Haven't really tried counting but maybe 5hz max. If it was 15-20 it'd probably achieve most of what I'd want.
A way to test this is to record the speedometer while driving and count how many updates/refreshes per second while accelerating.
I would also love to be able to see every MPH as I'm driving as well. Basically OP is wanting a higher polling rate for the speedometer. Dodge/Jeep/FIAT does a really good job with this. The refresh and polling rate on the speedometer is super quick. The Supra seems like it's polling the...