The Napier
The 1920s had dozens of cool cars, ranging from Rolls Royces to Chevys to Packards. Although I would have loved to have put my characters in the Rolls Royce Silver Ghost Torpedo (or a full racing car, for that matter!), for various reasons I couldn't. I settled instead on the Napier 40/50 HP. This British-built car had multiple body types and a max speed of 62 mph (100 kmh), although its normal running speed would not generally be above 40 mph (64.3 kmh). (Factoid: in 1907, its manufacturer used three Napier 60 HP cars to set the 24-hour speed record, averaging 65.9 mph, a record that lasted 18 years.) Here's a fun description of driving a Napier and a picture of a 1914 Napier, which is similar to what the protagonists would have driven.
The Western Desert
The setting of the second half of this chapter is Egypt's Western Desert--specifically, Mersa Matruh. Roughly, everything in Egypt west of the Nile is part of the Western Desert, although some maps show it as only being the southwest quarter of the country. During WWI, the Ottomans and Germans convinced the Senussi, a religious order/clan spanning the Libya-Egypt border, to attack the British in Egypt to divert their attention from Ottoman actions against the Suez Canal. The British used the town of Mersa Matruh as their base of operations since it was close enough to Alexandria to allow for supply lines and had a good water supply. Over the course of the war, thousands of members of the Western Frontier Force (WFF) passed through Mersa Matruh en route to fighting the Sanussi. (The location also was used as a British military base during WWII.)
What's cool about the WFF is how international it was. It included: 15th Sikhs, 1st New Zealand Rifle Brigade, 2 Troops Duke of Lancaster's Own Yeomanry, Composite Regiment Australian Light Horse, 1/6th Battalion Royal Scots, 15th Ludhiana Sikhs, 1st South African Infantry Brigade, Bikaner Camel Corps, Imperial Camel Corps, and Egyptian Artillery. But where did the garrison go after WWI was over? Did any of the units stay at Mersa Matruh? The answer is almost certainly no. Per this website, after Egypt gained its independence in 1922, the British troops that remained were in Abbassia (Cairo), Kasr-el-Nil (Cairo), Moascar (Ismailia), and Alexandria--no mention of Mersa Matruh. According to this website, it was only when the Italians joined WWII that the Brits moved back into the Western Desert to create the Western Desert Force. Thus in 1940 they re-opened the garrison at Mersa Matruh. So it seems likely that in 1923, the former British outpost would have been already pretty abandoned.
As noted in the chapter, Mersa Matruh is approximately 300 km from Alexandria. So if the protagonists traveled at 60 km/h to reach it (highly unlikely on the roads at the time), it would have taken them five hours. From Mersa Matruh to El Salloum is approximately 240 km, so it would have taken at least an additional four hours to reach it, and likely much longer.
Here are some more random factoids to go with the chapter:
-- If you've ever wondered what time the sun set on 11 December 1923, this helpful website claims it was 5:08 pm.
-- Here's an interesting Wikipedia article on the occupation of Constantinople.
-- In an earlier blog post, I noted the difficulty of finding what steam ships sailed to Egypt. I finally found a more helpful website here, although it's not comprehensive. If you love ship history, this link is for you! While we're at it, here's a page on the Khedivial Mail Line.