OK, that would be a 10% speed difference, significant and too much to handle without adjustment.
You would obviously have to use alternative tapes with a different stripe count to affect the speed. I'd first measure the scale MPH of each locomotive over a measured distance and calculate the speed error. I'd suspect that one will probably be the offender, but it's hard to say. In any case, once you have that data, you can then set about correcting the issue with a new custom tach tape. Let's say that one is running too slow at 45 SMPH and the other one at the correct speed 50 SMPH speed, this is probably the easiest situation to address. Since you can only affect the speed with whole increments of a stripe, at 50 SMPH, each stripe accounts for about 2 SMPH. A good place to start would be a tape with 19 stripes to replace the 24 stripe "standard" and see where that gets you.
Here's a link to a set of custom tape images: Custom Tach Tape Image Post In DCS Documentation Thread. The easy way to do this is to print these on 1/2 page gummed shipping labels using the "label" print paper type, that's how I do it.
In order to pick the correct one, just take the exact image of a stock tape for your size flywheel and print the images on plain paper at first at full size. Match the exact length of the 24 stripes from the stock tape to a 19 stripe image in the custom tape files. Get as close as you can, you should be able to get pretty close.
Print that file on the gummed label paper using the "label" setting and cut it out for your custom tape.