My cart’s top end speed seems to have slowed down since I bought it 6 years ago.
It’s a 48 volt club car with 4 12 volt batteries. 2 batteries were new last year and the other two this year. It stays charged for a long time it just isn’t as fast as it used to be. It now runs 18 MPH wide open.
We live in a small town and they are actually licensed for street use, we can run all over town in it it’s just not as fast as it once was.
Does anyone have any suggestions on where to start looking for a reason why it would have slowed down?
It has a lift kit and bigger than stock tires on it, but it had those when I bought it too. I have replaced the tires but they are the same size as the old ones.

Might need the MCOR replaced.

As far as electric carts go, always buy them in sets. In your case (I get it, its expensive), but you will damage the new ones, trying to make up for the older units. MCOR ,might be, depending on how old it is. Cables, can get corroded badly at the connections, clean is mean.

I have an 04 CC 48 like yours, I use at home.

I have one also that I switched from the 8 six volt to 4 twelve volt DC batteries a few years ago, mainly because of weight and also the cost. I had/have the same problem of the cart slowing down . Load testing the batteries didnt tell me much till I did a hydrometer test and found that one cell was bad/low. I only change the one thats bad and the speed comes back every time. I know that logically I should change all four batteries, but financially thats not always possible. As long as the other three test ok Im good with just swapping out the bad one. I dont know if its just coincidence or not, but it is always the first cell by the positive lug on the first battery.

I put new cables on it every year.

look at your throttle linkage, if it has a pedal position sensor make sure it's going all the way.

They make a rundown tester that duplicates the battery discharge of the motor. They are worth their weight in gold for testing golf car battery condition. A simple load tester works just as well but with that you only test one battery at a time. With a rundown tester you test the bank as a whole. While it's running you can check your cabling. A lot of times I find that one or two cables will get corroded internally and will get warm. Dead giveaway is the heat. Resistance is heat. Have done a few lately with bad speed sensors. On CC typically when the sensor goes south the reverse speed is roughly the same as forward. "Usually" you can test them by turning the key OFF, then push the car forward by hand. It should automatically apply the "brake" (uses the motor to slow the car). If the beeper works it should also beep. If it doesn't your speed sensor may be dead.

yes on batteries, if you had one dead and replace it and the others are "good", they're damaged too. Always replace them in sets just like a a diesel pickup, if one is dead it will suck the other one down too and it too will die soon. Thats a big issue with electric golf cars.

The speed sensor's job is to limit the maximum rpm of the motor, and it works in conjunction with the controller on newer cars. It acts like a governor of sorts. So on some of the newer cars I've been working on, people complain of slower top speed when in fact the motor RPM isn't changing any, or hasn't changed, but the tires are flat or they put different (shorter) tires on or whatever, drivetrain related. Just something I mention, for something to think about.

Working on these electric carts, makes me not appreciate electric vehicle movement....I'd just as soon fix 1000 gas carts than 1 electric cart and that doesn't even start to consider the cost difference to fix gas vs electric...

18mph, I'd be happy with that honestly. A lot of newer carts won't do much more than 15 without playing with the controller, gearing, etc. Did a drive2 a couple months ago (gears, motor, controller the whole thing) and it's up close to 30mph, but it was NOT cheap!