i ran a 4.5 on an 18 with no issues, so 22 on a 7.5 should be fine.
either drivedrain binding to put the motor/speedo under excess strain, or you have an issue with the speedo.
I would recommend a strip down and rebuilt of the car and ensure it all rolls freely. There used to be an issue of the diff gears rubbing inside the gearbox housings, so make sure everything is free and don't overtighten anything.
Also make sure the gear mesh of the pinion and spur isn't too tight, there should always be a little movement between the teeth
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