Well, they're car tires, so... sidewall says max 40 psi, and since air pressure is a function of weight, I run 34 psi front and rear.
Your mileage may vary...
Yeah, and those car tires with a Maximum Pressure of 40 psi on the sidewalls generally also show that they only need that pressure to carry their Maximum Load; usually they are capable of carrying maybe
THREE TIMES the weight of your Spyder, or maybe more,
EACH, at 40 psi!! Only most cars don't actually weigh so much to need their tires to run at their Maximum Pressure all the time, so since, as you say, the necessary air pressure is related to the weight they are carrying, the Manufacturer's recommended pressure
for that tire on that car is likely to be somewhat less, maybe even into the high 20's/low 30's, because the whole car weighs in at something less than 4 times the Maximum Load on the tires (4 times cos it's got 4 wheels!) The recommended pressures for the front and the rear may even vary a bit, because of the different weights those tires will be carrying.
And our Spyders total loaded weight with rider, pillion, and all the gear aboard comes in at
maybe a Third the weight of your car, so why are you still using a pressure that is suitable/required for something that weighs maybe Three times as much?! Running car tires on your Spyder at or near the same pressures those tires need when carrying a car that weighs a helluva lot more means that your Spyder tires will be Grossly
OVER-inflated for the load, and it's likely that you'll pay the penalty one way or another, hopefully, without taking out anyone else in the process!
Do a Search on Tires and spend some time reading, there's a lot of knowledge and experience that's been shared here, even some 'Rules of Thumb' and 'rough calculations' on how to arrive at a better and far more suitable pressure for your car tires on a Spyder, there's even some detail on more complex calculations too (
all of which basically support/agree on the 18 psi that's frequently recommended/mentioned/discussed here as being suitable for most readily available car tires that fit/suit Spyders!) instead of just sticking with the same sort of pressures needed for that tire to carry a car weighing maybe Three times as much! Or, since it's your Spyder, you can continue to run over-inflated tires for the weight/load your car tires are carrying, and suffer the greater wear, harsher ride, less traction, and other issues that doing so can bring - I only hope you don't suffer some/any of the
more drastic consequences that running over-inflated tires can bring!
Good Luck!