well simple actually...
I can see from the pic, that you have a TC installed.. looks to be a series 30, which means it has an 10% overdrive at top speed..
so with 19" tyres that's 32.3 mph already;
say you have your engine set to govern at 3800 instead of 3600 (which is indeed possible )
then your top speed should be 34 mph..
37... difficult tbh that's above 4000 rpm.. a point at which it may not be too pleasant of an outcome ... I think the governor wheel itself (tiny plastic piece with steel inserts) is not safe above 4000-4100 rpm and you're already getting really close...
so let's hope it was a misreading on your wife's part
'sid
PS use a proper calculator perhaps

I'm not saying you should use kartcalc.net from your mobilephone (since it'd also has a gps tach if you're really curious and it doesn't even cost anything) since that'd be blunt advertising for my own stuff.. and I don't do that
