Tire sizes for my Lotus Unique
 

I' curious whether I can use 700mm tire on my Lotus Unique?
Right now it has 27" tires on a UKAI 27"X1" alloy wheel.

It's not 700mm, it's 700c. The 'c' is for a series. Long story. It's just a size. And it's not compatible with 27". The difference is 8mm in diameter. It's a small difference. Often you can replace 27" wheels with 700c wheels, and the advantage is that you have a bigger selection of tires available. But you can't put 700c tires on a 27" wheel.

There are a very limited selection of decent tires available for 27" wheels. For regular road use, I would recommend Panaracer Paselas in 27x1-1/4" size.

They happen to be very good tires. Continental also makes a couple of good models in that size. I would stay away from Kenda tires in that size.

Still lots of very good or even NOS Araya or Ukai 700c wheels out there. Also, brand new Alex X rim wheels are available on eBay, but always check C list first. Frequently see brand new wheels avail for cheap because seller got 27" which wasn't what they had on their bike, and a couple times 700c offered on C list for same reason.

Needed a 27" rear last Summer, so I answered an ad for a brand new front or rear for $20 ea & seller told me to just take them both for $20 as I was only one who responded in a month. Don

Also in 27" if you like a white wall tire, the Schwalbe HS159. Not high performance, but long lasting and classic looking. I admit I like the ride of the Panaracer Paselas better; they are my main tire.

If you're determined to drop down a size to 700c and buy new wheels, @robertj298, make sure your brake pads have a little room to drop down with them.