Tim,

Over the years, after the diesel to gas conversion, I've installed many of
these. I've found that moderate (understatement here) vacuum at 65+ mph
limits the control effectiveness. I've added valved vacuum reserve canisters
- helped. Unfortunately the high engine compartment temperatures invariably
lead to failure of the vacuum diaphragm and/or the control valving. A couple
of years ago I switched to the Rostra (bought the Audiovox cruise control
line) Ultracruise II line. This uses a DC electric motor to drive the
throttle cable. No vacuum requirement. Has worked flawlessly with no upper
speed limit (only courage).

Frank Grunthaner