.co.uk/tyre-size-calculatorBy inputting the current size tyre and a possible alternative, you can get a very accurate idea of what is possible and the effect on the ride height and the speedo accuracy.
For instance using a 185/70 x 15 would give an increase in diameter of 1% , which is well within the accuracy needed to keep the speed-reading legal.
Look at the thread below on the Ferrari Blue +4 and the recommendations there are for the 165/80 x 15 Vredstein . This would almost 2% larger in diameter but less than 2% "fast" on the speedo at 70mph. It might be enough to help you with the infamous bumps, and will hep the ride quality of the car.
I too live in an area which is infested with speed bumps, and have to be very careful making a slow transit across them. You can adjust the ride height at the front of the car by fitting the SSL front suspension modification. Not cheap at almost £1000 including fitting, but a very worthwhile modification, and this is what I have done. I continue to use 205/60 x 15 on my +8 (but this is much too big for a +4)
