As I slowly grind on with the Bajaab project, and now have “big” all terrain tires to fit, I wanted to know how much the taller tires would affect gearing, and if this would diminish the car’s pulling power, particularly in 1st gear.
I found a fun gear ratio calculator, and began plugging all the numbers in. I found out that luckily, the 195/80 R15 tires don’t have as much of an effect on the gearing as I thought they would vs. the 165/80 R15 tires it has currently. In fact in top gear, theoretical top speed increased by only 7mph. In 1st gear the speed difference was a fraction of a mph at the engine speed I was interested in. The Saab’s Ford V4 reaches peak torque at 2500rpm, so that’s what I was looking at.
Although I have no plans to take the Saab out into any really hairy terrain, it is after all only 2 wheel drive, and I have no desire to relive the actual Baja team’s experience plugging through silt beds for hours on end - I also had no real idea whether 12mph for this sort of engine speed was good or bad comparatively. So I figured the Jeep Wrangler would be an interesting yardstick for comparison. Since the specs for the TJ came up first, that’s what I went with.
Plugging in the axle and gearbox ratios for a TJ with Dana 44 and nv3550 transmission, and (purportedly stock) 28.3" tall tires, I got the speeds above. Not really that far off from what the Saab is doing. In fact even with the taller than stock tires, the Saab 96 is shorter legged than the 5-speed TJ Rubicon, pushing out 11.9mph at 2500 rpm vs. the TJ’s 12.8 - interestingly, the Saab is slower in every gear but of course, unlike a Wrangler there is no low range to drop into when 1st gear isn’t short enough.