Unfortunately I don't think there is any way to make the Premier Highwayman to look or fit like the Speedway, because the way the Speedway jacket is made is different than rest of the Aero jacket construction.
There is a lot of curves in the Speedway:
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I got some Aero jackets and even the overall dimensions is similar to the Speedway, the fit is totally different because there the way the Speedway is assembled. The Speedway can't be flattened like the other Aero jackets. I think this contribute to the fit more than just the numbers.
One other thing to keep in mind is the big over hang of the Speedway in the front. Mine is size 42 with added length to 24" in the back, and this translate to 22" in the front with a 21" zipper.
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If you don't mind coming down to Kerrisdale I can let you try on the Speedway and PHwayman and another secret jacket or two. I think the Premier Highwayman can fit like the Flat Head copy if lengthened, but never like the Speedway fit.
They can do it, but might not with the Highwayman Label. All needs to be done is to curve the center back panel, and fill the two side panels accordingly. Right now their curve comes from two side panels not the center back panel:Thanks, I may have to take up on your offer, but first I want to see what Aero has to say