What is the best Diff Ratios?

[the bear]

What is the best diff ratio i.e. 3.92 / 3.62 / 3.38 etc for the different types of use (track, road) and what is the likely difference.

for example.

What ratio would give the best economy in a car used for touring?

What ratio would be best suited for sprinting where acceleration rather than top speed is important?

[Blatman]

Rather depends on what you've done with the gearbox ratios.

For standard Type 9 amd MT75 ratios in a road car, IMO the best diff is a 3.62. This will also be acceptable for track day use, and I have used this combination for a few sprints and did OK.

For a race/sprint car, you are far more likely to change the gearbox ratios so that you can hold on to first for *much* longer, and then when you do change gear, the resulting drop in revs isn't too large. The so called close ratio 'box. When using a car for race or sprint use only with no consideration for road manners, then you want as long a first as you can get. The longest is around a 2.1, with 2.39 being fairly common as well. With these two first gears, anything longer than a 3.9 would be tiresome in traffic, requiring lots of clutch slip and revs to move the car along at walking pace. Shorten the diff to 4.1, 4.4 or even 4.6/4.7 and you can recover some road manners whilst still retaining the usefulness of the long first. The shorter the diff, the closer the revs will be when you change gear too as swapping the diff affects the way all the ratios behave. The downside is that with a short (numerically high numbers) diff, you have a low top speed. One of my cars uses a 2.39 first gear with a 4.4 diff. Max speed at 7500rpm is around 129mph. This is good though 'cos I can actually achieve Vmax in top gear, eventually...

Most road cars with standard gearbox ratios and 3.6 or 3.9 diffs have theoretical top speeds the wrong side of 150mph, and therefore not much chance of actually making it all the way there. You'll get closer with an aeroscreen than with a full screen, but you'd be lucky to max out a "standard" ratio'd car in 5th gear.

So to summarise, for a mostly road going car with standard gearbox ratios, 3.6 or 3.9 diff is what you want.

For a race/sprint car, thenthe diff choice will depend on what you do with the gearbox ratios.

For a sprint/race car that you also want to use on the road, and assuming you have the "race" gearbox ratios, then you'll be wanting a 4.1, 4.4 or lower diff to retain the usefulness of the car in both road and track environments.