I don't think you are measuring from the correct side. offset is always referred from backspace. the distance from the inside of the rim to the hub. for example on my neon race car, I use 2" back space on the right and 4" back space on the left. this is a crucial dimension to clear calipers and strut. measuring from the front side is really irrelevant. 40mm is only approx 1.5" 40/25.4 (i didn't feel like opening calculator for an exact answer but you get the idea). as I recall the stock back space is around 4" but don't hold me to that exact value.

 

alot of times tire shops will do this calculation for you for free.

 

The offset of a vehicle's wheel is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel. It can thus be either positive or negative, and is typically measured in millimeters. Offset has a significant effect on many elements of a vehicle's suspension, including suspension geometry, clearance between the tire and suspension elements, the scrub radius of the steering system, and visually, the width of the wheel faces relative to the car's bodywork.

Zero Offset - The plane of the hub mounting surface is even with the centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.

"Deep dish" wheels typically have negative offset or a very low positive offset.

To maintain handling characteristics and avoid undue loads on bushings and ball joints, the car manufacturer's original offset should be maintained when choosing new wheels unless there are overriding clearance issues.

Wheels are usually stamped with their offset using the German prefix "ET", meaning "Einpresstiefe" or, literally, "insertion depth". An example would be "ET45" for a 45mm offset.

The 7mm difference may/may not cause some rub on the inner liner and/or suspension parts...strut/spring or coilover. Also I would test fit 1 tire on the rim put it on the front because the tire size will or wont allow rotation and turn steering to full lock to check make sure all is clear.

or calculate like this First, measure the overall width of the wheel (remember, just because a wheel is 18x7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement between the outboard flange and the inboard flange is 7.5”). Next, divide that width of the wheel by two; this will give you the centerline of the wheel.

Overall width/2 = Centerline

After determining the centerline, measure from the hub-mounting surface of the hub to the edge of the inboard flange (if the wheel were laying flat on the ground – face up – your measurement would be from the ground to the hub-mounting surface). This is your back spacing.

Back spacing - Centerline = Offset