Date registered: May 2013
Vehicle: 1961 190SL
Location: Northern California, USA
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"It's not the engine it's the gears, it sure needs another gear to bring the revs down thus the new cars having 5.6 or 8 gears."
This is the kind of challenge that makes the car hobby interesting. You have options, but none of them are quickly accomplished in an afternoon:
1. Add a Gear Vendors type of overdrive to your present transmission. This will probably involve a adaptor to your transmission and shortening the driveshaft. These add on overdrives are tried and true and work really well.
2. Substitute a transmission with an overdrive gear. As I mentioned on another subject on this forum an old friend of mine put a GM 200R4 overdrive transmission in a Studebaker Avanti and he loves the reduced RPM. I have a 1932 Ford roadster street rod that I originally used a TH350 transmission in and I replaced it with a 2004R overdrive automatic also for reduced RPM on the highway. Now at highway speeds the engine is just running at a fast idle. At highway speeds it's now getting 20+ MPG but no tweaks I've made have gotten me to 21 MPG but that's OK on a hot rod.
3. Change the ring gear and pinion in the diff to a set with a gear ratio more to your liking or substitute in a diff that fits or can be made to fit and already has a gear ratio you like. The ring gear and pinion installation requires adjustments to get the correct tooth contact pattern and "backlash". That task is well within the capability of a motivated amateur mechanic, requiring only some artist's oil paint to smear on the gears to show the tooth contact pattern, a dial indicator to verify correct gear "backlash" and some of your time.