RV-4 Posted November 13, 2023 Share Posted November 13, 2023 I have a 1969 Lotus s3 LSB2436. I have a spare 3.9 and 4.1 diffs. I want to cruise with reduced RPM. which ratio is best suited for cruising ? I have a Crossflow 1600 cc. Cheers George Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 13, 2023 Member Share Posted November 13, 2023 The lower the final drive ratio the lower the engine speed, all else being equal. I recommend using one of the online calculators to see how the numbers play out with your gearbox and tyre size. Jonathan Link to comment Share on other sites More sharing options...
OldAndrewE Posted November 13, 2023 Share Posted November 13, 2023 Bit of confusion over terminology there JK. I would say it is the other way round, i.e. the higher the ratio the lower the engine speed - think of it in terms of gearbox ratios, top is certainly higher than first. Of the two RV-4 mentions the 3.9 is higher (and is certainly the one I would use for cruising) Link to comment Share on other sites More sharing options...
rj Posted November 13, 2023 Share Posted November 13, 2023 The simple answer without confusing anybody: 3.9 will result in lower engine revs than 4.1 Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 13, 2023 Member Share Posted November 13, 2023 4 minutes ago, rj said: The simple answer without confusing anybody: 3.9 will result in lower engine revs than 4.1 Yes. By lower I meant smaller numerically. Sorry If I caused confusion. Jonathan 1 Link to comment Share on other sites More sharing options...
john g Posted November 13, 2023 Share Posted November 13, 2023 So, if the 3.9 is used besides lowering revs would that mean slower acceleration and reduced top speed? Link to comment Share on other sites More sharing options...
OldAndrewE Posted November 13, 2023 Share Posted November 13, 2023 Flat out acceleration will be slower, don't know how noticeable it will be. Top speed - I think it depends on what is limiting top speed at the moment Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 13, 2023 Member Share Posted November 13, 2023 8 minutes ago, john g said: So, if the 3.9 is used besides lowering revs would that mean slower acceleration and reduced top speed? It will reduce acceleration within a particular gear, other things being equal. But changing the final drive ratio affects the overall gearing all of the gears, so it might bring a more appropriate gear into play. Using a gear calculator lets you play with these comparisons and with shift points etc. Jonathan 1 Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 13, 2023 Member Share Posted November 13, 2023 6 minutes ago, OldAndrewE said: Top speed - I think it depends on what is limiting top speed at the moment Yes. There have been production road cars which are over-geared and have a higher maximum speed in the gear below top. Jonathan Link to comment Share on other sites More sharing options...
RV-4 Posted November 15, 2023 Author Share Posted November 15, 2023 Thank you so much for the comments on the final ratio 3.9. George Alberta Canada Link to comment Share on other sites More sharing options...
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