Planning a GSL rear axle swap. Want to install lower gear set at same time. Going into a 85GS FB with FC S4 trans and "fat" Nikki carb. Car has stock port 12A.

Looking for opinion on the highway cruise RPM / gas mileage between 3.91, 4.10, and 4.30. Is there a significant autoX/track performance difference between ratios?

Car is not a DD, but gets driven to autoX, track, and rotary car meets so highway performance is important.

 

With the stock setup on my SA (3.9 rear) the cruising rpm for 80MPH is 4000 rpm exactly, 75MPH is about 3500 rpm and 65MPH is ~3000 rpm. I know this because my speedo is off slightly due to age and tire size.

 

Rough guess with 4.30 rear gear 5k rpm at 80,with current trans 5th gear ratio. Would have to look into S4 gearset specs but recall them having a lower numerical 5th gear like SE did as they're behind a 13B with more torque and power than a 12A. Might be able to run the 4.30 gearset with the overdrive of S4 box,certainly wouldn't be as bad as 12A box. If you already have S4 box,i'd go with 4.10 ratio. What size are tires(circumference) on back of car,will also affect final drive ratio.

 

If autox and track is your goal, do the following: Drive down the highway in 4th gear right now.

Is that acceptable fuel mileage and noise level for you? If no, don't do anything.

If yes, find yourself a used NA Miata gear box and swap those gears into your transmission. Then get a Kia Sportage rear diff with 4.77 gears to put in your rear end. This will give you identical 1st and 2nd gear that you have now (because miata 1st and 2nd are pretty tall), and then it will compress 3rd, 4th, and 5th into your current span between 3rd and 4th. It's the best ratio you're gonna get for the track with a N/A 12a, and will keep you out of the lower revs. Around town, you'll basically be driving in 5th gear. If you don't do the miata gears, any of the different final drives you pick are going to make 1st and 2nd really short (not so good for autox).

 

Many years ago I created a spreadsheet that allows you to input different gearing and tyre size to determine the effect on speedometer error, cruising speed etc.

See here: Wheel Sizing, Gearing, and Speedometer Spreadsheet - AusRotary

FWIW, my advice is to go shorter than that. You want at least 4.44 for a stock 12A. They need all the help they can get to get up and boogie. When I switched from 3.9 to 4.44 on my stock 12A, I was able to finally able to chirp the tyres between gears and cruise uphill without gearing down so often. For me, fuel consumption actually went down slightly, even after taking into account odometer correction, most likely due to the fact far less throttle was required under all conditions. But you may find yourself using driving a bit harder as it is much tempter to rev out in each gear.

 

i believe the mileage claim, my old 79 would get better mileage in the 3500-4000rpm range than in the 3000-3500rpm range, shorter gears would put it in the better rpm range at a slightly lower mph.

interestingly the Rx8 was like this too, it would get better mpg at 80 than at 65...