You could reduce it a bit by putting the common parts of it into a view:
VIEW: select * from car, cardetails, comments where car.regno = cardetails.regno and car.regno = comments.regno
Then if your view name is "carquery":
Select * from carquery where make like '$make' and model like '$model' and transmission like '$transmission' and fuel like '$fuel' and mileage >= '$mileagelow' and mileage <= '$mileagehigh' and price >= '$pricelow' and price <= '$pricehigh'
Which would be a little bit better.